Figure 1 Nuclear fuel cycle diagram Figure 1 includes the storage, reprocessing and final disposal of spent fuel. The percentage in the figure represents the content of U-235.
The substances accounting for 3% of the mass of spent fuel are uranium-235 and plutonium -239 fission products and their Decay chain Indirect product of. Although these substances are believed to be radioactive waste However, as they may have multiple industrial and material uses, they may still need to be further separated. The fission products of uranium and plutonium include Lanthanide The first peak is zirconium, ruthenium, molybdenum, technetium, ruthenium, rhodium palladium , silver, and the other peak is in the periodic table iodine Xenon, Cesium barium 、 lanthanum , cerium neodymium 。 Many fission products are not radioactive or have short life radio isotope However, a considerable number of products are medium to long-term radioisotopes, such as strontium-90, cesium-137, technetium-99 and iodine-129. Some countries have studied the separation methods of rare isotopes in fission waste. For example, precious metals such as silver and platinum group metals such as ruthenium, rhodium and palladium produced by fission can be more or less compensated for the cost of reprocessing, but these methods have not been commercialized. Fission products can change the heat conduction Performance. Lanthanide oxides will reduce the thermal conductivity of the fuel. 96% of the mass of spent nuclear fuel is residual unreacted uranium, most of which is uranium-238, and a small part is uranium-235. Generally, the mass fraction of uranium-235 is less than 0.83%, and the mass fraction of uranium-236 is about 0.4%.
Uranium-236 is a very troublesome long-lived radioactive waste.
Reprocessed uranium contains uranium-236, an isotope that does not exist in nature, and can be used as a marker for spent nuclear fuel.
If thorium fuel is used in reactors, the spent nuclear fuel produced will contain the uranium isotope uranium-233, with a half-life of 159200 years. It will affect the long-term radioactivity of spent nuclear fuel due to decay. and Mixed oxide nuclear fuel In contrast, due to the existence of uranium - 233 that has not decayed completely, the radioactivity of spent thorium fuel within one million years will be higher. About 1% of the mass of spent nuclear fuel is plutonium-239 and plutonium-240. These plutonium are produced by beta decay after uranium 238 captures neutrons. They are not only useful by-products, but also dangerous and difficult to treat wastes. In order to prevent nuclear proliferation, it is necessary to prohibit those countries that have not yet possessed nuclear weapons from using these plutonium to manufacture nuclear weapons. If the nuclear reactor works normally, the plutonium is reactor grade, not weapon grade. It contains more plutonium-240, and less than 80% of the plutonium is plutonium-239, making these plutonium not suitable for making nuclear weapons. However, it is not impossible to use these reactor grade plutonium to make nuclear weapons [1] 。 If the time of receiving radiation is relatively short, weapons grade plutonium will be produced. The proportion of plutonium-239 is higher than 80%, up to 93%.
When the nuclear reactor is shut down, nuclear chain reaction However, due to beta decay of decay products, spent fuel will still emit a lot of heat. Therefore, when the nuclear reactor is shut down, the thermal power released by decay is about 7% of the power when the nuclear reactor is stable. One hour after the reactor is shut down, the decay heat power is about 1.5% of the power in stable operation; 0.4% after one day; It became 0.2% after one week. The decay heat power will continue to decrease slowly with time. The spent nuclear fuel removed from the nuclear reactor is usually stored in the spent nuclear fuel pool filled with water, which needs to be kept for one year or even longer to cool it, and provide shielding for its radioactivity. The design of spent nuclear fuel pool used in practice usually does not rely on passive cooling, but needs to use a heat exchanger to circulate water in it, so as to take away the heat generated by decay.
The spent nuclear fuel cooled to a certain extent will be removed from the spent nuclear fuel pool and put into the special Dry storage drum Or wet intermediate storage equipment to make room for spent nuclear fuel pool, and as an alternative before final disposal. The specific activity of spent fuel is very high. A large amount of decay heat is also released. It must be stored for a period of time until the radioactivity and residual heat are reduced to a certain extent before operation and treatment. According to the storage mode, spent fuel storage can be divided into wet storage (pool storage) and dry storage
Wet storage [2] Pool storage is adopted. The spent fuel unloaded from the reactor in the nuclear power plant is temporarily stored in the spent fuel pool, so each nuclear power plant will have its own spent fuel pool. The spent fuel pool is generally filled with boron containing water of a certain concentration to prevent chain reaction. A cooling system is installed in the pool to carry out the decay heat of spent fuel.
The storage tank has two structures:
Concrete structure spent fuel storage pool made of stainless steel (2) A storage pool constructed from underground caverns. The CLAB plant in Sweden belongs to this category, which consists of spent fuel receiving, storage and auxiliary building. Only the storage pool is built in the cave. The rock cave is 120m long, 21m wide and 27m high. It is composed of four pools divided by the whole rock. The rock cave has good performance of preventing external impact, and can also isolate the environment in case of internal accidents, so as to prevent the environment from pollution.
The dry storage facilities that have been built around the world mainly include air cooling storage rooms, dry concrete containers, dry wells and metal containers.
(1) Air cooled storage room
Storage of spent fuel in Heavy concrete Shielded air cooling storage room, where the air flows through the decay zone of spent fuel through natural convection. It is discharged from the chimney. The storage room is divided into several cylindrical channels. The spent fuel assembly with the outer packaging container is stored vertically in the duct. The spacing of spent fuel assemblies shall ensure that nuclear criticality does not occur. The storage room can be set on the ground or underground. The repository is equipped with a gas monitoring system to monitor radioactivity and leakage of packaging containers. (2) Dry concrete container
By cylindrical reinforced concrete The body and top cover are composed. Air enters from its bottom. Discharge from the top. Take away the waste heat released by spent fuel. Containers containing spent fuel assemblies can be stored in ordinary ground buildings. The storage plant shall be equipped with loading equipment room, transfer channel, container loading workshop, control room, etc. All working rooms shall be of reinforced concrete structure. (3) Dry well
It is made of concrete and contains a carbon steel shaft. There are concrete plugs at the wellhead. The racks containing spent fuel assemblies are stored in dry wells. Dry well storage is generally composed of receiving, transferring and storage. Spent fuel assemblies with outer packaging are placed on racks in receiving facilities. Enter the shielded transport container in the transfer facility, and finally transport it to the dry well for storage with a gantry crane. The storage area is also equipped with continuous radioactive aerosol monitors.
(4) Metal container
It is made of nodular cast iron or forged steel lined with stainless steel sleeve. There are cooling fins outside the wall. The cover is divided into two layers, the inner layer is a shielding layer, and the outer layer plays a role in fixing. The container is equipped with grids made of boron containing aluminum plates for loading spent fuel assemblies. The designed metal container shall be subject to safety analysis according to shielding, critical calculation, thermal and strength analysis and tests under normal and accident conditions.
P3+dry storage drum for spent fuel
As the wet and dry storage capacity of spent fuel in nuclear power plants is limited and can only be used as temporary storage, these spent fuel must be transported to spent fuel reprocessing plant or other places for spent fuel reprocessing. Therefore, the transportation of spent fuel is an indispensable link, and because of the particularity of spent fuel, there are special regulations for the transportation of spent fuel.
Under safety protection measures, spent fuel transportation is a process of transferring spent fuel from one place to another with special containers and special transportation tools. Countries have the following requirements for the transportation of spent fuel:
(1) Transport must be carried out in strict accordance with the International Atomic Energy Agency's Regulations for the Safe Transport of Radioactive Materials and relevant national regulations;
(2) According to the characteristics of the country, the specific transportation approval system is stipulated. It generally stipulates that the type, quantity, transportation route, tools and measures that may cause accidents of the transported objects must be designed and reviewed and approved by the relevant departments; The design and manufacture of transport containers must be recognized and registered with relevant departments; Before sending spent fuel, the tightness, surface radiation level, surface pollution degree, means of transport and fixing method of the container must be checked and approved; The transportation route, especially the safety measures and emergency measures in case of accidents must be recognized again;
(3) The spent fuel transport container belongs to type B cargo package, which must be tested under normal transport conditions and accident transport conditions, and can only be used after passing the test;
(4) Operators shall receive technical training and can only operate after graduation.
The spent fuel assembly can be transported by road, railway and sea if it meets the requirements of the regulations. As the safety requirements for the transportation of spent fuel assemblies are getting higher and higher, the weight of containers is getting larger and larger. There are special transportation vehicles and ships, but no special roads, railways and docks are needed. Only by controlling the quality of containers and strictly organizing the transportation process, can the transportation safety be ensured.
Dry storage drum for spent fuel The main purposes of nuclear fuel reprocessing are:
(1) Recover the remaining fissile nuclide uranium-235, newly generated plutonium-239 and convertible nuclide uranium-233 or thorium-232.
(2) Useful fission products can be extracted when necessary. Such as strontium-90, cesium-137 and transuranic elements such as neptunium, americium and curium.
(2) Remove long-life Radionuclide And fission products with large neutron absorption cross section for the treatment and safe disposal of radioactive waste containing only short-lived nuclides. Post treatment process
The post treatment process of irradiated spent fuel can be divided into two categories: water method and dry method. The so-called water method is to dissolve spent fuel in acid, and then separate uranium, plutonium and fission products by precipitation, solvent extraction, ion exchange or adsorption, because each process is water phase operation. Therefore, it is called water law. The so-called dry process, i.e. high-temperature metallurgy or fluorination volatilization, does not need to be operated in the aqueous phase. Whether the water method or the dry method, the raw materials processed are solid, and the products are uranium and plutonium oxides. Water method has been widely used in industry, mainly by Solvent extraction However, the high-temperature metallurgy method or fluorinated volatilization method is in the stage of research and development. The solvent extraction method can effectively remove fission products, and is suitable for processing natural uranium, low enriched uranium, high enriched uranium, high-temperature gas cooled reactor elements and fast reactor elements. Irradiated fuel (spent fuel) contains a large amount of radioactive substances. With the extension of storage time, the radioactive activity and heat release rate will decrease after natural decay. The spent fuel is generally cooled in the spent fuel storage pool. The cooling time of spent fuel of power reactor is generally no less than 3-5 years. The cooling of spent fuel to reduce radioactivity can ease the technical difficulties of spent fuel reprocessing process.
(a) Water post-treatment
The sedimentation method was adopted in the early water method post-treatment plant. All post-treatment plants in the world use solvent extraction process. Since the process technology is mature and has accumulated rich experience. In the future, this process will still be widely used.
Water reprocessing process mainly includes: head end treatment, chemical separation and tail end treatment of uranium and plutonium.
1) Head end processing. The head end treatment includes mechanical treatment and chemical treatment.
2) Mechanical treatment. The mechanical treatment at the head end cuts the spent fuel assembly into small segments, exposing uranium from the cladding for chemical dissolution of the fuel core. Spent fuel is leached with nitric acid at boiling or non boiling temperature to dissolve Uranium dioxide 。 Dissolved Uranyl nitrate There is insoluble residue in the solution, which needs to be removed through clarification and filtration. The clarified solution obtained through filtration is sent to the chemical separation process for treatment after adjusting the valence state of plutonium and neptunium. 3) Chemical separation. The chemical separation process is to separate uranium, plutonium and radioactive fission products, and to separate and purify uranium and plutonium. The chemical separation process of reprocessing plants all over the world adopts purex solvent extraction process. With 30% tributyl phosphate (TBP) as the extractant and n-dodecane or hydrogenated kerosene as the diluent, liquid-liquid extraction is carried out. Generally, it goes through three solvent extraction cycles, namely the standard process of co decontamination separation cycle, uranium line two and three cycles, and plutonium line two and three cycles, There are also two extraction cycles.
4) Uranium and ring tail end treatment. Uranium and ring tail end treatment is to make uranyl nitrate and plutonium nitrate solution into oxides to produce crystals, and uranyl nitrate is made into uranium dioxide by fluidized bed denitration. Plutonium nitrate is prepared by oxalic acid precipitation and calcination Plutonium dioxide 。 (b) Dry post-treatment
Dry reprocessing is a process of nuclear fuel reprocessing under non-aqueous conditions. There are two categories of dry post-treatment: volatile method and high-temperature method:
1) Volatilization method, which can be divided into Fluoride volatilization method And chloride volatilization method. 2) High temperature method can be divided into physical method and chemical method. Physical methods include fractional distillation, fractional crystallization and molten metal extraction; Chemical methods include molten metal extraction, molten salt extraction, molten salt electrolysis and melting refining.
In the process of reprocessing, the separation, purification and recovery of main components in spent fuel must meet certain requirements. The product recovery rate is an important economic and technical indicator of the reprocessing plant. The recovery rate of uranium and plutonium in the general water process can reach 99.8% and 99.5% respectively. Product radioactivity is one of the main quality indicators of the reprocessing plant. It should be proposed after comprehensive consideration of the characteristics of spent fuel, product use, element reprocessing technology, economy, safety and other aspects.
Post processing technology
The spent fuel reprocessing technology is to chemically convert the used uranium waste (spent fuel) into plutonium The separation from fission products is called spent fuel re dissolution and reprocessing technology. The recovered uranium and plutonium can be recycled in mixed oxide fuel of nuclear power plants to produce more energy So as to make full use of uranium resources and reduce enrichment demand. Post treatment also reduces High-level waste The volume of and the removal of plutonium contribute to the final disposal of waste. Spent fuel reprocessing technology is a high technology under the condition of high radioactivity. The nuclear fuel processing and storage of nuclear power plants in the world is a very difficult thing in itself. With this technology, its significance is not only to make full use of the function of nuclear fuel, improve the utilization ability of nuclear fuel, benefit mankind, but also to reduce the volume and radioactivity, It has created conditions for the preservation of nuclear waste and made a great contribution to the environment.
On December 21, 2010, China's first intermediate test plant for spent fuel reprocessing of power reactors - CNNC 404 pilot test project achieved success in hot commissioning. The success of hot commissioning has realized nuclear fuel The goal of closed cycle strongly promotes the rapid development of nuclear fuel industry and nuclear power, provides an important research and experiment platform for the development of China's advanced reprocessing engineering technology, and marks that China has mastered Spent fuel reprocessing technology for power reactor 。 Long life nuclear waste (including spent fuel) must be isolated from human beings and the environment for a long time. It is widely accepted that spent fuel, high-level radioactive waste from reprocessing of nuclear fuel and plutonium waste need to be stored in properly designed places for tens of thousands to one million years to reduce the environmental pollution caused by its radioactivity. At the same time, it must be ensured that plutonium and highly enriched uranium are not used for military purposes. A basic consensus is that it is safer to store spent fuel hundreds of meters underground than to stack it on the ground. Therefore, it is a feasible scheme to store these wastes in the artificially constructed underground repository in a stable geological structure, which is the final disposal mode of spent fuel, also called deep geological disposal.
The final disposal of spent fuel refers to the radioactive nuclear waste storage site excavated in a stable geological structure, which is generally 300 meters below the ground. Many factors, such as the shape of nuclear waste, its packaging, the sealing and seepage prevention of the site, and the geological conditions, determine the success of the storage site. The basic requirement for deep geological disposal is to isolate the nuclear waste from the environment for a long time, while requiring little or no maintenance. The time scale of deep geological disposal is very large, usually from tens of thousands to one million years. In deep geological disposal, nuclear waste in containers is sealed in some way and stored in tunnels. The outermost protective mechanism is the geological structure itself (such as rock stratum).
