Интегральный реактор на быстрых нейтронах: разработка и перспективы

Nuclear reactor design

The integral fast reactor (IFR, originally advanced liquid metal reactor) is a design for a nuclear reactor using fast neutrons and no neutron moderator (a "fast" reactor). IFR would breed more fuel and is distinguished by a nuclear fuel cycle that uses reprocessing via electrorefining at the reactor site. The U. S. Department of Energy began designing an IFR in 1984 and built a prototype, the Experimental Breeder Reactor II. On April 3, 1986, two tests demonstrated the safety of the IFR concept. These tests simulated accidents involving loss of coolant flow. Even with its normal shutdown devices disabled, the reactor shut itself down safely without overheating anywhere in the system. The IFR project was canceled by the US Congress in 1994, three years before completion. The proposed Generation IV Sodium Cooled Fast Reactor is its closest surviving fast breeder reactor design. Other countries have also designed and operated fast reactors. S PRISM (from SuperPRISM), also called PRISM (Power Reactor Innovative Small Module), is the name of a nuclear power plant design by GE Hitachi Nuclear Energy (GEH) based on the IFR. In 2022, GE Hitachi Nuclear Energy and TerraPower began exploring locating 5 Natrium sodium fast reactors based nuclear power plant design incorporating a PRISM reactor based on the IFR plus Terrapower's Traveling Wave design with a molten salt storage system in Kemmerer, Wyoming.

Research on IFR reactors began in 1984 at Argonne National Laboratory in Argonne, Illinois. as a part of the U. S. Department of Energy's national laboratory system, and currently operated on a contract by the University of Chicago. Argonne previously had a branch campus named "Argonne West" in Idaho Falls, Idaho that is now part of the Idaho National Laboratory. In the past, at the branch campus, physicists from Argonne West built what was known as the Experimental Breeder Reactor II (EBR II). In the meantime, physicists at Argonne designed the IFR concept, and it was decided that the EBR II would be converted to an IFR. Charles Till, a Canadian physicist from Argonne, was the head of the IFR project, and Yoon Chang was the deputy head. Till was positioned in Idaho, while Chang was in Illinois.

With the election of President Bill Clinton in 1992, and the appointment of Hazel O'Leary as the Secretary of Energy, there was pressure from the top to cancel the IFR. Sen. John Kerry (D MA) and O'Leary led the opposition to the reactor, arguing that it would be a threat to non proliferation efforts, and that it was a continuation of the Clinch River Breeder Reactor Project that had been canceled by Congress. Simultaneously, in 1994 Energy Secretary O'Leary awarded the lead IFR scientist with $10,000 and a gold medal, with the citation stating his work to develop IFR technology provided "improved safety, more efficient use of fuel and less radioactive waste." IFR opponents also presented a report by the DOE's Office of Nuclear Safety regarding a former Argonne employee's allegations that Argonne had retaliated against him for raising concerns about safety, as well as about the quality of research done on the IFR program. The report received international attention, with a notable difference in the coverage it received from major scientific publications. The British journal Nature entitled its article "Report backs whistleblower", and also noted conflicts of interest on the part of a DOE panel that assessed IFR research. In contrast, the article that appeared in Science was entitled "Was Argonne Whistleblower Really Blowing Smoke?". Despite support for the reactor by then Rep. Dick Durbin (D IL) and U. S. Senators Carol Moseley Braun (D IL) and Paul Simon (D IL), funding for the reactor was slashed, and it was ultimately canceled in 1994, at a greater cost than finishing it. When this was brought to President Clinton's attention, he said "I know; it's a symbol."

In 2001, as part of the Generation IV roadmap, the DOE tasked a 242 person team of scientists from DOE, UC Berkeley, Massachusetts Institute of Technology (MIT), Stanford, ANL, Lawrence Livermore National Laboratory (LLNL), Toshiba, Westinghouse, Duke, EPRI, and other institutions to evaluate 19 of the best reactor designs on 27 different criteria. The IFR ranked #1 in their study which was released April 9, 2002. At present, there are no Integral Fast Reactors in commercial operation. However, the BN 800 reactor, a very similar fast reactor operated as a burner of plutonium stockpiles, became commercially operational in 2014.

The IFR is cooled by liquid sodium and fueled by an alloy of uranium and plutonium. The fuel is contained in steel cladding with liquid sodium filling in the space between the fuel and the cladding. A void above the fuel allows helium and radioactive xenon to be collected safely without significantly increasing pressure inside the fuel element, and also allows the fuel to expand without breaching the cladding, making metal rather than oxide fuel practical. The advantages of liquid sodium coolant, as opposed to liquid metal lead, are that liquid sodium is far less dense and far less viscous (reduced pumping costs), is not corrosive (via dissolution) to common steels, and creates essentially no radioactive neutron activation byproducts. The disadvantage of sodium coolant, as opposed to lead coolant, is that sodium is chemically reactive, especially with water or air. Lead may be substituted for the eutectic alloy of lead and bismuth as used for coolant in Soviet Alfa class submarines' reactors.

Metal fuel with a sodium filled void inside the cladding to allow fuel expansion has been demonstrated in EBR II. Metallic fuel makes pyroprocessing the reprocessing technology of choice. Fabrication of metallic fuel is easier and cheaper than ceramic (oxide) fuel, especially under remote handling conditions.

Breeder reactors (such as the IFR) could in principle extract almost all of the energy contained in uranium or thorium, decreasing fuel requirements by nearly two orders of magnitude compared to traditional once through reactors, which extract less than 0.65% of the energy in mined uranium, and less than 5% of the enriched uranium with which they are fueled. This could greatly dampen concern about fuel supply or energy used in mining. What is more important today is why fast reactors are fuel efficient: because fast neutrons can fission or "burn out" all the transuranic waste (TRU) waste components (actinides: reactor grade plutonium and minor actinides) many of which last tens of thousands of years or longer and make conventional nuclear waste disposal so problematic. Most of the radioactive fission products (FPs) the reactor produces have much shorter half lives: they are intensely radioactive in the short term but decay quickly. Through many cycles, the IFR ultimately causes 99.9% of the uranium and transuranium elements to undergo fission and produce power; so its waste is just the fission products. These have much shorter half lives; so in 300 years, their radioactivity will fall below that of the original uranium ore. The fact that 4th generation reactors are being designed to use the waste from 3rd generation plants could change the nuclear story fundamentally—potentially making the combination of 3rd and 4th generation plants a more attractive energy option than 3rd generation by itself would have been, both from the perspective of waste management and energy security. "Integral" refers to on site reprocessing by electrochemical pyroprocessing. This separates spent fuel into 3 fractions: 1. Uranium, 2. Plutonium isotopes and other Transuranium elements, and 3. Nuclear fission products. The uranium and transuranium elements are recycled into new fuel rods, and the fission products are eventually converted to glass and metal blocks for safer disposal. Because fractions 2 and 3 (the combined transuranium elements and the fission products) are highly radioactive, fuel rod transfer and reprocessing operations use robotic or remote controlled equipment. This is also claimed to be a feature; not a bug; since fissile material that never leaves the facility (and would be lethal to handle if it did) greatly reduces the proliferation potential of possible diversion of fissile material.

In traditional light water reactors (LWRs) the core must be maintained at a high pressure to keep the water liquid at high temperatures. In contrast, since the IFR is a liquid metal cooled reactor, the core could operate at close to ambient pressure, dramatically reducing the danger of a loss of coolant accident. The entire reactor core, heat exchangers, and primary cooling pumps are immersed in a pool of liquid sodium or lead, making a loss of primary coolant extremely unlikely. The coolant loops are designed to allow for cooling through natural convection, meaning that in the case of a power loss or unexpected reactor shutdown, the heat from the reactor core would be sufficient to keep the coolant circulating even if the primary cooling pumps were to fail. The IFR also has passive safety advantages as compared with conventional LWRs. The fuel and cladding are designed such that when they expand due to increased temperatures, more neutrons would be able to escape the core, thus reducing the rate of the fission chain reaction. In other words, an increase in the core temperature will act as a feedback mechanism that decreases the core power. This attribute is known as a negative temperature coefficient of reactivity. Most LWRs also have negative reactivity coefficients; however, in an IFR, this effect is strong enough to stop the reactor from reaching core damage without external action from operators or safety systems. This was demonstrated in a series of safety tests on the prototype. Pete Planchon, the engineer who conducted the tests for an international audience quipped "Back in 1986, we actually gave a small [20 MWe] prototype advanced fast reactor a couple of chances to melt down. It politely refused both times." Liquid sodium presents safety problems because it ignites spontaneously on contact with air and can cause explosions on contact with water. This was the case at the Monju Nuclear Power Plant in a 1995 accident and fire. To reduce the risk of explosions following a leak of water from the steam turbines, the IFR design (as with other sodium cooled fast reactors) includes an intermediate liquid metal coolant loop between the reactor and the steam turbines. The purpose of this loop is to ensure that any explosion following the accidental mixing of sodium and turbine water would be limited to the secondary heat exchanger and not pose a risk to the reactor itself. Alternative designs use lead instead of sodium as the primary coolant. The disadvantages of lead are its higher density and viscosity, which increases pumping costs, and radioactive activation products resulting from neutron absorption. A lead bismuth eutectate, as used in some Russian submarine reactors, has lower viscosity and density, but the same activation product problems can occur.

IFR style reactors could produce much less waste than LWR reactors, and could even utilize other waste as fuel. The primary argument for pursuing IFR style technology today is that it provides the best solution to the existing nuclear waste problem because fast reactors can be fueled from the waste products of existing reactors as well as from the plutonium used in weapons, as is the case in the operating, as of 2014, BN 800 reactor. Depleted uranium (DU) waste can also be used as fuel in fast reactors. The waste products of IFR reactors either have a short half life, which means that they decay quickly and become relatively safe, or a long halflife, which means that they are only slightly radioactive. Due to pyroprocessing the total volume of true waste/fission products is 1/20th the volume of spent fuel produced by a light water plant of the same power output, and is often all considered to be waste. 70% of fission products are either stable or have half lives under one year. Technetium 99 and iodine 129, which constitute 6% of fission products, have very long half lives but can be transmuted to isotopes with very short half lives (15.46 seconds and 12.36 hours) by neutron absorption within a reactor, effectively destroying them (see more Long lived fission products). Zirconium 93, another 5% of fission products, could in principle be recycled into fuel pin cladding, where it does not matter that it is radioactive. Excluding the contribution from Transuranic waste(TRU) – which are isotopes produced when Uranium 238 captures a slow thermal neutron in an LWR but does not fission, all the remaining high level waste/fission products("FP") left over from reprocessing out the TRU fuel, is less radiotoxic(in Sieverts) than natural uranium(in a gram to gram comparison) within 400 years, and it continues its decline following this. The two forms of IFR waste produced, contain no plutonium or other actinides. The radioactivity of the waste decays to levels similar to the original ore in about 300–400 years. In fact, in the U. S. most spent LWR fuel has remained in storage at the reactor site instead of being transported for reprocessing or placement in a geological repository. The smaller volumes of high level waste from reprocessing could stay at reactor sites for some time, but are intensely radioactive from medium lived fission products(MLFPs) and need to be stored securely, like in the present Dry cask storage vessels. In its first few decades of use, before the MLFP's decay to lower heat producing levels, geological repository capacity is constrained not by volume but by heat generation, and decay heat generation from medium lived fission products is about the same per unit power from any kind of fission reactor, limiting early repository emplacement. The potential complete removal of plutonium from the waste stream of the reactor reduces the concern that now exists with spent nuclear fuel from most other reactors, namely that a spent fuel repository could be used as a plutonium mine at some future date. Some believe that actinide removal would offer few if any significant advantages for disposal in a geologic repository because some of the fission product nuclides of greatest concern in scenarios such as groundwater leaching actually have longer half lives than the radioactive actinides. These concerns do not consider the plan to store such materials in insoluble Synroc, and do not measure hazards in proportion to those from natural sources such as medical x rays, cosmic rays, or natural radioactive rocks (such as granite). These persons are concerned with radioactive fission products such as technetium 99, iodine 129, and cesium 135 with half lives between 213,000 and 15.7 million years" Some of which are being targeted for transmutation to belay even these comparatively low concerns, for example the IFR's positive void coefficient could be reduced to an acceptable level by adding technetium to the core, helping destroy the long lived fission product technetium 99 by nuclear transmutation in the process. (see more Long lived fission products)

IFRs use virtually all of the energy content in the uranium fuel whereas a traditional light water reactor uses less than 0.65% of the energy in mined uranium and less than 5% of the energy in enriched uranium.

The IFR uses metal alloy fuel (uranium/plutonium/zirconium) which is a good conductor of heat, unlike the LWR's (and even some fast breeder reactors') uranium oxide which is a poor conductor of heat and reaches high temperatures at the center of fuel pellets. The IFR also has a smaller volume of fuel, since the fissile material is diluted with fertile material by a ratio of 5 or less, compared to about 30 for LWR fuel. The IFR core requires more heat removal per core volume during operation than the LWR core; but on the other hand, after a shutdown, there is far less trapped heat that is still diffusing out and needs to be removed. However, decay heat generation from short lived fission products and actinides is comparable in both cases, starting at a high level and decreasing with time elapsed after shutdown. The high volume of liquid sodium primary coolant in the pool configuration is designed to absorb decay heat without reaching fuel melting temperature. The primary sodium pumps are designed with flywheels so they will coast down slowly (90 seconds) if power is removed. This coast down further aids core cooling upon shutdown. If the primary cooling loop were to be somehow suddenly stopped, or if the control rods were suddenly removed, the metal fuel can melt as accidentally demonstrated in EBR I, however, the melting fuel is then extruded up the steel fuel cladding tubes and out of the active core region leading to permanent reactor shutdown and no further fission heat generation or fuel melting. With metal fuel, the cladding is not breached and no radioactivity is released even in extreme overpower transients. Self regulation of the IFR's power level depends mainly on thermal expansion of the fuel which allows more neutrons to escape, damping the chain reaction. LWRs have less effect from thermal expansion of fuel (since much of the core is the neutron moderator) but have strong negative feedback from Doppler broadening (which acts on thermal and epithermal neutrons, not fast neutrons) and negative void coefficient from boiling of the water moderator/coolant; the less dense steam returns fewer and less thermalized neutrons to the fuel, which are more likely to be captured by U 238 than induce fissions. However, the IFR's positive void coefficient could be reduced to an acceptable level by adding technetium to the core, helping destroy the long lived fission product technetium 99 by nuclear transmutation in the process. but the IFR fuel cycle has some design features that would make proliferation more difficult than the current PUREX recycling of spent LWR fuel. For one thing, it may operate at higher burnups and therefore increase the relative abundance of the non fissile, but fertile, isotopes plutonium 238, plutonium 240 and plutonium 242. Unlike PUREX reprocessing, the IFR's electrolytic reprocessing of spent fuel did not separate out pure plutonium, and left it mixed with minor actinides and some rare earth fission products which make the theoretical ability to make a bomb directly out of it considerably dubious. Plutonium produced in the fuel of a breeder reactor generally has a higher fraction of the isotope plutonium 240, than that produced in other reactors, making it less attractive for weapons use, particularly in first generation nuclear weapon designs similar to Fat Man. This offers an intrinsic degree of proliferation resistance, but the plutonium made in the blanket of uranium surrounding the core, if such a blanket is used, is usually of a high Pu 239 quality, containing very little Pu 240, making it highly attractive for weapons use. "Although some recent proposals for the future of the ALMR/IFR concept have focused more on its ability to transform and irreversibly use up plutonium, such as the conceptual PRISM (reactor) and the in operation(2014) BN 800 reactor in Russia, the developers of the IFR acknowledge that it is 'uncontested that the IFR can be configured as a net producer of plutonium'." As mentioned above, if operated not as a burner, but as a breeder, the IFR has a clear proliferation potential "if instead of processing spent fuel, the ALMR system were used to reprocess irradiated fertile (breeding) material (that is if a blanket of breeding U 238 was used), the resulting plutonium would be a superior material, with a nearly ideal isotope composition for nuclear weapons manufacture."

A commercial version of the IFR, S PRISM, can be built in a factory and transported to the site. This small modular design (311 MWe modules) reduces costs and allows nuclear plants of various sizes (311 MWe and any integer multiple) to be economically constructed. Cost assessments taking account of the complete life cycle show that fast reactors could be no more expensive than the most widely used reactors in the world – water moderated water cooled reactors.