Кіріспе
Бақыланбайтын ядролық бөліну тізбекті реакциясы
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.
Критикалық апат – бұл бақыланбайтын ядролық бөліну тізбекті реакциясы. Оны кейде сындық ауытқу, сындық қуат ауытқуы, дивергенттік тізбекті реакция немесе жай ғана сын деп те атайды. Мұндай кез келген оқиға байытылған уран немесе плутоний сияқты сынық материалдың сынық массасының қасақана жиналуына немесе орналасуына байланысты болады. Егер олар қорғалмаған ортада орын алса, қауіпті апаттар өлімге әкелетін сәулелену дозаларын шығаруы мүмкін. Әдеттегі жағдайларда, сынық реакциясы (өздігінен қуаттылықпен қамтамасыз етілетін немесе қуаттылығы артатын) тек реактордың активтік аймағы немесе қолайлы сынақ ортасы сияқты қауіпсіз қорғалған жерде ғана болуы керек. Критикалық апат, егер реакция қасақана орындалмаса, мысалы, қауіпсіз емес ортада немесе реакторды күтіп-жөндеу кезінде орын алады. Ядролық қару жасау үшін қажетті барлық ерекшеліктер кездейсоқ түрде пайда бола алмайды. Кейбір жағдайларда тізбекті реакция кезінде бөлінетін жылу бөлініп шығатын (және басқа да жақын материалдардың) кеңеюіне себеп болады. Мұндай жағдайларда тізбекті реакция төмен қуатты тұрақты күйге түсуі немесе тіпті уақытша немесе тұрақты түрде тоқтауы (сындық емес) мүмкін. Атом энергиясын дамыту тарихында кем дегенде 60 критикалық апат болды, оның 22-сі процестік ортада, ядролық реактордың активтік аймағынан немесе тәжірибелік құралымдардан тыс, ал 38-і шағын тәжірибелік реакторларда және басқа да сынақ құралымдарында тіркелді. Реакторлардың сыртында болатын процестік апаттар үлкен сәуле шығарумен сипатталса да, шығарылымдар жергілікті болады. Дегенмен, осы оқиғаларға жақын адамдар сәулеленуге ұшырап, 20-дан астам адам қаза тапты. Реактор және сынақ құралымдарының кейбір апаттарында босаған энергия едәуір механикалық зақымдануға немесе будың жарылысына әкелді.
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 тамыз 1945 || Лос-Аламос || Ғалым Гарри Даглиан сәулеленуден уланып, 25 күннен кейін плутоний шаршасына вольфрам карбиді кірпішін кездейсоқ түсіргеннен кейін қайтыс болды, бұл кірпіш кейін "шайтан ядросы" деп аталды. Кірпіш нейтрондық рефлектор ретінде әрекет етіп, массаны сындық деңгейге жеткізді. Бұл өлімге әкелген алғашқы белгілі критикалық апат еді.
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 мамыр 1946 || Лос-Аламос || Ғалым Луи Слотин Даглиан апатына қатысқан плутонийдің "шайтан ядросы" сферасын пайдаланып, ұқсас оқиға кезінде (сол кезде "Паджарито апаты" деп аталды) өзіне кездейсоқ сәуле шағылыстырды. Слотин плутоний шаршасын жоғары және төмен орналасқан нейтронды шағылыстыратын бериллийден жасалған 9 дюймдік диаметрлі екі жарты шармен қоршады. Ол ыдыстарды сәл бөлек ұстап, құрастырманы сындық емес күйде ұстау үшін бұранданы пайдаланды, бұл қалыпты ережелерге қайшы келеді. Бұранда кездейсоқ сырғығанда, ыдыстар плутонийдың айналасын жауып, құрастырманы сындық деңгейге жеткізді. Слотин құрылғыны жылдам бөлшектеді, бұл бөлмедегі басқаларды өлімге әкелетін сәулеленуден сақтап қалды, бірақ Слотин өзі тоғыз күннен кейін сәулеленуден уланып қайтыс болды. "Шайтан ядросы" ерітіліп, материал келесі жылдары басқа бомба сынақтарында қайта пайдаланылды.
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 маусым 1958 || Оук Ридж, Теннесси || Y-12 зауытында уранды өңдеуге байланысты алғашқы тіркелген оқиға болды. Әдеттегі тесікті тексеру кезінде, білместен, 55 галлондық барабанға бөлінгіш ерітінді жинауға рұқсат етілді. Апат шамамен 20 минутқа созылды және нәтижесінде сегіз жұмысшы едәуір сәулеленуге ұшырады. Өлім-жітім болған жоқ, бірақ бесеуі 44 күн бойы ауруханада жатты. Барлық сегіз жұмыскер ақырында жұмысқа оралды.
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 қазан 1958 || Винча ядролық институты || Борис Кидрич ядролық институтындағы Винчадағы ауыр сулы РБ реакторында критикалық апат болды, нәтижесінде бір адам қаза тауып, бесеуі жарақат алды. Алғашқы тірі қалғандарға Еуропада алғаш рет сүйек миін трансплантация жасалды.
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 желтоқсан 1958 || Лос-Аламос || Плутонийді тазартумен айналысатын химиялық оператор Сесил Келли ірі араластыру резервуарындағы араластырғышты қосып, резервуарда айналым пайда болды. Органикалық еріткіште еріген плутоний тұнбаның ортасына ағып түсті. Процедуралық қатеге байланысты қоспада 3,27 кг плутоний болды, ол 200 микросекундқа дейін сындық деңгейге жетті. Кейінгі бағалауларға сәйкес, Келли 3900-4900 рад (36,385-45,715 Св) сәулелену алды. Басқа операторлар ақшыл көк жарықты көрді және Келлиді сыртта тауып: "Мен күйіп жатырмын! Мен күйіп жатырмын!" Ол 35 сағаттан кейін қайтыс болды.
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 қаңтар 1961 || SL-1, Айдахо Фоллсынан 64 км батысқа қарай || АҚШ армиясының тәжірибелік ядролық қуат реакторы SL-1 орталық басқару таяқшасын дұрыс емес қолмен шығару салдарынан будың жарылысына және активтік аймақтың бұзылуына ұшырады, нәтижесінде үш оператор жарылыс күшінен қаза тауып, денелеріне тікелей тиді.
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 желтоқсан 1968 || Маяк || Орталық Ресейдегі ядролық отынды өңдеу орталығында платонийді тазарту техникалары әртүрлі еріткіштерді қолдану арқылы тәжірибе жасалып жатты. Осы еріткіштердің кейбіреулері резервуарға құйылуы тиіс емес резервуарға көшті және бұл резервуар үшін бөлінгіш қауіпсіз шектің артығын құрады. Ережеге қайшы келіп, смена бастығы екі операторға резервуардың көлемін азайтуды және еріткішті басқа ыдысқа көшіруді бұйырды. Екі оператор "импровизацияланған және бекітілмеген операцияда уақытша сақтау ыдысы ретінде платонийлі органикалық ерітіндіні сақтау үшін қолайсыз геометриялы ыдысты" пайдаланды, яғни операторлар платоний ерітінділерін дұрыс емес – тіпті пішіні жағынан да – ыдысқа құйып жатты. Еріткіш ерітіндісінің көп бөлігі құйылғаннан кейін, жарық пен жылудың жарқырауы болды. "Қорқып, оператор бөтелкені тастап, баспалдақтан төмен қашты, бөлмеден шықты". Кешен эвакуацияланғаннан кейін, смена бастығы мен сәулеленуді бақылау бастығы ғимаратқа қайта кірді. Смена бастығы сәулеленуді бақылау бастығын алдап, оқиға орнына кірді; одан кейін смена бастығын сәулеленудің өлімге әкелетін дозасымен ұрып, мүмкін, ерітіндіні еденге құюға тырысқандықтан, үшінші және ең ірі критикалық апат болды.
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 қыркүйек 1983 || Буэнос-Айрес, Аргентина, Centro Atomico Constituyentes || RA-2 зерттеу реакторындағы оператор реактордағы суды жаңадан орналастыру кезінде 3700 рад (37 Гр) сәулеленудің өлімге әкелетін дозасын алды, екеуі жарақат алды.
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 тамыз 1985 || Чжажма шығанағы, Владивосток || Ядролық қуатпен жұмыс істейтін советтік суасты қайығы К-431-дің реакторлық бак қақпағы отынды ауыстырғаннан кейін ауыстырылып жатты. Қақпа дұрыс орнатылмады және басқару таяқшаларымен бірге қайтадан көтерілуі керек болды. Сәуледен қорғау үшін қақпаны тым жоғары көтерілмейтіндей етіп, сәуледен қорғау бағанасы орнатылуы керек еді, бірақ бұл бағана дұрыс орнатылмады және басқару таяқшаларымен бірге қақпа тым жоғары көтерілді. Сағат 10:55-те бортық реактор жылдам сындық деңгейге жетті, нәтижесінде шамамен 5·1018 бөліну және жылу/будың жарылысы болды. Жарылыс жаңа отынды шығарып, машиналық бөлімдерді жойды, суасты қайығының қысым корпусын және артқы бөлігін бұзды және 70 метр қашықтықта суға түскен шатырды жартылай жойды. 4 сағаттан кейін өрт болды, одан кейін радиоактивтік ластануды бағалау жұмыстары басталды. Он адам қаза тауып, 49 адам сәулеленуден зардап шекті, ал Дунай түбегінің солтүстік-батыс бөлігі ауыр ластанды.
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 ||
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| 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 ||
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| 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 ||
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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.
Көк сәуле
Көптеген сынды апаттар көк түсті жарқын шығаратыны байқалды. Осы себепті ауадағы электр ұшқындары, соның ішінде найзағай да көк түсті болып көрінеді. Чернобыль апаттық жағдайды жойғандар озон иісін қоршаған ортадағы жоғары радиоактивтіліктің белгісі деп есептеген. Бұл көк жарқын немесе "көк сәуле" су сынды жүйеге қатысқанда немесе көк жарқынды адам көзімен қабылдағанда Черенков сәулеленуімен де байланысты болуы мүмкін. Алайда, ядролық ыдыраудан шығатын төмен энергиялы зарядталған бөлшектерге қатысты емес.
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.
Жылу әсерлері
Кейбір адамдар "жылу толқыны" сезінгенін хабарлады. Бұл жағдайға психосоматикалық реакция болуы мүмкін бе (яғни, өлімге әкелетін сәулелену дозасынан болатын сөзсіз өлімнің жоғары ықтималдығын түсінуге байланысты), әлде бұл радиацияның нәтижесінде қызудың (немесе терідегі жылу сезгішін жүйкелердің термиялық емес стимуляциясы) физикалық әсері ме? Көз куәгерлерінің баяндамалары бойынша барлық сынды жағдайларды қарап шығу көрсетті, жылу толқындары тек флуоресцентті көк сәуле (Шеренков емес жарық, жоғарыда қараңыз) байқалған кезде ғана тіркелген. Бұл екеуінің арасындағы байланыс болуы мүмкін екенін көрсетеді, және олардың бірін анықтауға болады. Тығыз ауада азот пен оттегінің шығарылым сызықтарының 30% -дан астамы ультракүлгін диапазонда, ал 45% -ы инфрақызыл диапазонда. Көрінетін диапазонда тек 25% ғана. Тері жарықты (көрінетін немесе көрінбейтін) тері бетін қыздыру арқылы сезінеді, сондықтан бұл құбылыс жылу толқынының сезіміне түсіндіру беруі мүмкін. Дегенмен, бұл түсіндірме расталмады және куәгерлер баяндаған жарықтың қарқындылығы мен сезілген жылудың қарқындылығы арасындағы айқайшылықтар болуы мүмкін. Осы мәселені зерттеуге адамдардың осы жағдайларға куә болып, өз тәжірибелері мен байқауларын егжей-тегжейлі баяндауға жеткілікті уақыт тіршілік еткен жағдайлардың аздығы кедергі келтіреді.
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.