Spent nuclear fuel composition and reprocessing methods. Nuclear Fuel Cycle: Spent Nuclear Fuel

MOSCOW, June 21 – RIA Novosti. The enterprise of the state corporation "Rosatom" "Production Association "Mayak" (Ozersk, Chelyabinsk region) plans by 2020 to become the first enterprise in the world to master the technologies for reprocessing spent nuclear fuel (SNF) of any type, the deputy General Director of Mayak for strategic development Dmitry Kolupaev.

The organizer of Atomexpo 2017 is the state corporation Rosatom. The general information partner of the forum is the RIA Novosti agency (the flagship resource of MIA Rossiya Segodnya).

Reprocessing of spent nuclear fuel is a high-tech process aimed at minimizing the radiation hazard of spent nuclear fuel, safe disposal of unused components, separation of useful substances and ensuring their further use. Industrial reprocessing of spent nuclear fuel is carried out in three countries - Russia, France, and Great Britain.

Mayak is implementing a project to expand the range of spent fuel it reprocesses. In particular, the technology for reprocessing spent fuel from Russian VVER-1000 reactors has been mastered. This project will enable the enterprise in the next one and a half to two years to become the only enterprise in the world that can reprocess any type of spent nuclear fuel, including spent nuclear fuel of foreign design, as well as defective fuel assemblies. This will give Rosatom additional competitive advantages in world markets.

Mayak is the first industrial facility in the domestic nuclear industry. It was created to produce weapons-grade plutonium necessary for the creation of Soviet atomic weapons. The priority areas of Mayak's work at present are the reprocessing of spent nuclear fuel, the production of isotopes and radiation monitoring equipment, and the implementation of state defense orders.

"Omnivorous" complex

“In recent years, Mayak has made significant progress in terms of reprocessing spent nuclear fuel from research reactors. The reprocessing of several fuel compositions has been mastered, but the key project will probably be the reprocessing of uranium-zirconium fuel. Production facilities for this should be ready this year.” , said Kolupaev.

He explained that this will be a pilot plant, which will first allow us to test the necessary technologies, and then actually become a production plant.

"There is relatively little such fuel, and this is, first of all, the spent fuel of our nuclear icebreakers. It is located in a dry container storage facility in the North, but it cannot be used for any length of time. Therefore, the problem of reprocessing this type of spent nuclear fuel must be solved, and this does not require large production capacities,” the agency’s interlocutor noted.

Experimental reprocessing of uranium-zirconium spent nuclear fuel should be implemented by 2018, Kolupaev added. “This will actually make Mayak an absolute technological leader in terms of the range of fuel compositions that our enterprise will be able to process, because after mastering this technology, we will be able to process any fuel composition,” he said.

“And the final point will, perhaps, be the development of reprocessing of spent fuel from the AMB reactors of the first stage of the Beloyarsk NPP. The problem there is not so much in the fuel compositions themselves (several dozen types of fuel were used in the first and second units of the station), but in the geometric dimensions of the spent fuel assemblies.” , - said Kolupaev.

These assemblies reach a length of 14 meters, and in order to cut them, it is necessary special installation, he explained.

“It is planned to be created by 2020. And then an “omnivorous” reprocessing complex will be fully created at Mayak, both for different types of spent nuclear fuel and for the size of spent fuel assemblies,” noted the deputy general director of Mayak.

Reprocessing of radioactive waste

In addition to spent fuel reprocessing, Mayak is actively developing reprocessing technology radioactive waste, Kolupaev recalled.

“In the near future, the enterprise plans to begin operating a facility for solidifying long-lived intermediate-level waste, mainly plutonium-containing waste, for which cementation, as, say, our colleagues in the UK do, is not optimal. Our approach is based on the use of a ceramic-like matrix, which has a high durability and good waste capacity,” he said.

Last year was a kind of “start-up” for Mayak in terms of implementing a project to reprocess ionizing radiation sources, Kolupaev noted.

"We have fully fulfilled our obligations in terms of the volume of sources returned. This year, the volumes of sources returned for recycling will be significantly greater. We are optimizing the technology for source recycling to make it cheaper and more attractive to customers. This is a very important area that will allow our partners to receive a complete the cycle of services - from the moment of supply of sources to their complete disposal,” he added.

The planet's population, as well as its need for energy, is only growing every year, along with the prices of gas and oil, the processing of which, by the way, has its sad and irreversible consequences for the ecology of the earth. And nuclear energy today does not have a worthy alternative, either in terms of profitability or the ability to meet global energy needs.

Despite the fact that such statements sound very abstract, in practice, the abandonment of nuclear energy will mean a sharp rise in price for such things as necessary for everyone, such as food, clothing, medicine, comfortable Appliances, education, medicine, the ability to move freely around the world and much more. In such a situation, the best solution is to focus efforts on making nuclear energy as safe and efficient as possible.

Not everyone knows this fact: fresh nuclear fuel does not pose any danger to humans. Before the widespread introduction of industrial automation, uranium dioxide fuel pellets were driven into assembly rods by hand. The radioactivity of fuel increases several million times after irradiation in a nuclear reactor. It is at this moment that it becomes dangerous to humans and the environment.

Like any production, nuclear power plants generate waste. At the same time, the amount of waste produced by nuclear power plants is significantly less compared to other industries, but due to its high danger to the environment, it requires special handling. And here it is necessary to clarify some confusion between the concepts of RW (radioactive waste) and SNF (spent nuclear fuel), which often arises in the media.

By Russian classification, SNF refers to spent fuel elements removed from the reactor. Let us trace the path along which natural uranium mined in mines is converted into spent nuclear fuel. As we know, natural uranium consists of the isotopes uranium-235 and uranium-238. Modern nuclear power plants operate on uranium - 235. But due to low content 235 isotope (only 0.7%), for use as nuclear fuel, uranium extracted from the bowels of the earth must be enriched to a few percent. Uranium used in reactors is placed in fuel elements (fuel elements), from which fuel assemblies are assembled in the form of hexagonal rods. They are immersed in the reactor until a critical mass is reached. Before starting the reactor, the fuel rods contain 95% uranium-238 and 5% uranium-235. As a result of the operation of the reactor, fission products - radioactive isotopes - appear in place of uranium-235. The rods are removed, but as spent nuclear fuel.

SNF has rich resource potential. First, radioisotopes from spent fuel that can be chemically extracted have wide application for medical and scientific purposes. And not only for medical purposes - metals platinum group, formed in a reactor during the fission of uranium, are cheaper than the same metals obtained from ore. Secondly, the spent fuel contains uranium-238, which is considered worldwide as the main fuel element of future nuclear power plants. Thus, reprocessed spent nuclear fuel becomes not only the richest source for obtaining fresh nuclear fuel, but also solves the environmental problems of uranium deposits: there is no point in developing uranium mines, because at the moment 22 thousand tons of spent nuclear fuel have already been accumulated in Russia. At the same time, the content of radioactive elements in spent fuel, which cannot be reprocessed and require reliable isolation from the environment, is only 3%. For reference: reprocessing 50 tons of spent nuclear fuel saves 1.6 billion cubic meters natural gas or 1.2 million tons of oil.

Radioactive waste (RAW) also contains radioisotopes. The difference is that it is not possible to extract them, or the costs of extracting them are not economically feasible. At the moment, depending on the type of radioactive waste, there are several ways to manage radioactive waste. The sequence of actions is as follows: first, the volume of radioactive waste is reduced. In this case, for solid radioactive waste, pressing or combustion is used, for liquid radioactive waste – coagulation and evaporation, processing through mechanical or ion exchange filters. After treatment using special fabric or fiber filters, the volume of gaseous radioactive waste is reduced. The next stage is immobilization, that is, placing radioactive waste in a durable matrix of cement, bitumen, glass, ceramics or other materials that reduce the likelihood of radioactive waste being released into the environment. The resulting masses are placed in special containers and then stored. The final stage is the movement of containers with radioactive waste to the disposal site.

According to scientists, the most effective method for disposing of radioactive waste today is in stable geological formations of the earth’s crust. This method provides an effective insulating barrier for a period of tens of thousands to millions of years. Published in the electronic bulletin of the European Atomic Society, the results of joint research by the Subatech laboratory in France and the SCK-CEN research center in Belgium showed that the period during which blocks with nuclear waste can maintain their integrity exceeds 100 thousand years. The researchers came to this conclusion after making probabilistic estimates of the possible dissolution of buried nuclear waste from open and closed fuel cycles over various periods of time.

At the recent international scientific and practical conference “Safety, Efficiency and Economics of Nuclear Energy” held in Moscow, the pressing problems of spent nuclear fuel management were also discussed. In Russia, the storage and reprocessing of spent nuclear fuel is currently carried out by the Mayak production association (Ozersk, Chelyabinsk region) and the Mining and Chemical Combine (Zheleznogorsk, Krasnoyarsk region), which are part of the nuclear and radiation safety complex of the State Corporation Rosatom. Advisor to the State Corporation "Rosatom" I.V. Gusakov-Stanyukovich spoke about the departmental “Program for creating infrastructure and handling spent nuclear fuel for 2011-2020 and for the period until 2030.” According to him, today, of the 22,000 tons of spent fuel available, most of it is located at nuclear power plants. At the same time, the amount that is removed for storage during the year is less than what the nuclear power plant manages to produce during this time. And if spent fuel from those stations that use VVER-type reactors (water-cooled power reactor) is transported for storage at the Federal State Unitary Enterprise Mining and Chemical Combine or for reprocessing at the Federal State Unitary Enterprise PA Mayak, then the main problem Currently, this is spent fuel from RBMK reactors (high power channel reactor), the amount of which is 12.5 thousand tons. The dry storage facility for RBMK spent fuel at the Mining and Chemical Combine recently began operating, and in the spring of 2012 the first train with spent fuel from the Leningrad NPP arrived there. In the future, conditioned SNF from Leningrad, Kursk and Smolensk NPPs will be sent to the Mining and Chemical Combine, and substandard SNF will be sent to PA Mayak.

The implementation of the program for creating infrastructure and handling spent nuclear fuel by 2018 will make it possible to increase the volume of annual removal of spent nuclear fuel from nuclear power plant sites, which will exceed the annual production of spent nuclear fuel by 1.5 times. And by 2030, all 100% of the spent fuel from RBMK-1000 and VVER-1000 reactors will be placed for long-term centralized storage at the MCC site, after which the main specialization of the MCC will be the production of MOX fuel. As for plans for spent fuel from VVER-440 and BN-600 reactors, as well as transport and research reactors, the reprocessing of these spent fuels will be carried out at Mayak. An exception will be the Bilibino NPP, where it is impractical to transport spent fuel to centralized reprocessing facilities due to its geographical remoteness, so it will be buried on site.

Spent nuclear fuel from power reactors The initial stage of the post-reactor stage of the nuclear fuel cycle is the same for open and closed nuclear fuel cycles.

It involves removing fuel rods with spent nuclear fuel from the reactor, storing it in an on-site pool (“wet” storage in underwater cooling pools) for several years and then transporting it to a reprocessing plant. In the open version of the nuclear fuel cycle, spent fuel is placed in specially equipped storage facilities (“dry” storage in an inert gas or air environment in containers or chambers), where it is kept for several decades, then processed into a form that prevents the theft of radionuclides and prepared for final disposal.

In the closed version of the nuclear fuel cycle, the spent fuel is supplied to a radiochemical plant, where it is processed to extract fissile nuclear materials.

Spent nuclear fuel (SNF) is a special type of radioactive materials - raw materials for the radiochemical industry.

Irradiated fuel elements removed from the reactor after their exhaustion have significant accumulated activity. There are two types of spent nuclear fuel:

1) SNF from industrial reactors, which has a chemical form of both the fuel itself and its cladding, convenient for dissolution and subsequent processing;

2) Fuel rods for power reactors.

SNF from industrial reactors is reprocessed without fail, while SNF is not always reprocessed. Power generation spent fuel is classified as high-level waste if it is not subjected to further processing, or to valuable energy raw materials, if processed. In some countries (USA, Sweden, Canada, Spain, Finland), SNF is completely classified as radioactive waste (RAW). In England, France, Japan - to energy raw materials. In Russia, part of the spent fuel is considered radioactive waste, and part is sent for reprocessing to radiochemical plants (146).

Due to the fact that not all countries adhere to closed-loop tactics nuclear cycle, SNF in the world is constantly increasing. The practice of countries adhering to a closed uranium fuel cycle has shown that partial closure of the nuclear fuel cycle of light water reactors is unprofitable, even with a possible 3-4 times increase in the price of uranium in the next decades. Nevertheless, these countries are closing the nuclear fuel cycle of light water reactors, covering the costs by increasing electricity tariffs. On the contrary, the United States and some other countries refuse to reprocess spent nuclear fuel, keeping in mind the future final disposal of spent nuclear fuel, preferring its long-term storage, which turns out to be cheaper. However, it is expected that by the twenties the reprocessing of spent nuclear fuel in the world will increase.

The fuel assemblies with spent nuclear fuel removed from the core of a power reactor are stored in a cooling pool at a nuclear power plant for 5-10 years to reduce heat generation and decay of short-lived radionuclides. On the first day after its unloading from the reactor, 1 kg of spent nuclear fuel from a nuclear power plant contains from 26 to 180 thousand Ci of radioactivity. After a year, the activity of 1 kg of spent fuel decreases to 1 thousand Ci, after 30 years, to 0.26 thousand Ci. A year after removal, as a result of the decay of short-lived radionuclides, the activity of spent fuel is reduced by 11 - 12 times, and after 30 years - by 140 - 220 times and then slowly decreases over hundreds of years 9 (146).

If natural uranium was initially loaded into the reactor, then 0.2 - 0.3% 235U remains in the spent fuel. Re-enrichment of such uranium is not economically feasible, so it remains in the form of so-called waste uranium. The waste uranium can later be used as breeding material in fast neutron reactors. When low-enriched uranium is used to load nuclear reactors, spent fuel contains 1% 235U. Such uranium can be further enriched to its original content in nuclear fuel and returned to the nuclear fuel cycle. The reactivity of nuclear fuel can be restored by adding other fissile nuclides to it - 239Pu or 233U, i.e. secondary nuclear fuel. If 239Pu is added to depleted uranium in an amount equivalent to enriching the fuel with 235U, then a uranium-plutonium fuel cycle is implemented. Mixed uranium-plutonium fuel is used in both thermal and fast neutron reactors. Uranium-plutonium fuel ensures the fullest use of uranium resources and expanded reproduction of fissile material. For nuclear fuel regeneration technology, the characteristics of the fuel unloaded from the reactor are extremely important: chemical and radiochemical composition, content of fissile materials, activity level. These characteristics of nuclear fuel are determined by the power of the reactor, the burnup of the fuel in the reactor, the duration of the campaign, the reproduction rate of secondary fissile materials, the holding time of the fuel after unloading it from the reactor, and the type of reactor.

Spent nuclear fuel unloaded from reactors is transferred for reprocessing only after a certain period of time. This is due to the fact that among the fission products there are a large number of short-lived radionuclides, which determine a large share of the activity of the fuel discharged from the reactor. Therefore, freshly unloaded fuel is kept in special storage facilities for a time sufficient for the decay of the main amount of short-lived radionuclides. This greatly facilitates the organization of biological protection, reduces the radiation impact on chemical reagents and solvents during the reprocessing of treated nuclear fuel, and reduces the set of elements from which the main products must be purified. Thus, after two to three years of exposure, the activity of irradiated fuel is determined by long-lived fission products: Zr, Nb, Sr, Ce and other rare earth elements, Ru and α-active transuranium elements. 96% of spent nuclear fuel is uranium-235 and uranium-238, 1% is plutonium, 2-3% is radioactive fission fragments.

The spent fuel holding time is 3 years for light water reactors, 150 days for fast neutron reactors (155).

The total activity of fission products contained in 1 ton of VVER-1000 spent fuel after three years of aging in the spent fuel pool (SP) is 790,000 Ci.

When SNF is stored in an on-site storage facility, its activity monotonically decreases (by about an order of magnitude over 10 years). When activity drops to standards that determine the safety of transporting spent fuel by rail, it is removed from their storage facilities and moved either to a long-term storage facility or to a fuel reprocessing plant. At the processing plant, fuel rod assemblies are reloaded from containers into the factory buffer storage pool using loading and unloading mechanisms. Here the assemblies are stored until they are sent for processing. After holding in the pool for a period selected at a given plant, the fuel assemblies are unloaded from storage and sent to the fuel preparation department for extraction for the operation of opening spent fuel rods.

Reprocessing of irradiated nuclear fuel is carried out with the aim of extracting fissile radionuclides from it (primarily 233U, 235U and 239Pu), purifying uranium from neutron-absorbing impurities, separating neptunium and some other transuranic elements, and obtaining isotopes for industrial, scientific or medical purposes. Nuclear fuel reprocessing refers to the reprocessing of fuel rods from power, scientific or transport reactors, as well as the reprocessing of breeder reactor blankets. Radiochemical reprocessing of spent fuel is the main stage of the closed version of the nuclear fuel cycle, and a mandatory stage in the production of weapons-grade plutonium (Fig. 35).

Processing of fissile material irradiated with neutrons in a nuclear fuel reactor is carried out to solve problems such as

Obtaining uranium and plutonium for the production of new fuel;

Obtaining fissile materials (uranium and plutonium) for the production of nuclear weapons;

Obtaining a variety of radioisotopes that are used in medicine, industry and science;

Rice. 35. Some stages of spent nuclear fuel reprocessing at Mayak PA. All operations are carried out using manipulators and chambers protected by a 6-layer lead glass (155).

Receiving income from other countries that are either interested in the first and second, or do not want to store large volumes of spent nuclear fuel;

Solving environmental problems associated with the disposal of radioactive waste.

In Russia, irradiated uranium from breeder reactors and fuel rods from VVER-440, BN and some ship engines are processed; Fuel rods of the main types of power reactors VVER-1000, RBMK (any type) are not recycled and are currently accumulated in special storage facilities.

Currently, the amount of spent fuel is constantly increasing and its regeneration is the main task of radiochemical technology for reprocessing spent fuel rods. During the reprocessing process, uranium and plutonium are separated and purified from radioactive fission products, including neutron-absorbing nuclides (neutron poisons), which, when fissile materials are reused, can prevent the development of a chain reaction in the reactor. nuclear reaction.

Radioactive fission products contain a large number of valuable radionuclides that can be used in the field of small-scale nuclear energy (radioisotopic heat sources for thermoelectric power generators), as well as for the manufacture of sources of ionizing radiation. Transuranium elements are used, resulting from side reactions of uranium nuclei with neutrons. Radiochemical technology for reprocessing spent nuclear fuel must ensure the extraction of all nuclides useful from a practical point of view or of scientific interest (147 43).

The process of chemical reprocessing of spent fuel is associated with solving the problem of isolating from the biosphere a large amount of radionuclides generated as a result of the fission of uranium nuclei. This problem is one of the most serious and difficult to solve problems in the development of nuclear energy.

The first stage of radiochemical production includes fuel preparation, i.e. to free it from the structural parts of the assemblies and destroy the protective shells of the fuel rods. The next stage is associated with the transfer of nuclear fuel into the phase from which chemical processing will be carried out: into a solution, into a melt, into the gas phase. Conversion into solution is most often done by dissolving in nitric acid. In this case, uranium goes into the hexavalent state and forms a uranyl ion, UO 2 2+, and plutonium partially in the hexavalent state and into the tetravalent state, PuO 2 2+ and Pu 4+, respectively. Transfer to the gas phase is associated with the formation of volatile uranium and plutonium halides. After the transfer of nuclear materials, the corresponding phase involves a series of operations directly related to the isolation and purification of valuable components and the release of each of them in the form of a commercial product (Fig. 36).

Fig.36. General scheme for the circulation of uranium and plutonium in closed loop (156).

Reprocessing (reprocessing) of spent nuclear fuel involves the extraction of uranium, accumulated plutonium and fractions of fragmentation elements. 1 ton of spent fuel at the time of removal from the reactor contains 950-980 kg of 235U and 238U, 5.5-9.6 kg of Pu, as well as a small amount of α-emitters (neptunium, americium, curium, etc.), the activity of which can reach 26 thousand Ci per 1 kg of spent fuel. It is these elements that must be isolated, concentrated, purified and converted into the required chemical form during a closed nuclear fuel cycle.

The technological process of spent nuclear fuel reprocessing includes:

Mechanical fragmentation (cutting) of fuel assemblies and fuel rods in order to open the fuel material;

Dissolution;

Cleaning solutions of ballast impurities;

Extraction separation and purification of uranium, plutonium and other commercial nuclides;

Release of plutonium dioxide, neptunium dioxide, uranyl nitrate hexahydrate and uranium oxide;

Processing of solutions containing other radionuclides and their separation.

The technology for separating uranium and plutonium, separating them and purifying them from fission products is based on the process of extracting uranium and plutonium with tributyl phosphate. It is carried out on multi-stage continuous extractors. As a result, uranium and plutonium are purified from fission products millions of times. Reprocessing of spent nuclear fuel is associated with the formation of a small volume of solid and gaseous radioactive waste with an activity of about 0.22 Ci/year (the maximum permissible release is 0.9 Ci/year) and a large amount of liquid radioactive waste.

All construction materials of fuel rods are characterized by chemical resistance, and their dissolution poses a serious problem. In addition to fissile materials, fuel rods contain various storage devices and coatings consisting of stainless steel, zirconium, molybdenum, silicon, graphite, chromium, etc. When nuclear fuel is dissolved, these substances do not dissolve in nitric acid and create a large amount of suspensions and colloids in the resulting solution.

The listed features of fuel rods have necessitated the development of new methods for opening or dissolving shells, as well as clarification of nuclear fuel solutions before extraction processing.

The fuel burnup of plutonium production reactors differs significantly from the fuel burnup of power reactors. Therefore, materials with a much higher content of radioactive fragmentation elements and plutonium per 1 ton U are received for reprocessing. This leads to increased requirements for the purification processes of the resulting products and for ensuring nuclear safety during the reprocessing process. Difficulties arise due to the need to process and dispose of large amounts of liquid high-level waste.

Next, uranium, plutonium and neptunium are isolated, separated and purified in three extraction cycles. In the first cycle, uranium and plutonium are jointly purified from the bulk of fission products, and then uranium and plutonium are separated. In the second and third cycles, uranium and plutonium are further separately purified and concentrated. The resulting products - uranyl nitrate and plutonium nitrate - are placed in buffer tanks before being transferred to conversion units. Oxalic acid is added to the plutonium nitrate solution, the resulting oxalate suspension is filtered, and the precipitate is calcined.

Powdered plutonium oxide is sifted through a sieve and placed in containers. In this form, plutonium is stored before it enters the plant for the production of new fuel rods.

Separation of fuel rod cladding material from the fuel cladding is one of the most difficult tasks in the nuclear fuel regeneration process. Existing methods can be divided into two groups: opening methods with separation of the cladding and core materials of fuel rods and opening methods without separating the cladding materials from the core material. The first group involves removing the cladding of fuel rods and removing structural materials before dissolving the nuclear fuel. Water-chemical methods involve dissolving shell materials in solvents that do not affect the core materials.

The use of these methods is typical for the processing of fuel rods made from uranium metal in shells made of aluminum or magnesium and its alloys. Aluminum easily dissolves in caustic soda or nitric acid, and magnesium - in dilute solutions of sulfuric acid when heated. After dissolving the shell, the core is dissolved in nitric acid.

However, fuel rods of modern power reactors have shells made of corrosion-resistant, poorly soluble materials: zirconium, zirconium alloys with tin (zircal) or niobium, stainless steel. Selective dissolution of these materials is only possible in highly aggressive environments. Zirconium is dissolved in hydrofluoric acid, in its mixtures with oxalic or nitric acids or NH4F solution. Stainless steel shell - in boiling 4-6 M H 2 SO 4. The main disadvantage of the chemical method of removing shells is the formation of a large amount of highly saline liquid radioactive waste.

To reduce the volume of waste from the destruction of shells and obtain this waste immediately in a solid state, more suitable for long-term storage, processes are being developed for the destruction of shells under the influence of non-aqueous reagents at elevated temperatures (pyrochemical methods). The zirconium shell is removed with anhydrous hydrogen chloride in a fluidized bed of Al 2 O 3 at 350-800 o C. Zirconium is converted into volatile ZrC l4 and is separated from the core material by sublimation, and then hydrolyzed, forming solid zirconium dioxide. Pyrometallurgical methods are based on the direct melting of shells or their dissolution in melts of other metals. These methods exploit differences in the melting temperatures of the shell and core materials or differences in their solubility in other molten metals or salts.

Mechanical methods Shell removal involves several stages. First, the end parts of the fuel assembly are cut off and disassembled into bundles of fuel rods and individual fuel rods. Then the shells are mechanically removed separately from each fuel rod.

Opening fuel rods can be carried out without separating the cladding materials from the core material.

When implementing water-chemical methods, the shell and core are dissolved in the same solvent to obtain a common solution. Co-dissolution is advisable when processing fuel with a high content of valuable components (235U and Pu) or when different types of fuel elements differing in size and configuration are processed at the same plant. In the case of pyrochemical methods, fuel rods are treated with gaseous reagents, which destroy not only the shell, but also the core.

A successful alternative to the methods of opening with simultaneous removal of the shell and methods of joint destruction of the shell and cores turned out to be the “cutting-leaching” method. The method is suitable for processing fuel rods in shells that are insoluble in nitric acid. Fuel rod assemblies are cut into small pieces, the exposed fuel rod core becomes accessible to chemical reagents and dissolves in nitric acid. Undissolved shells are washed from the remnants of the solution retained in them and removed in the form of scrap. Chopping fuel rods has certain advantages. The resulting waste - the remains of the shells - are in a solid state, i.e. there is no formation of liquid radioactive waste, as with chemical dissolution of the shell; there is no significant loss of valuable components, as during mechanical removal of shells, since sections of shells can be washed with a high degree of completeness; the design of cutting machines is simplified in comparison with the design of machines for mechanical removal of casings. The disadvantage of the cutting-leaching method is the complexity of the equipment for cutting fuel rods and the need for its remote maintenance. The possibility of replacing mechanical cutting methods with electrolytic and laser methods is currently being explored.

In spent fuel rods of high and medium burnup power reactors, large amounts of gaseous radioactive products accumulate, which represent a serious biological hazard: tritium, iodine and krypton. During the dissolution of nuclear fuel, they are mainly released and go with gas streams, but partially remain in solution, and are then distributed in a large number of products throughout the reprocessing chain. Tritium is especially dangerous, forming tritiated water HTO, which is then difficult to separate from ordinary water H2O. Therefore, at the stage of preparing fuel for dissolution, additional operations are introduced to free the fuel from the bulk of radioactive gases, concentrating them in small volumes of waste products. Pieces of oxide fuel are subjected to oxidative treatment with oxygen at a temperature of 450-470 o C. When the structure of the fuel lattice is rearranged due to the transition UO 2 -U 3 O 8, gaseous fission products - tritium, iodine, and noble gases - are released. Loosening of the fuel material during the release of gaseous products, as well as during the transition of uranium dioxide into nitrous oxide, helps to accelerate the subsequent dissolution of materials in nitric acid.

The choice of method for transferring nuclear fuel into solution depends on the chemical form of the fuel, the method of preliminary preparation of the fuel, and the need to ensure a certain productivity. Uranium metal is dissolved in 8-11M HNO 3, and uranium dioxide is dissolved in 6-8M HNO 3 at a temperature of 80-100 o C.

The destruction of the fuel composition upon dissolution leads to the release of all radioactive fission products. In this case, gaseous fission products enter the exhaust gas discharge system. The waste gases are cleaned before being released into the atmosphere.

Isolation and purification of target products

Uranium and plutonium, separated after the first extraction cycle, are further purified from fission products, neptunium, and each other to a level that meets the specifications of the nuclear fuel cycle and then converted into a commercial form.

Best results Further purification of uranium is achieved by combining different methods, such as extraction and ion exchange. However, on an industrial scale, it is more economical and technically simpler to use repeated extraction cycles with the same solvent - tributyl phosphate.

The number of extraction cycles and the depth of uranium purification are determined by the type and burnup of nuclear fuel supplied for reprocessing and the task of neptunium separation. To meet the technical specifications for the content of impurity α-emitters in uranium, the overall neptunium removal factor must be ≥500. After sorption purification, uranium is re-extracted into an aqueous solution, which is analyzed for purity, uranium content and degree of 235U enrichment.

The final stage of uranium refining is intended to convert it into uranium oxides - either by precipitation in the form of uranyl peroxide, uranyl oxalate, ammonium uranyl carbonate or ammonium uranate followed by calcination, or by direct thermal decomposition of uranyl nitrate hexahydrate.

After separation from the main mass of uranium, plutonium is subjected to further purification from fission products, uranium and other actinides to its own background for γ- and β-activity. The plants strive to produce plutonium dioxide as the final product, and then, in combination with chemical processing, to produce fuel rods, which avoids expensive transportation of plutonium, which requires special precautions especially when transporting solutions of plutonium nitrate. All stages of the technological process for purifying and concentrating plutonium require special reliability of nuclear safety systems, as well as the protection of personnel and the prevention of the possibility of environmental pollution due to the toxicity of plutonium and high levels of α-radiation. When developing equipment, all factors that can cause criticality are taken into account: mass of fissile material, homogeneity, geometry, reflection of neutrons, moderation and absorption of neutrons, as well as the concentration of fissile material in this process, etc. The minimum critical mass of an aqueous solution of plutonium nitrate is 510 g (if there is a water reflector). Nuclear safety during operations in the plutonium branch is ensured by the special geometry of the devices (their diameter and volume) and the limitation of the concentration of plutonium in the solution, which is constantly monitored at certain points of the continuous process.

The technology for the final purification and concentration of plutonium is based on successive cycles of extraction or ion exchange and an additional refining operation of plutonium precipitation followed by its thermal conversion into dioxide.

Plutonium dioxide enters the conditioning unit, where it is calcined, crushed, sifted, batched and packaged.

For the production of mixed uranium-plutonium fuel, the method of chemical coprecipitation of uranium and plutonium is advisable, which makes it possible to achieve complete homogeneity of the fuel. This process does not require separation of uranium and plutonium during spent fuel reprocessing. In this case, mixed solutions are obtained by partial separation of uranium and plutonium by displacement stripping. In this way it is possible to obtain (U, Pu)O2 for light water nuclear reactors on thermal neutrons with a PuO2 content of 3%, as well as for fast neutron reactors with a PuO2 content of 20%.

The discussion about the feasibility of reprocessing spent fuel is not only of a scientific, technical and economic nature, but also of a political nature, since the deployment of construction of reprocessing plants poses a potential threat of the proliferation of nuclear weapons. The central problem is ensuring complete safety of production, i.e. ensuring guarantees of controlled use of plutonium and environmental safety. Therefore, they are now creating efficient systems control of the technological process of chemical reprocessing of nuclear fuel, providing the ability to determine the amount of fissile materials at any stage of the process. Proposals of so-called alternative technological processes, for example the CIVEX process, in which plutonium is not completely separated from uranium and fission products at any stage of the process, which significantly complicates the possibility of its use in explosive devices, also serve to ensure guarantees of the non-proliferation of nuclear weapons.

Civex - reproduction of nuclear fuel without releasing plutonium.

To improve the environmental friendliness of SNF reprocessing, non-aqueous technological processes, which are based on differences in the volatility of the components of the system being processed. The advantages of non-aqueous processes are their compactness, the absence of strong dilutions and the formation of large volumes of liquid radioactive waste, and the lesser influence of radiation decomposition processes. The generated waste is in the solid phase and takes up a significantly smaller volume.

Currently, a variant of organizing a nuclear power plant is being studied, in which not identical units (for example, three of the same type of thermal neutron units) are built at the station, but different types (for example, two thermal and one fast reactor). First, fuel enriched in 235U is burned in a thermal reactor (with the formation of plutonium), then the fuel is transferred to a fast reactor, in which 238U is processed using the resulting plutonium. After the end of the use cycle, the spent fuel is supplied to the radiochemical plant, which is located directly on the territory of the nuclear power plant. The plant does not engage in complete fuel reprocessing - it is limited to separating only uranium and plutonium from spent fuel (by distilling off hexafluoride fluorides of these elements). The separated uranium and plutonium are used for the production of new mixed fuel, and the remaining spent fuel goes either to a plant for separating useful radionuclides or for disposal.

Before continuing the description of the closed nuclear fuel cycle, as I was convinced, it is worth talking in much more detail about the process of reprocessing SNF - spent nuclear fuel. And I have to agree: after all, most of the radiophobia, fueled by all sorts of opponents of nuclear energy, is based precisely on the myth about the terrible harmfulness of spent nuclear fuel, which simply knocks you down with incredible radioactivity and from day to day will destroy the entire planet and us, the “poor”, along with it . So, although I didn’t plan at first, I will have to write a cycle within a cycle - about the storage and reprocessing of spent nuclear fuel.

Part 3.

With processing, things weren’t always smooth sailing. Until the Purex process, patented in 1947 by the American Larned Brown Asprey, began to be introduced, both in the West and here we used the bismuth-phosphate process, developed in the same USA in 1943. The bismuth-phosphate process was used, first of all, to produce weapons-grade plutonium from spent fuel coming from breeder reactors, “tailored” for the creation of plutonium-239 specifically. Thanks to him, Nagasaki was “pleased” with the plutonium charge, and the same bismuth-phosphate process was used in the USSR to create our bombs. Both the Americans and we were in a hurry to forge a nuclear shield and sword, so we got around to mastering Asprey’s idea later than necessary.

The bismuth-phosphate process left us with a very bad memory: since 1957, from Ozersk to Pionersk, the East Ural radioactive trail stretched for more than 300 km, covering 23 thousand square kilometers and 272 thousand people living in this territory. Atheists talk about the wind rose, believers talk about the fact that someone or something is protecting Russia, there is no point in arguing: the East Ural trace did not touch Sverdlovsk and Chelyabinsk, cities with a population of over a million. But nuclear weapons took their bloody harvest - in the first 10 days, at least 200 people died from radiation, and the total number of victims is estimated at 250 thousand people. It is impossible not to talk about this in detail - you need to clearly understand how this became possible and whether everything has been done to ensure that this never happens again. So, of course, there will be a story about this accident at the Mayak plant. But let’s not do it right away—first, let’s try to understand in more detail what spent nuclear fuel is and how it is handled here and abroad in Russia. So let's start by studying how spent nuclear fuel is stored, and then we'll move on to methods for reprocessing it.

While browsing the websites of Greenpeace and other environmental activists, I sometimes came across the abbreviation SNF as “waste” nuclear fuel.

“Waste”?.. Let me remind you once again what we see in a conventional ton of spent nuclear fuel. 924 kg of uranium-238. Wow, a “waste”! After all, it was mined from natural ore, which often contains 99% or even more waste rock. They were pulled out of mines/quarries, purified mechanically, chemically, transported from remote corners, spun in centrifuges - and after all this, does anyone want to call it “waste”? Damn, no conscience... Next - about 8-9 kg of uranium-235, on which, in fact, all of our nuclear energy operates. From 10 to 12 kg are isotopes of plutonium, which simply does not exist in nature in any form; it can only “grow” in the reactor itself. 945 kilograms per ton are definitely useful substances obtained by man through enormous labor and a lot of money. Another 21 kg are transuranium elements.

“Transuranium” are those that are heavier than uranium, which also do not occur in nature, and which are also only “grown” in a nuclear reactor. Among them, for example, the isotope of neptunium-237 is an excellent starting material for the production of plutonium-238. And plutonium-238 is the basis of RTGs, radioactive sources of electricity: plutonium-238, when decaying, produces heat, and a thermoelectric generator converts it into electricity. RTGs power the equipment of spacecraft flying to places where solar panels are no longer useful. For example, the RTG provides electricity to the Quority Mars rover - now the RTG provides 125 watts of electrical power, in 14 years it will produce 100 watts. The Voyager equipment, as well as the equipment of the New Horizon launched to Pluto, worked and still works on RTGs. And also RTGs - navigation equipment along the Northern Sea Route, operating for years on the shores of seas with surprisingly gentle weather. RTGs are the work of weather stations in the same kind of places: they are set up once, and until the next call they have 20-30 years left. "Retreat"?..

Americium-241 is the basis of measuring instruments needed in a wide variety of industries. Only this element makes it possible, for example, to continuously measure the thickness of metal strips and sheet glass. With the help of americium-241, electrostatics are removed from plastics, synthetic films, and paper during their production; it is used in some smoke detectors. Americium-243 is even more promising - it can cause a chain reaction with a critical mass of only 3.78 kg. No, not for bombs, calm down, don't worry. 3.78 kilo is an ultra-compact reactor that quietly rises into orbit, from where a spacecraft can be launched into deep space at completely different speeds than today’s spacecraft. No, I’m not making up a fantastic story here: a ton of spent fuel contains about a kilogram of americium-241, from which almost a kilogram of americium-243 can be produced.

We can go on and on about transuranium atoms and their isotopes - many of them are already interesting, many open up the most tempting prospects. So I want to understand and forgive the person who calls spent nuclear fuel “waste”. I want to, but I can’t.

The entire radioactive danger is the remaining 30-35 kg of so-called “fission products”. A chain reaction is not just “one neutron knocking out two neutrons, and those, in turn, knocking out four more.” Neutrons are neutrons, but what happens to the atom into which this neutron deigns to crash? The impact causes the uranium-235 atom to fall apart, and the plutonium atom does the same. Yes, there is one more “secret” of nuclear energy that deserves a few words.

Remember how plutonium is formed in a reactor? From time to time, the “ballast” in the form of uranium-238 accepts a neutron and, after two beta decays, turns into plutonium-239. And plutonium enters into a chain reaction even more readily than uranium-235, and it does this as soon as it is formed. Plutonium “burns”, adding power to all our reactors - and this is good and useful. 1% of plutonium, which, on average, is contained in spent fuel, is the plutonium that did not have time to “burn out”, and it is produced twice as much during the time the fuel elements are in the reactor.

So, all the harmfulness of spent nuclear fuel is the fragments formed after neutrons strike uranium-235 nuclei and plutonium nuclei. Three - three and a half kilos of the rarest filth and abomination in each ton. Some of these elements begin to actively “eat” neutrons, slowing down the reaction. Some of these elements deteriorate the strength of the fuel pellet, making it brittle, and some are generally gases that cause the fuel pellets to “swell.” And all fission products (hereinafter – simply PD. No, just P and D, no need to add extra letters, even though they are asking for them!) – are obscenely radioactive. So, when we talk about spent fuel reprocessing, we talk about how to make these same 3–3.5% FP as safe as possible, how to reuse unburned uranium-235 and reactor plutonium. Just in case, I’ll repeat what “reactor plutonium” is: a mixture of plutonium isotopes with numbers 239, 240 and 241. Plutonium-240 is what makes reactor plutonium never become weapons-grade plutonium, that is, what makes spent nuclear fuel safe from the point of view of the proliferation of nuclear weapons.

I don’t want to theorize, let’s just look at the fate of the fuel rods after they were pulled out of the reactor. The assemblies “radiate” and heat up from the inside, since nuclear reactions continue in the PD. Where to put this “happiness”? Well, don't transport it! Water, the simplest water, slows down neutrons very well - that’s why fuel rods with spent nuclear fuel are placed in special on-site pools. After the radioactivity and temperature drop to values ​​that allow them to be transported, the rods are removed, placed in a special thick-walled container and transported to special “dry storage facilities”. “After” in the case of water-water reactors is three years, less is impossible. Transportation is not a trivial operation at all. Stick fuel rod assemblies into something made of cast iron and lead - that’s the weight! Therefore, the containers are simply steel, but they are filled with inert gases - they absorb neutrons and cool them at the same time. And now the containers themselves are sent to transport and packaging complexes, where steel is again, but already complete with concrete. They pulled them out of the pool, put them in containers, pumped gas into the containers, packed the containers and secured them into complexes, and only after that they drove them away. Only this way and no other way.

Where are they taking it? Dry spent fuel storage facilities have been implemented in Russia, the USA, Canada, Switzerland, Germany, Spain, Belgium, France, England, Sweden, Japan, Armenia, Slovakia, the Czech Republic, Romania, Bulgaria, Argentina, Romania, and Ukraine. All other countries are forced to somehow negotiate with them. However, why am I doing this? “Somehow” – yes, it’s clear how! Money. There are no options.

Technology for storing spent nuclear fuel in container-type storage facilities using dual-purpose containers (for storage and transportation), Photo: atomic-energy.ru

Dry storage is also a big topic. It's not so much a matter of quality as of quantity. More than 400 commercial reactors around the world, hundreds of experimental, experimental, research reactors, reactors for submarines of other aircraft carriers... Yeah. 378.5 thousand tons of spent fuel – as of today, for the summer of 2016. And 10.5 thousand tons annually. And 3-3.5% of them are PD. I didn’t just say that this abbreviation persistently asks for additional letters... A lot. So many. That’s why we need a lot of storage facilities; they require large volumes. Other requirements are clear: radiation safety, protection from any penetration, the maximum possible distance from large cities. Even after three years under water, the PD continues to be active - which means there is also a cooling system complete with a radiation safety system. In general, it’s troublesome, expensive, but there are no options.

Let's take a little more detail about how this is organized in Russia, since our dry spent fuel storage facility (with your permission - hereinafter referred to as the spent nuclear fuel storage facility) was put into operation quite recently, and it was the first to use technological innovations that make it unique today. And these words are not jingoistic patriotism, but a statement of fact on the part of the IAEA.

The construction of the spent nuclear fuel storage facility in Zheleznogorsk, at the Mining and Chemical Combine (hereinafter simply referred to as the Mining and Chemical Combine) began back in 2002, but six years passed before active work: everything changed dramatically after Russia adopted its first federal target program “Providing nuclear and radiation security for the period from 2008 to 2015." After this, the problem of financing was solved, and the General Director of the Mining and Chemical Complex, Petr Gavrilov, showed that in our times it is also possible to work with your sleeves rolled up, delivering results clearly on schedule and without boring financial frauds. In December 2011, the spent fuel storage facility at the Mining and Chemical Combine (wow, what a stream of acronyms that turned out to be) was put into operation. We made it! We met exactly within the estimate - 16 billion rubles, and let’s fix this figure more precisely, so that it is more convenient to compare with costs in countries that are now elegantly called “Western partners”. The ruble to dollar exchange rate in 2011 was an average of 31, so $516 million was invested in agriculture. The volume of the first stage of storage at the gas chemical complex is 8.129 thousand tons, that is, in Russia, arithmetic is 6 million 350 thousand dollars for storing 1 thousand tons of spent fuel (of course, these are only the initial costs).

And the word “managed” with an exclamation mark is also for a reason. The problem was that the Mayak production association did not reprocess spent fuel from RBMK-type reactors - only from VVER reactors. Accordingly, “wet” storage facilities for RBMK fuel were filled, filled, and filled. A large “wet” storage facility at the same gas-chemical complex saved the station from overflowing, but in 2011 it was also filled to capacity. Russian nuclear power plants produce 650 tons of spent fuel per year, and half of them are spent fuel from RBMKs, although their quantity is significantly less than that of VVER: reactor technology is such that the fuel burns up much less on RBMKs than on VVERs. The situation in 2011 was very tense because of this. For example, the “wet” storage facility at the Leningrad Nuclear Power Plant was 95% full at this point: one more fuel unloading, and the nuclear power plant would simply have to be shut down. The first train with spent fuel from St. Petersburg arrived in February 2012 - the problem was solved by “simply” maintaining the work schedule to the hour. Hey, Vostochny Cosmodrome!.. Look for Pyotr Gavrilov’s phone number, ask for a lecture on how to work. Since December 2011, the problem of spent fuel for the Leningrad, Kursk and Smolensk nuclear power plants has been resolved. SNF from the “wet” storage of the MCC itself is loaded into the dry storage facility, and SNF from these three nuclear power plants, which has been in storage for longer than the period after which transportation is possible, is transferred into it.

Why was the MCC chosen as the location for the central, main storage facility? Well, first of all, because of the extensive experience gained during the operation of the “wet” storage facility and because a spent fuel reprocessing plant with a capacity of 1,500 tons per year is planned and is being built at the MCC. Again, please pay attention to the numbers: annually Russian nuclear power plants produce 650 tons of spent fuel per year, Mayak reprocesses 600 of them, the plant at the Mining and Chemical Combine will reprocess another 1,500. The reprocessing rate is planned to be three times greater than the supply of spent fuel. For what? Russia will be able to accept spent fuel from Soviet-design reactors for reprocessing, and they are located in Ukraine, Armenia, Bulgaria, the Czech Republic, Finland, not to mention the new nuclear power plants that Rosatom is building around the world. The idea is obvious: to make money not only from building reactors and providing them with fuel, but also in, so to speak, the post-operation section.

But there are other reasons why the city of Zheleznogorsk (which was once Krasnoyarsk-26) was chosen for both storage and reprocessing of spent nuclear fuel. The security regime for this facility was built a long time ago and works without the slightest deviation. The seismic hazard for such objects is very important point, and Zheleznogorsk is located in one of the safest zones of our planet in this regard. Of course, no one forgot about earthquakes during construction: the SH building can withstand impacts of up to 9.7 points. True, in the history of the Earth there have been no such shocks in Siberia, but if we do do it, do it with a reserve. And, quite traditionally for Russian nuclear facilities, the crash of a plane onto the roof of the storage facility is also taken into account.

How concerned were you about radiation safety? The unfinished building of the RT-2 plant was carefully dismantled, and a completely new one was built on its foundation, after careful calculations. The new building is, for a moment, 80 thousand cubic meters of monolithic reinforced concrete. But these walls are just what they call the outer perimeter - important, but not the main one. SNF comes from nuclear power plants in special containers filled with inert gas and in which the “assemblies” are rigidly fixed. At the gas chemical plant they are placed in special canisters – again filled with inert gas. The “assemblies” continue to heat up, so there can’t be much cooling. In addition, inert gases completely eliminate corrosion, which, you see, is also important. The pencil cases are placed on racks, and placed at a distance from each other so as not to interfere with air convection. All these measures are designed to ensure that the farm continues to function quietly in the event of a complete lack of electricity and personnel - although I have no idea what would have to happen for such a case to occur. Well, perhaps a short circuit on the scale of the Krasnoyarsk Territory on the morning of January 1... In a word, NIKIMT-Atomstroy, which designed all this, did a great job. And there is no need to shy away from the abbreviation - Rosatom carefully preserves the names that appeared at the dawn of the atomic project! NIKIMT is the Research and Design Institute of Assembly Technology. Ufff!

Not only people from the IAEA visited the MCC. For example, the Japanese came - and tears of emotion flowed from them due to seismic safety. They asked about the guaranteed shelf life and refused to believe that it was only 50 years - we were sure that this was some kind of joke, since according to their standards it could not be less than 100 years. People came from the USA with calculators - they laughed at our meager GDP: storing spent nuclear fuel in Zheleznogorsk costs 5.5 times less than theirs. Various environmental activists and journalists arrived several times, ran with counters everywhere - no noise, no matter how hard you try. People were invited to public hearings as prescribed by all kinds of instructions - through the media, television, and the Internet. Social activists were not lazy - they came and examined. There is in Siberia the Public Environmental Chamber of the Civil Assembly of the Krasnoyarsk Territory (no, well, who comes up with such short names...), which summed up the results of the public hearings: “There are no grounds left for controversy around all types of safety at the spent nuclear fuel storage facility in Zheleznogorsk.”

Well, while everyone was running around and bitching, Pyotr Gavrilov and the head of the capital construction department of the plant, Alexey Vekentsev, continued to work - after all, in December 2011, only the first stage of the agricultural plant was completed. Having worked together with specialists from NIKIMT on the entire technological chain for reloading into canisters, ensuring the tightness of all seams on them, and so on, MCC with a clear conscience continued to work on expanding the storage facility. In December 2015, the State Commission signed an act of acceptance into operation of the agricultural complex “in full development” - a quiet, imperceptibly past event, confidently and reliably not noticed by our big media. What are some tens of thousands of cubes of concrete when it’s time to count the rhinestones in Kirkorov’s plume? world of the centralized dry storage complex for spent nuclear fuel." And again - exactly on schedule. And again – without corruption scandals.

“So far the only one in the world” – now with an emphasis on the word “for now”. Because in 2012 and to this day, decisions to build the same centralized dry storage facilities have already been made by Japan, Spain and South Korea. I emphasize - the same. The US Deputy Secretary of Energy also came to visit twice, but there is no doubt that “the same” will not appear there. They will add a porch, and it will instantly become epoch-making know-how. However, the situation with spent nuclear fuel in America deserves a separate note - everything there is very dramatic, although in places it is quite comical. Some kind of American “nuclear tradition” - doing serious projects in such a way that it is often impossible to look at it without smiling, I swear by the centrifuge!

Well, what does the completion of the construction of the full volume of agricultural production in Zheleznogorsk mean for Russia itself? Now there is enough space not only for spent fuel from RBMK reactors - there is also enough space for spent fuel from VVER reactors, and not only from nuclear power plants in Russia itself. The MCC is ready to accept spent fuel from the territory of Ukraine, Bulgaria, and the Czech Republic for storage; the “wet” storage facility for spent fuel at the Armenian NPP is preparing for partial unloading. But the ultimate goal is not the storage of spent nuclear fuel in itself, the ultimate goal is the very closure of the nuclear fuel cycle: work on the construction of a pilot demonstration center for spent nuclear fuel reprocessing is planned at the MCC. I will definitely return to spent nuclear fuel reprocessing, but after we briefly “examine” what is happening with spent nuclear fuel storage in various interesting countries.

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Nuclear fuel is a material used in nuclear reactors to carry out a controlled chain reaction. It is extremely energy-intensive and unsafe for humans, which imposes a number of restrictions on its use. Today we will learn what nuclear reactor fuel is, how it is classified and produced, and where it is used.

Progress of the chain reaction

During a nuclear chain reaction, the nucleus splits into two parts, which are called fission fragments. At the same time, several (2-3) neutrons are released, which subsequently cause the fission of subsequent nuclei. The process occurs when a neutron hits the nucleus of the original substance. Fission fragments have high kinetic energy. Their inhibition in matter is accompanied by the release of a huge amount of heat.

Fission fragments, together with their decay products, are called fission products. Nuclei that share neutrons of any energy are called nuclear fuel. As a rule, they are substances with an odd number of atoms. Some nuclei are fissioned purely by neutrons whose energy is above a certain threshold value. These are predominantly elements with an even number of atoms. Such nuclei are called raw material, since at the moment of capture of a neutron by a threshold nucleus, fuel nuclei are formed. The combination of combustible material and raw material is called nuclear fuel.

Classification

Nuclear fuel is divided into two classes:

1. Natural uranium. It contains fissile uranium-235 nuclei and uranium-238 feedstock, which is capable of forming plutonium-239 upon neutron capture.
2. A secondary fuel not found in nature. This includes, among other things, plutonium-239, which is obtained from fuel of the first type, as well as uranium-233, which is formed when neutrons are captured by thorium-232 nuclei.

From point of view chemical composition, there are the following types of nuclear fuel:

1. Metal (including alloys);
2. Oxide (for example, UO 2);
3. Carbide (for example PuC 1-x);
4. Mixed;
5. Nitride.

TVEL and TVS

Fuel for nuclear reactors is used in the form of small pellets. They are placed in hermetically sealed fuel elements (fuel elements), which, in turn, are combined into several hundred fuel assemblies (FA). Nuclear fuel is subject to high requirements for compatibility with fuel rod claddings. It must have a sufficient melting and evaporation temperature, good thermal conductivity, and not greatly increase in volume under neutron irradiation. The manufacturability of production is also taken into account.

Application

Fuel comes to nuclear power plants and other nuclear installations in the form of fuel assemblies. They can be loaded into the reactor both during its operation (in place of burnt-out fuel assemblies) and during a repair campaign. In the latter case, fuel assemblies are replaced in large groups. In this case, only a third of the fuel is completely replaced. The most burned-out assemblies are unloaded from the central part of the reactor, and in their place are placed partially burned-out assemblies that were previously located in less active areas. Consequently, new fuel assemblies are installed in place of the latter. This simple rearrangement scheme is considered traditional and has a number of advantages, the main one of which is ensuring uniform energy release. Of course, this is a schematic diagram that gives only a general idea of ​​the process.

Excerpt

After spent nuclear fuel is removed from the reactor core, it is sent to a cooling pool, which is usually located nearby. The fact is that spent fuel assemblies contain a huge amount of uranium fission fragments. After unloading from the reactor, each fuel rod contains about 300 thousand Curies of radioactive substances, releasing 100 kW/hour of energy. Due to this, the fuel self-heats and becomes highly radioactive.

The temperature of newly unloaded fuel can reach 300°C. Therefore, it is kept for 3-4 years under a layer of water, the temperature of which is maintained in the established range. As it is stored under water, the radioactivity of the fuel and the power of its residual emissions decreases. After about three years, self-heating of the fuel assembly reaches 50-60°C. Then the fuel is removed from the pools and sent for processing or disposal.

Uranium metal

Uranium metal is used relatively rarely as fuel for nuclear reactors. When a substance reaches a temperature of 660°C, a phase transition occurs, accompanied by a change in its structure. Simply put, uranium increases in volume, which can lead to the destruction of fuel rods. In the case of prolonged irradiation at a temperature of 200-500°C, the substance undergoes radiation growth. The essence of this phenomenon is the elongation of the irradiated uranium rod by 2-3 times.

The use of uranium metal at temperatures above 500°C is difficult due to its swelling. After nuclear fission, two fragments are formed, the total volume of which exceeds the volume of that very nucleus. Some fission fragments are represented by gas atoms (xenon, krypton, etc.). Gas accumulates in the pores of the uranium and forms internal pressure, which increases as the temperature increases. Due to an increase in the volume of atoms and an increase in gas pressure, nuclear fuel begins to swell. Thus, this refers to the relative change in volume associated with nuclear fission.

The strength of swelling depends on the temperature of the fuel rods and burnout. With increasing burnup, the number of fission fragments increases, and with increasing temperature and burnup, the internal gas pressure increases. If the fuel has higher mechanical properties, then it is less susceptible to swelling. Uranium metal is not one of these materials. Therefore, its use as fuel for nuclear reactors limits the burnup, which is one of the main characteristics of such fuel.

The mechanical properties of uranium and its radiation resistance are improved by alloying the material. This process involves adding aluminum, molybdenum and other metals to it. Thanks to doping additives, the number of fission neutrons required per capture is reduced. Therefore, materials that weakly absorb neutrons are used for these purposes.

Refractory compounds

Some refractory uranium compounds are considered good nuclear fuel: carbides, oxides and intermetallic compounds. The most common of these is uranium dioxide (ceramics). Its melting point is 2800°C, and its density is 10.2 g/cm 3 .

Since this material does not undergo phase transitions, it is less susceptible to swelling than uranium alloys. Thanks to this feature, the burnout temperature can be increased by several percent. At high temperatures, ceramics do not interact with niobium, zirconium, stainless steel and other materials. Its main disadvantage is its low thermal conductivity - 4.5 kJ (m*K), which limits the specific power of the reactor. In addition, hot ceramics are prone to cracking.

Plutonium

Plutonium is considered a low-melting metal. It melts at a temperature of 640°C. Due to its poor plastic properties, it is practically impossible to machine. The toxicity of the substance complicates the manufacturing technology of fuel rods. The nuclear industry has repeatedly attempted to use plutonium and its compounds, but they have not been successful. It is not advisable to use fuel for nuclear power plants containing plutonium due to an approximately 2-fold reduction in the acceleration period, which standard reactor control systems are not designed for.

For the manufacture of nuclear fuel, as a rule, plutonium dioxide, alloys of plutonium with minerals, and a mixture of plutonium carbides and uranium carbides are used. Dispersion fuels, in which particles of uranium and plutonium compounds are placed in a metal matrix of molybdenum, aluminum, stainless steel and other metals, have high mechanical properties and thermal conductivity. The radiation resistance and thermal conductivity of the dispersion fuel depend on the matrix material. For example, at the first nuclear power plant, the dispersed fuel consisted of particles of a uranium alloy with 9% molybdenum, which were filled with molybdenum.

As for thorium fuel, it is not used today due to difficulties in the production and processing of fuel rods.

Production

Significant volumes of the main raw material for nuclear fuel - uranium - are concentrated in several countries: Russia, the USA, France, Canada and South Africa. Its deposits are usually located near gold and copper, so all these materials are mined at the same time.

The health of people working in mining is at great risk. The fact is that uranium is a toxic material, and the gases released during its mining can cause cancer. And this despite the fact that the ore contains no more than 1% of this substance.

Receipt

The production of nuclear fuel from uranium ore includes the following stages:

1. Hydrometallurgical processing. Includes leaching, crushing and extraction or sorption recovery. The result of hydrometallurgical processing is a purified suspension of oxyuranium oxide, sodium diuranate or ammonium diuranate.
2. Conversion of a substance from oxide to tetrafluoride or hexafluoride, used to enrich uranium-235.
3. Enrichment of a substance by centrifugation or gas thermal diffusion.
4. Conversion of enriched material into dioxide, from which fuel rod “pellets” are produced.

Regeneration

During operation of a nuclear reactor, fuel cannot be completely burned out, so free isotopes are reproduced. In this regard, spent fuel rods are subject to regeneration for the purpose of reuse.

Today, this problem is solved through the Purex process, consisting of the following stages:

1. Cutting fuel rods into two parts and dissolving them in nitric acid;
2. Cleaning the solution from fission products and shell parts;
3. Isolation of pure compounds of uranium and plutonium.

After this, the resulting plutonium dioxide is used for the production of new cores, and the uranium is used for enrichment or also for the production of cores. Reprocessing nuclear fuel is a complex and expensive process. Its cost has a significant impact on the economic feasibility of using nuclear power plants. The same can be said about the disposal of nuclear fuel waste that is not suitable for regeneration.