Введение
Неконтролируемая цепная реакция ядерного деления
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
Авария критичности — это случайная неконтролируемая цепная реакция ядерного деления. Она также известна как критическая экскурсия, критическая экскурсия мощности, дивергентная цепная реакция или просто критическое состояние. Любое подобное событие связано с непреднамеренным накоплением или расположением критической массы делящегося материала, например, обогащенного урана или плутония. Аварии критичности могут привести к потенциально смертельным дозам радиации, если они происходят в незащищенной среде. В нормальных условиях критическая или сверхкритическая реакция деления (самоподдерживающаяся по мощности или увеличивающаяся по мощности) должна происходить только внутри надежно защищенного объекта, такого как активная зона реактора или подходящая экспериментальная установка. Авария критичности возникает, если та же реакция достигается непреднамеренно, например, в небезопасной среде или во время технического обслуживания реактора. Хотя опасные и часто смертельные для людей в непосредственной близости, образовавшаяся критическая масса не способна вызвать мощный ядерный взрыв, подобный взрыву ядерной бомбы. Это связано с тем, что все конструктивные особенности, необходимые для создания ядерного боеприпаса, не могут возникнуть случайно. В некоторых случаях тепло, выделяемое цепной реакцией, приводит к расширению делящегося (и другого находящегося поблизости) материала. В таких случаях цепная реакция может либо перейти в устойчивое состояние с низкой мощностью, либо временно или постоянно прекратиться (стать субкритической). За всю историю развития атомной энергетики произошло по меньшей мере 60 аварий критичности, включая 22 в технологических процессах, за пределами активных зон ядерных реакторов или экспериментальных установок, и 38 в небольших экспериментальных реакторах и других испытательных сборках. Хотя аварии, происходящие за пределами реакторов, характеризуются значительными выбросами радиации, эти выбросы локализованы. Тем не менее, люди, находившиеся вблизи этих событий, получили смертельные дозы радиации, в результате чего погибло более 20 человек. В нескольких авариях на реакторах и критических экспериментальных установках высвобожденная энергия вызвала значительные механические повреждения или паровые взрывы.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
21 августа 1945 года ученый из Лос-Аламоса Гарри Даглиан получил смертельное радиационное отравление и умер через 25 дней после того, как случайно уронил кирпич из карбида вольфрама на сферу плутония, которая позже (см. следующую запись) получила прозвище «ядро демона». Кирпич действовал как нейтронный отражатель, приведя массу к критическому состоянию. Это была первая известная авария критичности, приведшая к смерти.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
21 мая 1946 года ученый из Лос-Аламоса Луи Слотин случайно облучил себя во время аналогичного инцидента (в то время называвшегося «аварией Паджарито») с использованием той же сферы плутония «ядро демона», которая была вовлечена в аварию Даглиана. Слотин окружил плутониевую сферу двумя полусферическими чашками диаметром 9 дюймов из нейтронно-отражающего материала бериллия, по одной сверху и снизу. Он использовал отвертку, чтобы удерживать чашки слегка раздвинутыми, тем самым поддерживая сборку в субкритическом состоянии, вопреки обычным протоколам. Когда отвертка случайно соскользнула, чашки сомкнулись вокруг плутония, переводя сборку в сверхкритическое состояние. Слотин быстро разобрал устройство, вероятно, спасая других в комнате от смертельного облучения, но сам Слотин умер от отравления радиацией через девять дней. «Ядро демона» было расплавлено, а материал был повторно использован в других испытаниях бомб в последующие годы.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
16 июня 1958 года на заводе Y12 в Оук-Ридже, штат Теннесси, произошла первая зарегистрированная авария критичности, связанная с переработкой урана. Во время обычной проверки на утечки делящийся раствор непреднамеренно скопился в 55-галлонном барабане. Экскурсия длилась около 20 минут и привела к значительному облучению восьми рабочих. Никто не погиб, хотя пятеро были госпитализированы на 44 дня. Все восемь рабочих в конечном итоге вернулись к работе.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
15 октября 1958 года в Институте ядерных исследований в Винче произошла критическая экскурсия в тяжеловодном реакторе RB в Институте ядерных исследований Бориса Кидрича в Винче, Югославия, в результате чего один человек погиб и пятеро получили ранения. Первые выжившие получили первую в Европе трансплантацию костного мозга.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
30 декабря 1958 года в Лос-Аламосе Сесил Келли, химик-оператор, работавший над очисткой плутония, включил мешалку в большом смесительном баке, что создало воронку в баке. Плутоний, растворенный в органическом растворителе, стекал в центр воронки. Из-за процедурной ошибки смесь содержала 3,27 кг плутония, который достиг критического состояния примерно на 200 микросекунд. По более поздним оценкам, Келли получил от 3900 до 4900 рад (36,385–45,715 Зв). Другие операторы сообщили, что видели яркую вспышку синего света и обнаружили Келли снаружи, кричащего: «Я горю! Я горю!» Он умер через 35 часов.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
3 января 1961 года SL-1, экспериментальный ядерный реактор Армии США, расположенный в 64 км к западу от Айдахо-Фолс, подвергся паровому взрыву и разрушению активной зоны из-за неправильного ручного извлечения центрального управляющего стержня, в результате чего три оператора погибли от силы взрыва и были пронзены.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
10 декабря 1968 года в Маяке, на ядерном топливном заводе в центральной России, проводились эксперименты с методами очистки плутония с использованием различных растворителей для экстракции. Некоторые из этих растворителей попали в резервуар, не предназначенный для их хранения, и превысили допустимый предел делящегося вещества для этого резервуара. Вопреки процедуре, начальник смены приказал двум операторам уменьшить объем резервуара и перекачать растворитель в другой сосуд. Два оператора использовали «неблагоприятный по геометрии сосуд в импровизированной и несанкционированной операции в качестве временного сосуда для хранения органического раствора плутония»; другими словами, операторы переливали растворы плутония в контейнер неправильного — и, что более важно, формы. После того, как большая часть растворителя была вылита, произошла вспышка света и тепла. «Испугавшись, оператор уронил бутылку, сбежал вниз по лестнице и вышел из комнаты». После эвакуации комплекса начальник смены и руководитель группы радиационного контроля вновь вошли в здание. Начальник смены затем обманул руководителя группы радиационного контроля и вошел в комнату, где произошел инцидент; за этим последовала третья и самая крупная авария критичности, которая облучила начальника смены смертельной дозой радиации, возможно, из-за попытки начальника смены вылить раствор в напольный слив.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
23 сентября 1983 года оператор исследовательского реактора RA-2 в Буэнос-Айресе, Аргентина, получил смертельную дозу радиации в 3700 рад (37 Гр) во время изменения конфигурации топливных стержней с использованием замедляющей воды в реакторе. Двое других получили ранения.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
10 августа 1985 года в заливе Чажма, Владивосток, заменяли крышку реакторного отсека советской атомной подводной лодки К-431 после ее дозаправки. Крышка была установлена неправильно и ее пришлось поднять снова вместе с управляющими стержнями. Предполагалось, что балка должна была предотвратить слишком сильное поднятие крышки, но эта балка была установлена неправильно, и крышка с управляющими стержнями была поднята слишком высоко. В 10:55 утра правый реактор стал быстро критическим, что привело к критической экскурсии примерно в 5·1018 делений и тепловому/паровому взрыву. Взрыв выбросил новую загрузку топлива, разрушил машинные отделения, прорвал корпус подводной лодки и кормовую переборку и частично разрушил укрытие для дозаправки, крыша которого упала в воду на расстоянии 70 метров. После этого последовал пожар, который был потушен через 4 часа, после чего началась оценка радиоактивного загрязнения. Погибло десять человек, 49 получили радиационные травмы, и большая территория к северо-западу от полуострова Дунай была сильно загрязнена.
Uncontrolled nuclear fission chain reaction
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment. Under normal circumstances, a critical or supercritical fission reaction (one that is self sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical). In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions. |
| 21 August 1945 || Los Alamos || Scientist Harry Daghlian suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.|| 0 || 1 ||
|
| 21 May 1946 || Los Alamos || Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium involved in the Daghlian accident. Slotin surrounded the plutonium sphere with two 9 inch diameter hemispherical cups of the neutron reflecting material beryllium, one above and one below. He was using a screwdriver to keep the cups slightly apart and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||
|
| 16 June 1958 || Oak Ridge, Tennessee || The first recorded uranium processing–related criticality occurred at the Y 12 Plant. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for 44 days. All eight workers eventually returned to work. || 8 || 0 ||
|
| 15 October 1958 || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||
|
| 30 December 1958 || Los Alamos || Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||
|
| 3 January 1961 || SL 1, 40 mile (64 km) west of Idaho Falls||SL 1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||
|
| 10 December 1968 || Mayak || The nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques using different solvents for solvent extraction. Some of these solvents carried over to a tank not intended to hold them, and exceeded the fissile safe limit for that tank. Against procedure a shift supervisor ordered two operators to lower the tank inventory and remove the solvent to another vessel. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type—more importantly, shape—of container. After most of the solvent solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident; this was followed by the third and largest criticality excursion that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.|| 1 || 1 ||
|
| 23 September 1983 || Centro Atomico Constituyentes || An operator at the RA 2 research reactor in Buenos Aires, Argentina, received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured. || 2 || 1 ||
|
|
| 10 August 1985 || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K 431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·1018 fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||
|
| 30 September 1999 || Tōkai || At the Japanese uranium reprocessing facility in Ibaraki Prefecture, technicians working on producing fuel for the Jōyō fast reactor poured a uranyl nitrate solution into a precipitation tank which was not designed to hold a solution of this uranium enrichment, causing an eventual critical mass to be formed, resulting in the death of two workers from severe radiation exposure. ||1 || 2 ||
|}
<gallery>
File:Partially reflected plutonium sphere. jpeg|The sphere of plutonium surrounded by neutron reflecting tungsten carbide blocks in a re enactment of Harry Daghlian's 1945 experiment Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) "emphasized that the nuclear reactors won't explode." By 23 March 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radioactivity release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.
Синий свет
Было замечено, что многие аварии с развитием критичности сопровождаются синей вспышкой света. Именно поэтому электрические искры в воздухе, включая молнию, кажутся электрически-голубыми. Ликвидаторы Чернобыльской аварии считали запах озона признаком высокой фоновой радиоактивности. Эта синяя вспышка или "голубое свечение" также может быть объяснена излучением Черенкова, если вода присутствует в критической системе или если синяя вспышка воспринимается человеческим глазом, но не для частиц с низкой энергией, испускаемых при ядерном распаде.
It has been observed that many criticality accidents emit a blue flash of light. This is also the reason electric sparks in air, including lightning, appear electric blue. The smell of ozone was said to be a sign of high ambient radioactivity by Chernobyl liquidators. This blue flash or "blue glow" can also be attributed to Cherenkov radiation, if either water is involved in the critical system or when the blue flash is experienced by the human eye. but not for the lower energy charged particles emitted from nuclear decay.
Тепловые эффекты
Некоторые люди сообщали об ощущении "тепловой волны" во время критического события. Неизвестно, является ли это психосоматической реакцией на осознание произошедшего (то есть высокой вероятности неминуемой смерти от смертельной дозы радиации), или физическим эффектом нагрева (либо нетермической стимуляцией теплочувствительных нервов кожи) из-за излучения, испускаемого при критическом событии. Анализ всех случаев критических аварий с показаниями очевидцев показывает, что тепловые волны наблюдались только одновременно с флуоресцентным синим свечением (не свечением Черенкова, см. выше). Это указывает на возможную связь между этими явлениями, и их действительно можно установить. В плотном воздухе более 30% спектральных линий излучения азота и кислорода приходится на ультрафиолетовый диапазон, а около 45% – на инфракрасный. Лишь около 25% находится в видимом диапазоне. Поскольку кожа ощущает свет (видимый или невидимый) через нагрев поверхности, возможно, именно это явление объясняет восприятие тепловых волн. Однако это объяснение не подтверждено и может не соответствовать интенсивности света, о которой сообщают свидетели, по сравнению с интенсивностью воспринимаемого тепла. Дальнейшие исследования затруднены из-за малого количества данных, полученных в редких случаях, когда люди становились свидетелями этих инцидентов и выживали достаточно долго, чтобы дать подробный отчет о своем опыте и наблюдениях.
Some people reported feeling a "heat wave" during a criticality event. It is not known whether this may be a psychosomatic reaction to the realization of what has just occurred (i. e. the high probability of inevitable impending death from a fatal radiation dose), or if it is a physical effect of heating (or non thermal stimulation of heat sensing nerves in the skin) due to radiation emitted by the criticality event. A review of all of the criticality accidents with eyewitness accounts indicates that the heat waves were only observed when the fluorescent blue glow (the non Cherenkov light, see above) was also observed. This would suggest a possible relationship between the two, and indeed, one can be potentially identified. In dense air, over 30% of the emission lines from nitrogen and oxygen are in the ultraviolet range, and about 45% are in the infrared range. Only about 25% are in the visible range. Since the skin feels light (visible or otherwise) through its heating of the skin surface, it is possible that this phenomenon can explain the heat wave perceptions. However, this explanation has not been confirmed and may be inconsistent with the intensity of light reported by witnesses compared to the intensity of heat perceived. Further research is hindered by the small amount of data available from the few instances where humans have witnessed these incidents and survived long enough to provide a detailed account of their experiences and observations.