Atomic fuel. How nuclear fuel is made (29 photos). Negative sides of nuclear power plants

Nuclear energy is used in thermal power engineering, when energy is obtained from nuclear fuel in reactors in the form of heat. It is used to generate electricity in nuclear power plants (NPP), for power plants of large sea vessels, for desalination of sea water.

Nuclear energy owes its appearance, first of all, to the nature of the neutron discovered in 1932. Neutrons are part of all atomic nuclei, except for the hydrogen nucleus. Bound neutrons in the nucleus exist indefinitely. In their free form, they are short-lived, since they either decay with a half-life of 11.7 minutes, turning into a proton and emitting an electron and a neutrino, or are quickly captured by the nuclei of atoms.

Modern nuclear power is based on the use of energy released during the fission of a natural isotope uranium-235. At nuclear power plants, a controlled nuclear fission reaction is carried out in nuclear reactor. According to the energy of neutrons that produce nuclear fission, distinguish between thermal and fast neutron reactors.

The main unit of a nuclear power plant is a nuclear reactor, the diagram of which is shown in fig. 1. Energy is obtained from nuclear fuel, and then it is transferred to another working fluid (water, metallic or organic liquid, gas) in the form of heat; then it is converted into electricity in the same way as in conventional ones.

They control the process, maintain the reaction, stabilize the power, start and stop the reactor using special mobile control rods 6 and 7 from materials that intensively absorb thermal neutrons. They are driven by a control system 5 . Actions control rods are manifested in a change in the power of the neutron flux in the core. By channels 10 water circulates, cooling the biological protection concrete

The control rods are made of boron or cadmium, which are thermally, radiation and corrosion resistant, mechanically strong, and have good heat transfer properties.

Inside a massive steel case 3 there is a basket 8 with fuel elements 9 . The coolant enters through the pipeline 2 , passes through the core, washes all fuel elements, heats up and through the pipeline 4 enters the steam generator.

Rice. 1. Nuclear reactor

The reactor is placed inside a thick concrete biological containment device. 1 , which protects the surrounding space from the flow of neutrons, alpha, beta, gamma radiation.

Fuel elements (fuel rods) is the main part of the reactor. A nuclear reaction directly takes place in them and heat is released, all other parts serve to insulate, control and remove heat. Structurally, fuel elements can be made of rod, plate, tubular, spherical, etc. Most often they are rod, up to 1 meter long, 10 mm in diameter. They are usually assembled from uranium pellets or from short tubes and plates. Outside, the fuel rods are covered with a corrosion-resistant, thin metal sheath. Zirconium, aluminum, magnesium alloys, as well as alloyed stainless steel are used for the shell.

The transfer of heat released during a nuclear reaction in the reactor core to the working fluid of the engine (turbine) of power plants is carried out according to single-loop, double-loop and three-loop schemes (Fig. 2).

Rice. 2. Nuclear power plant
a - according to a single-circuit scheme; b - according to the two-circuit scheme; c - according to the three-circuit scheme
1 - reactor; 2, 3 - biological protection; 4 - pressure regulator; 5 - turbine; 6 - electric generator; 7 - capacitor; 8 - pump; 9 - reserve capacity; 10 – regenerative heater; 11 – steam generator; 12 - pump; 13 - intermediate heat exchanger

Each circuit is a closed system. Reactor 1 (in all thermal circuits) placed inside the primary 2 and secondary 3 biological defenses. If the nuclear power plant is built according to a single-circuit thermal scheme, the steam from the reactor through the pressure regulator 4 enters the turbine 5 . The turbine shaft is connected to the generator shaft 6 in which electric current is generated. The exhaust steam enters the condenser, where it is cooled and completely condensed. Pump 8 directs condensate to a regenerative heater 10 , and then it enters the reactor.

With a two-circuit scheme, the coolant heated in the reactor enters the steam generator 11 , where heat is transferred by surface heating to the coolant of the working fluid (feed water of the secondary circuit). In pressurized water reactors, the coolant in the steam generator is cooled by approximately 15 ... 40 ° C and then by a circulation pump 12 back to the reactor.

With a three-loop scheme, the coolant (usually liquid sodium) from the reactor is sent to an intermediate heat exchanger 13 and from there by the circulation pump 12 returns to the reactor. The coolant in the secondary circuit is also liquid sodium. This circuit is not irradiated and therefore non-radioactive. Sodium of the second circuit enters the steam generator 11 , gives off heat to the working fluid, and then the circulation pump is sent back to the intermediate heat exchanger.

The number of circulation circuits determines the type of reactor, the coolant used, its nuclear-physical properties, and the degree of radioactivity. The single-loop scheme can be used in boiling water reactors and in gas-cooled reactors. The most widespread double circuit when used as a heat carrier of water, gas and organic liquids. The three-circuit scheme is used at nuclear power plants with fast neutron reactors using liquid metal coolants (sodium, potassium, sodium-potassium alloys).

Nuclear fuel can be uranium-235, uranium-233 and plutonium-232. Raw materials for obtaining nuclear fuel - natural uranium and thorium. During the nuclear reaction of one gram of fissile material (uranium-235), energy equivalent to 22×10 3 kWh (19×10 6 cal) is released. To obtain this amount of energy, it is necessary to burn 1900 kg of oil.

Uranium-235 is readily available, its energy reserves are about the same as fossil fuels. However, using nuclear fuel with such low efficiency as it is now, the available uranium sources will be depleted in 50-100 years. At the same time, there are practically inexhaustible "deposits" of nuclear fuel - this is uranium dissolved in sea water. It is hundreds of times more abundant in the ocean than on land. The cost of obtaining one kilogram of uranium dioxide from sea water is about $60-80, and in the future it will drop to $30, while the cost of uranium dioxide produced in the richest deposits on land is $10-20. Therefore, after some time, the costs on land and "on sea water" will become of the same order.

The cost of nuclear fuel is about half that of fossil coals. At coal-fired power plants, 50-70% of the cost of electricity falls to the share of fuel, and at nuclear power plants - 15-30%. A modern thermal power plant with a capacity of 2.3 million kW (for example, Samara GRES) consumes about 18 tons of coal (6 trains) or 12 thousand tons of fuel oil (4 trains) daily. The nuclear one, of the same power, consumes only 11 kg of nuclear fuel during the day, and 4 tons during the year. However, a nuclear power plant is more expensive than a thermal one in terms of construction, operation, and repair. For example, the construction of a nuclear power plant with a capacity of 2–4 million kW costs approximately 50–100% more than a thermal one.

It is possible to reduce capital costs for NPP construction by:

1. standardization and unification of equipment;
2. development of compact reactor designs;
3. improvement of management and regulation systems;
4. reducing the duration of the shutdown of the reactor for refueling.

An important characteristic of nuclear power plants (nuclear reactor) is the efficiency of the fuel cycle. To improve the economy of the fuel cycle, you should:

• to increase the depth of nuclear fuel burnup;
• raise the breeding ratio of plutonium.

With each fission of the uranium-235 nucleus, 2-3 neutrons are released. Of these, only one is used for further reaction, the rest are lost. However, it is possible to use them for the reproduction of nuclear fuel by creating fast neutron reactors. When the reactor is operating on fast neutrons, it is possible to simultaneously obtain approximately 1.7 kg of plutonium-239 for 1 kg of burned uranium-235. In this way, the low thermal efficiency of nuclear power plants can be covered.

Fast neutron reactors are ten times more efficient (in terms of the use of nuclear fuel) than fuel neutron reactors. They have no moderator and use highly enriched nuclear fuel. Neutrons emitted from the core are absorbed not by structural materials, but by uranium-238 or thorium-232 located around.

In the future, the main fissile materials for nuclear power plants will be plutonium-239 and uranium-233, obtained respectively from uranium-238 and thorium-232 in fast neutron reactors. The conversion of uranium-238 into plutonium-239 in reactors will increase the resources of nuclear fuel by about 100 times, and thorium-232 into uranium-233 by 200 times.

On fig. Figure 3 shows a diagram of a fast neutron nuclear power plant.

Distinctive features of a nuclear power plant on fast neutrons are:

1. the change in the criticality of a nuclear reactor is carried out by reflecting part of the fission neutrons of nuclear fuel from the periphery back to the core using reflectors 3 ;
2. reflectors 3 can rotate, changing the leakage of neutrons and, consequently, the intensity of fission reactions;
3. nuclear fuel is reproduced;
4. removal of excess thermal energy from the reactor is carried out using a cooler-radiator 6 .

Rice. 3. Scheme of a nuclear power plant on fast neutrons:
1 - fuel elements; 2 – renewable nuclear fuel; 3 – fast neutron reflectors; 4 - nuclear reactor; 5 - consumer of electricity; 6 - refrigerator-emitter; 7 - converter of thermal energy into electrical energy; 8 - radiation protection.

Converters of thermal energy into electrical energy

According to the principle of using thermal energy generated by a nuclear power plant, converters can be divided into 2 classes:

1. machine (dynamic);
2. machineless (direct converters).

In machine converters, a gas turbine plant is usually connected to the reactor, in which the working fluid can be hydrogen, helium, helium-xenon mixture. The efficiency of converting heat supplied directly to the turbogenerator into electricity is quite high - the efficiency of the converter η = 0,7-0,75.

A diagram of a nuclear power plant with a dynamic gas turbine (machine) converter is shown in fig. four.

Another type of machine converter is a magnetogasdynamic or magnetohydrodynamic generator (MGDG). A diagram of such a generator is shown in fig. 5. The generator is a channel of rectangular cross section, two walls of which are made of a dielectric, and two of which are made of an electrically conductive material. An electrically conductive working fluid moves through the channels - liquid or gaseous, which is penetrated by a magnetic field. As you know, when a conductor moves in a magnetic field, an EMF arises, which along the electrodes 2 transferred to the consumer of electricity 3 . The energy source of the working heat flow is the heat released in the nuclear reactor. This thermal energy is spent on the movement of charges in a magnetic field, i.e. is converted into the kinetic energy of the current-carrying jet, and the kinetic energy is converted into electrical energy.

Rice. 4. Scheme of a nuclear power plant with a gas turbine converter:
1 - reactor; 2 – circuit with liquid metal coolant; 3 – heat exchanger for heat supply to gas; 4 - turbine; 5 - electric generator; 6 - compressor; 7 - radiator-radiator; 8 – heat removal circuit; 9 - circulation pump; 10 - heat exchanger for heat removal; 11 - heat exchanger-regenerator; 12 - circuit with the working fluid of the gas turbine converter.

Direct converters (machineless) of thermal energy into electrical energy are divided into:

1. thermoelectric;
2. thermionic;
3. electrochemical.

Thermoelectric generators (TEGs) are based on the Seebeck principle, which means that in a closed circuit consisting of dissimilar materials, a thermoelectric power arises if a temperature difference is maintained at the points of contact of these materials (Fig. 6). To generate electricity, it is advisable to use semiconductor TEGs, which have a higher efficiency, while the temperature of the hot junction must be brought up to 1400 K and higher.

Thermionic converters (TEC) make it possible to obtain electricity as a result of the emission of electrons from a cathode heated to high temperatures (Fig. 7).

Rice. 5. Magnetogasdynamic generator:
1 – magnetic field; 2 - electrodes; 3 - consumer of electricity; 4 - dielectric; 5 - conductor; 6 - working fluid (gas).

Rice. 6. Scheme of thermoelectric generator operation

Rice. 7. Scheme of operation of the thermionic converter

To maintain the emission current, heat is supplied to the cathode Q one . The electrons emitted by the cathode, having overcome the vacuum gap, reach the anode and are absorbed by it. During the "condensation" of electrons at the anode, energy is released equal to the work function of electrons with the opposite sign. If we ensure a continuous supply of heat to the cathode and its removal from the anode, then through the load R direct current will flow. Electron emission proceeds efficiently at cathode temperatures above 2200 K.

Safety and reliability of NPP operation

One of the main issues in the development of nuclear energy is to ensure the reliability and safety of nuclear power plants.

Radiation safety is ensured by:

1. the creation of reliable structures and devices for the biological protection of personnel from exposure to radiation;
2. purification of air and water leaving the NPP premises beyond its limits;
3. extraction and reliable localization of radioactive contamination;
4. daily dosimetric control of NPP premises and individual dosimetric control of personnel.

NPP premises, depending on the mode of operation and the equipment installed in them, are divided into 3 categories:

1. strict regime zone;
2. restricted zone;
3. normal mode zone.

Personnel are constantly in the rooms of the third category; these rooms at the station are radiation safe.

Nuclear power plants generate solid, liquid and gaseous radioactive waste. They must be disposed of in such a way that no pollution of the environment is created.

The gases removed from the room during ventilation may contain radioactive substances in the form of aerosols, radioactive dust and radioactive gases. The ventilation of the station is built in such a way that air flows pass from the most “clean” to “polluted”, and cross-flows in the opposite direction are excluded. In all rooms of the station, a complete replacement of air is carried out within no more than one hour.

During the operation of nuclear power plants, the problem of removal and disposal of radioactive waste arises. Fuel elements spent in reactors withstand a certain time in water pools directly at nuclear power plants until stabilization of isotopes with a short half-life occurs, after which the fuel elements are sent to special radiochemical plants for regeneration. There, nuclear fuel is extracted from the fuel rods, and radioactive waste is subject to burial.

The life cycle of nuclear fuel based on uranium or plutonium begins at mining enterprises, chemical plants, in gas centrifuges, and does not end at the moment the fuel assembly is unloaded from the reactor, since each fuel assembly has a long way to go through disposal and then reprocessing.

Extraction of raw materials for nuclear fuel

Uranium is the heaviest metal on earth. About 99.4% of the earth's uranium is uranium-238, and only 0.6% is uranium-235. A report by the International Atomic Energy Agency called "Red Book" contains data on the growth of production and demand for uranium, despite the accident at the Fukushima-1 nuclear power plant, which made many think about the prospects for nuclear energy. In the last few years alone, explored uranium reserves have increased by 7%, which is associated with the discovery of new deposits. Kazakhstan, Canada and Australia remain the largest producers, producing up to 63% of the world's uranium. In addition, there are metal reserves in Australia, Brazil, China, Malawi, Russia, Niger, USA, Ukraine, China and other countries. Earlier, Pronedra wrote that in 2016, 7.9 thousand tons of uranium were mined in the Russian Federation.

Today, uranium is mined in three different ways. The open method does not lose its relevance. It is used in cases where the deposits are close to the surface of the earth. In the open pit method, bulldozers create a quarry, then the ore with impurities is loaded into dump trucks for transportation to processing complexes.

Often the ore body lies at great depths, in which case an underground mining method is used. A mine breaks out up to two kilometers deep, the rock, by drilling, is mined in horizontal drifts, transported upward in freight elevators.

The mixture, which is thus taken out to the top, has many components. The rock must be crushed, diluted with water and removed excess. Next, sulfuric acid is added to the mixture to carry out the leaching process. During this reaction, chemists get a yellow precipitate of uranium salts. Finally, uranium with impurities is refined at the refinery. Only after this is uranium oxide obtained, which is traded on the stock exchange.

There is a much safer, environmentally friendly and cost-effective way, which is called borehole in-situ leaching (SIL).

With this method of field development, the territory remains safe for personnel, and the radiation background corresponds to the background in large cities. To mine uranium by leaching, you need to drill 6 holes at the corners of the hexagon. Sulfuric acid is pumped into the uranium deposits through these wells, it mixes with its salts. This solution is extracted, namely, it is pumped out through a well in the center of the hexagon. To achieve the desired concentration of uranium salts, the mixture is passed several times through sorption columns.

Nuclear fuel production

The production of nuclear fuel is unimaginable without gas centrifuges, which are used to produce enriched uranium. After reaching the required concentration, so-called tablets are pressed from uranium dioxide. They are created using lubricants that are removed during firing in furnaces. The firing temperature reaches 1000 degrees. After that, the tablets are checked for compliance with the stated requirements. The quality of the surface, the moisture content, the ratio of oxygen and uranium matter.

At the same time, tubular shells for fuel elements are being prepared in another workshop. The above processes, including subsequent dosing and packaging of tablets in shell tubes, sealing, decontamination, are called fuel fabrication. In Russia, the creation of fuel assemblies (FA) is carried out by the enterprises "Machine-Building Plant" in the Moscow Region, "Novosibirsk Plant of Chemical Concentrates" in Novosibirsk, "Moscow Plant of Polymetals" and others.

Each batch of fuel assemblies is created for a specific type of reactor. European fuel assemblies are made in the form of a square, and Russian - with a hexagonal section. In the Russian Federation, reactors of the VVER-440 and VVER-1000 types are widely used. The first fuel elements for VVER-440 began to be developed in 1963, and for VVER-1000 - in 1978. Despite the fact that new reactors with post-Fukushima safety technologies are being actively introduced in Russia, there are many old-style nuclear facilities operating throughout the country and abroad, so fuel assemblies for different types of reactors remain equally relevant.

For example, to provide fuel assemblies for one active zone of the RBMK-1000 reactor, more than 200 thousand components made of zirconium alloys, as well as 14 million sintered pellets of uranium dioxide, are needed. Sometimes the cost of manufacturing a fuel assembly can exceed the cost of the fuel contained in the cells, which is why it is so important to ensure a high energy return from each kilogram of uranium.

Production process costs in %

Separately, it should be said about fuel assemblies for research reactors. They are designed in such a way as to make the observation and study of the neutron generation process as comfortable as possible. Such fuel rods for experiments in the fields of nuclear physics, production of isotopes, radiation medicine in Russia are produced by the Novosibirsk Plant of Chemical Concentrates. TVS are created on the basis of seamless elements with uranium and aluminum.

The production of nuclear fuel in the Russian Federation is carried out by the fuel company TVEL (a division of Rosatom). The enterprise is working on the enrichment of raw materials, the assembly of fuel elements, and also provides fuel licensing services. The Kovrov Mechanical Plant in the Vladimir Region and the Ural Gas Centrifuge Plant in the Sverdlovsk Region create equipment for Russian fuel assemblies.

Features of transportation of fuel rods

Natural uranium is characterized by a low level of radioactivity, however, before the production of fuel assemblies, the metal undergoes an enrichment procedure. The content of uranium-235 in natural ore does not exceed 0.7%, and the radioactivity is 25 becquerels per 1 milligram of uranium.

The uranium pellets placed in the fuel assemblies contain uranium with a uranium-235 concentration of 5%. Finished fuel assemblies with nuclear fuel are transported in special high-strength metal containers. For transportation, rail, road, sea and even air transport is used. Each container contains two assemblies. Transportation of non-irradiated (fresh) fuel does not pose a radiation hazard, since the radiation does not go beyond the zirconium tubes into which pressed uranium pellets are placed.

A special route is developed for a batch of fuel, the cargo is transported accompanied by the security personnel of the manufacturer or the customer (more often), which is primarily due to the high cost of equipment. In the entire history of nuclear fuel production, not a single transport accident involving fuel assemblies has been recorded that would affect the radiation background of the environment or lead to casualties.

Fuel in the reactor core

A unit of nuclear fuel - TVEL - is capable of releasing a huge amount of energy for a long time. Neither coal nor gas can compare with such volumes. The life cycle of fuel at any nuclear power plant begins with the unloading, removal and storage of fresh fuel in the fuel assembly warehouse. When the previous batch of fuel in the reactor burns out, the personnel completes the fuel assemblies for loading into the core (the working zone of the reactor, where the decay reaction takes place). As a rule, the fuel is partially reloaded.

The fuel is fully loaded into the core only at the time of the first start of the reactor. This is due to the fact that the fuel elements in the reactor burn out unevenly, since the neutron flux varies in intensity in different zones of the reactor. Thanks to accounting devices, the station staff has the ability to monitor the degree of burn-up of each unit of fuel in real time and replace it. Sometimes, instead of loading new fuel assemblies, assemblies are moved among themselves. In the center of the active zone, burnout occurs most intensively.

TVS after nuclear power plant

Uranium that has worked out in a nuclear reactor is called irradiated or burnt out. And such fuel assemblies - spent nuclear fuel. SNF is positioned separately from radioactive waste, since it has at least 2 useful components - unburned uranium (metal burnout never reaches 100%) and transuranium radionuclides.

Recently, physicists have begun to use radioactive isotopes accumulated in SNF in industry and medicine. After the fuel has worked out its campaign (the time spent by the assembly in the reactor core under conditions of operation at rated power), it is sent to the spent fuel pool, then to storage directly in the reactor compartment, and after that - for processing or disposal. The cooling pool is designed to remove heat and protect against ionizing radiation, since the fuel assemblies remain dangerous after being removed from the reactor.

In the US, Canada or Sweden, SNF is not sent for reprocessing. Other countries, including Russia, are working on a closed fuel cycle. It allows to significantly reduce the cost of nuclear fuel production, since part of the SNF is reused.

The fuel rods are dissolved in acid, after which the researchers separate plutonium and unused uranium from the waste. About 3% of raw materials cannot be reused; these are high-level wastes that undergo bituminization or vitrification procedures.

From spent nuclear fuel, 1% of plutonium can be obtained. This metal does not need to be enriched, Russia uses it in the process of producing innovative MOX fuel. A closed fuel cycle makes it possible to make one fuel assembly cheaper by about 3%, but this technology requires large investments in the construction of industrial units, so it has not yet become widespread in the world. Nevertheless, the Rosatom fuel company does not stop research in this direction. Recently, Pronedra wrote that the Russian Federation is working on a fuel capable of utilizing americium, curium and neptunium isotopes in the reactor core, which are included in the very 3% of highly radioactive waste.

Nuclear fuel producers: rating

1. Until recently, the French company Areva provided 31% of the world market for fuel assemblies. The company is engaged in the production of nuclear fuel and the assembly of components for nuclear power plants. In 2017, Areva experienced a qualitative upgrade, new investors came to the company, and the colossal loss of 2015 was reduced by 3 times.
2. Westinghouse is the American division of the Japanese company Toshiba. It actively develops the market in Eastern Europe, supplies fuel assemblies to Ukrainian NPPs. Together with Toshiba, it provides 26% of the world market for the production of nuclear fuel.
3. Fuel company TVEL of the state corporation Rosatom (Russia) is in third place. TVEL provides 17% of the world market, has a ten-year contract portfolio worth $30 billion and supplies fuel to more than 70 reactors. TVEL develops fuel assemblies for VVER reactors, and also enters the market for nuclear installations of Western design.
4. Japan Nuclear Fuel Limited, according to the latest data, provides 16% of the world market, supplies fuel assemblies to most of the nuclear reactors in Japan itself.
5. Mitsubishi Heavy Industries is a Japanese giant that manufactures turbines, tankers, air conditioners, and, more recently, nuclear fuel for Western-style reactors. Mitsubishi Heavy Industries (a division of the parent company) is engaged in the construction of APWR nuclear reactors, research activities together with Areva. It is this company that was chosen by the Japanese government to develop new reactors.

Nuclear power is a modern and rapidly developing way of generating electricity. Do you know how nuclear power plants are arranged? What is the principle of operation of a nuclear power plant? What types of nuclear reactors exist today? We will try to consider in detail the scheme of operation of a nuclear power plant, delve into the structure of a nuclear reactor and find out how safe the atomic method of generating electricity is.

How is a nuclear power plant organized?

Any station is a closed area far from the residential area. There are several buildings on its territory. The most important building is the reactor building, next to it is the turbine hall from which the reactor is controlled, and the safety building.

The scheme is impossible without a nuclear reactor. An atomic (nuclear) reactor is a device of a nuclear power plant, which is designed to organize a chain reaction of neutron fission with the obligatory release of energy in this process. But what is the principle of operation of a nuclear power plant?

The entire reactor plant is placed in the reactor building, a large concrete tower that hides the reactor and, in the event of an accident, will contain all the products of a nuclear reaction. This large tower is called containment, hermetic shell or containment.

The containment zone in the new reactors has 2 thick concrete walls - shells.
An 80 cm thick outer shell protects the containment area from external influences.

The inner shell with a thickness of 1 meter 20 cm has special steel cables in its device, which increase the strength of concrete by almost three times and will not allow the structure to crumble. On the inside, it is lined with a thin sheet of special steel, which is designed to serve as additional protection for the containment and, in the event of an accident, prevent the contents of the reactor from being released outside the containment area.

Such a device of a nuclear power plant can withstand the fall of an aircraft weighing up to 200 tons, an 8-magnitude earthquake, tornado and tsunami.

The first pressurized enclosure was built at the American nuclear power plant Connecticut Yankee in 1968.

The total height of the containment area is 50-60 meters.

What is a nuclear reactor made of?

To understand the principle of operation of a nuclear reactor, and hence the principle of operation of a nuclear power plant, you need to understand the components of the reactor.

• active zone. This is the area where the nuclear fuel (heat releaser) and the moderator are placed. Atoms of fuel (most often uranium is the fuel) perform a fission chain reaction. The moderator is designed to control the fission process, and allows you to carry out the reaction required in terms of speed and strength.
• Neutron reflector. The reflector surrounds the active zone. It consists of the same material as the moderator. In fact, this is a box, the main purpose of which is to prevent neutrons from leaving the core and getting into the environment.
• Coolant. The coolant must absorb the heat that was released during the fission of fuel atoms and transfer it to other substances. The coolant largely determines how a nuclear power plant is designed. The most popular coolant today is water.
  Reactor control system. Sensors and mechanisms that bring the nuclear power plant reactor into action.

Fuel for nuclear power plants

What does a nuclear power plant do? Fuel for nuclear power plants are chemical elements with radioactive properties. At all nuclear power plants, uranium is such an element.

The design of stations implies that nuclear power plants operate on complex composite fuel, and not on a pure chemical element. And in order to extract uranium fuel from natural uranium, which is loaded into a nuclear reactor, you need to carry out a lot of manipulations.

Enriched uranium

Uranium consists of two isotopes, that is, it contains nuclei with different masses. They were named by the number of protons and neutrons isotope -235 and isotope-238. Researchers of the 20th century began to extract uranium 235 from the ore, because. it was easier to decompose and transform. It turned out that there is only 0.7% of such uranium in nature (the remaining percentages went to the 238th isotope).

What to do in this case? They decided to enrich uranium. Enrichment of uranium is a process when there are many necessary 235x isotopes and few unnecessary 238x isotopes left in it. The task of uranium enrichers is to make almost 100% uranium-235 from 0.7%.

Uranium can be enriched using two technologies - gas diffusion or gas centrifuge. For their use, uranium extracted from ore is converted into a gaseous state. In the form of gas, it is enriched.

uranium powder

Enriched uranium gas is converted into a solid state - uranium dioxide. This pure solid uranium 235 looks like large white crystals that are later crushed into uranium powder.

Uranium tablets

Uranium pellets are solid metal washers, a couple of centimeters long. In order to mold such tablets from uranium powder, it is mixed with a substance - a plasticizer, it improves the quality of tablet pressing.

Pressed washers are baked at a temperature of 1200 degrees Celsius for more than a day to give the tablets special strength and resistance to high temperatures. The way a nuclear power plant works directly depends on how well the uranium fuel is compressed and baked.

Tablets are baked in molybdenum boxes, because. only this metal is able not to melt at "hellish" temperatures over one and a half thousand degrees. After that, uranium fuel for nuclear power plants is considered ready.

What is TVEL and TVS?

The reactor core looks like a huge disk or pipe with holes in the walls (depending on the type of reactor), 5 times larger than a human body. These holes contain uranium fuel, the atoms of which carry out the desired reaction.

It’s impossible to simply throw fuel into a reactor, well, if you don’t want to get an explosion of the entire station and an accident with consequences for a couple of nearby states. Therefore, uranium fuel is placed in fuel rods, and then collected in fuel assemblies. What do these abbreviations mean?

• TVEL - fuel element (not to be confused with the same name of the Russian company that produces them). In fact, this is a thin and long zirconium tube made of zirconium alloys, into which uranium pellets are placed. It is in fuel rods that uranium atoms begin to interact with each other, releasing heat during the reaction.

Zirconium was chosen as a material for the production of fuel rods due to its refractoriness and anti-corrosion properties.

The type of fuel elements depends on the type and structure of the reactor. As a rule, the structure and purpose of fuel rods does not change; the length and width of the tube can be different.

The machine loads more than 200 uranium pellets into one zirconium tube. In total, about 10 million uranium pellets work simultaneously in the reactor.
FA - fuel assembly. NPP workers call fuel assemblies bundles.

In fact, these are several TVELs fastened together. Fuel assemblies are ready-made nuclear fuel, what a nuclear power plant runs on. It is fuel assemblies that are loaded into a nuclear reactor. About 150 - 400 fuel assemblies are placed in one reactor.
Depending on which reactor the fuel assembly will operate in, they come in different shapes. Sometimes the bundles are folded into a cubic, sometimes into a cylindrical, sometimes into a hexagonal shape.

One fuel assembly for 4 years of operation generates the same amount of energy as when burning 670 wagons of coal, 730 tanks with natural gas or 900 tanks loaded with oil.
Today, fuel assemblies are produced mainly at factories in Russia, France, the USA and Japan.

In order to deliver fuel for nuclear power plants to other countries, fuel assemblies are sealed in long and wide metal pipes, air is pumped out of the pipes and delivered on board cargo aircraft by special machines.

Nuclear fuel for nuclear power plants weighs prohibitively much, tk. uranium is one of the heaviest metals on the planet. Its specific gravity is 2.5 times that of steel.

Nuclear power plant: principle of operation

What is the principle of operation of a nuclear power plant? The principle of operation of nuclear power plants is based on a chain reaction of fission of atoms of a radioactive substance - uranium. This reaction takes place in the core of a nuclear reactor.

If you do not go into the intricacies of nuclear physics, the principle of operation of a nuclear power plant looks like this:
After the nuclear reactor is started, absorbing rods are removed from the fuel rods, which prevent the uranium from reacting.

As soon as the rods are removed, the uranium neutrons begin to interact with each other.

When neutrons collide, a mini-explosion occurs at the atomic level, energy is released and new neutrons are born, a chain reaction begins to occur. This process releases heat.

The heat is transferred to the coolant. Depending on the type of coolant, it turns into steam or gas, which rotates the turbine.

The turbine drives an electric generator. It is he who, in fact, generates electricity.

If you do not follow the process, uranium neutrons can collide with each other until the reactor is blown up and the entire nuclear power plant is blown to smithereens. Computer sensors control the process. They detect an increase in temperature or a change in pressure in the reactor and can automatically stop the reactions.

What is the difference between the principle of operation of nuclear power plants and thermal power plants (thermal power plants)?

Differences in work are only at the first stages. In nuclear power plants, the coolant receives heat from the fission of atoms of uranium fuel, in thermal power plants, the coolant receives heat from the combustion of organic fuel (coal, gas or oil). After either the atoms of uranium or the gas with coal have released heat, the schemes of operation of nuclear power plants and thermal power plants are the same.

Types of nuclear reactors

How a nuclear power plant works depends on how its nuclear reactor works. Today there are two main types of reactors, which are classified according to the spectrum of neurons:
A slow neutron reactor, also called a thermal reactor.

For its operation, 235 uranium is used, which goes through the stages of enrichment, the creation of uranium tablets, etc. Today, slow neutron reactors are in the vast majority.
Fast neutron reactor.

These reactors are the future, because they work on uranium-238, which is a dime a dozen in nature and it is not necessary to enrich this element. The disadvantage of such reactors is only in very high costs for design, construction and launch. Today, fast neutron reactors operate only in Russia.

The coolant in fast neutron reactors is mercury, gas, sodium or lead.

Slow neutron reactors, which are used today by all nuclear power plants in the world, also come in several types.

The IAEA organization (International Atomic Energy Agency) has created its own classification, which is used most often in the world nuclear industry. Since the principle of operation of a nuclear power plant largely depends on the choice of coolant and moderator, the IAEA has based its classification on these differences.

From a chemical point of view, deuterium oxide is an ideal moderator and coolant, because its atoms most effectively interact with the neutrons of uranium compared to other substances. Simply put, heavy water performs its task with minimal losses and maximum results. However, its production costs money, while it is much easier to use the usual “light” and familiar water for us.

A few facts about nuclear reactors...

It is interesting that one nuclear power plant reactor is built for at least 3 years!
To build a reactor, you need equipment that runs on an electric current of 210 kilo amperes, which is a million times the current that can kill a person.

One shell (structural element) of a nuclear reactor weighs 150 tons. There are 6 such elements in one reactor.

Pressurized water reactor

We have already found out how the nuclear power plant works in general, in order to “sort it out” let's see how the most popular pressurized nuclear reactor works.
All over the world today, generation 3+ pressurized water reactors are used. They are considered the most reliable and safe.

All pressurized water reactors in the world over all the years of their operation in total have already managed to gain more than 1000 years of trouble-free operation and have never given serious deviations.

The structure of nuclear power plants based on pressurized water reactors implies that distilled water circulates between the fuel rods, heated to 320 degrees. To prevent it from going into a vapor state, it is kept under a pressure of 160 atmospheres. The NPP scheme calls it primary water.

The heated water enters the steam generator and gives off its heat to the water of the secondary circuit, after which it “returns” to the reactor again. Outwardly, it looks like the pipes of the primary water circuit are in contact with other pipes - the water of the second circuit, they transfer heat to each other, but the waters do not contact. Tubes are in contact.

Thus, the possibility of radiation getting into the water of the secondary circuit, which will further participate in the process of generating electricity, is excluded.

Nuclear power plant safety

Having learned the principle of operation of nuclear power plants, we must understand how safety is arranged. The design of nuclear power plants today requires increased attention to safety rules.
The cost of nuclear power plant safety is approximately 40% of the total cost of the plant itself.

The NPP scheme includes 4 physical barriers that prevent the release of radioactive substances. What are these barriers supposed to do? At the right time, be able to stop the nuclear reaction, ensure constant heat removal from the core and the reactor itself, and prevent the release of radionuclides from the containment (containment zone).

• The first barrier is the strength of uranium pellets. It is important that they do not collapse under the influence of high temperatures in a nuclear reactor. In many ways, how a nuclear power plant works depends on how the uranium pellets were "baked" at the initial stage of production. If the uranium fuel pellets are baked incorrectly, the reactions of the uranium atoms in the reactor will be unpredictable.
• The second barrier is the tightness of fuel rods. Zirconium tubes must be tightly sealed, if the tightness is broken, then at best the reactor will be damaged and work stopped, at worst everything will fly into the air.
• The third barrier is a strong steel reactor vessel a, (that same large tower - a containment area) which "holds" all radioactive processes in itself. The hull is damaged - radiation will be released into the atmosphere.
• The fourth barrier is emergency protection rods. Above the active zone, rods with moderators are suspended on magnets, which can absorb all neutrons in 2 seconds and stop the chain reaction.

If, despite the construction of a nuclear power plant with many degrees of protection, it is not possible to cool the reactor core at the right time, and the fuel temperature rises to 2600 degrees, then the last hope of the safety system comes into play - the so-called melt trap.

The fact is that at such a temperature the bottom of the reactor vessel will melt, and all the remnants of nuclear fuel and molten structures will flow into a special “glass” suspended above the reactor core.

The melt trap is refrigerated and refractory. It is filled with the so-called "sacrificial material", which gradually stops the fission chain reaction.

Thus, the NPP scheme implies several degrees of protection, which almost completely exclude any possibility of an accident.

Spent nuclear fuel is uranium that has worked in a nuclear reactor and contains radioactive fission products. Therefore, it is also called irradiated or burnt nuclear fuel.

How is SNF different from radioactive waste (RW)? First of all, the fact that SNF is a valuable product containing 2 useful components - unburned uranium and transuranium elements. In addition, fission products contain radionuclides (radioactive isotopes), which can be successfully used in industry, medicine, and also in scientific research.

After being removed from the reactor, spent nuclear fuel (SNF) retains radioactivity and releases heat. Therefore, for some time, such fuel is kept in pools under water to remove heat and protect against ionizing radiation. The next step could be:

• final disposal is the completion of an open fuel cycle as is done in the USA, Canada and Sweden.
• reprocessing of spent nuclear fuel for further use - a closed fuel cycle. The path of the closed fuel cycle was chosen by Russia, Great Britain, France and Japan.

Spent nuclear fuel is initially stored directly in the reactor building. Then it is moved to another location in special "dry storage" warehouses. In the closed fuel cycle for today's light water reactors, the fuel travels exactly the same path. Starting from uranium mines and factories, uranium goes through all the stages of transformation and enrichment for the manufacture of reactor fuel.After the fuel is removed from the reactor, the fuel rods are processed in refineries where they are crushed and dissolved in acid. After a special chemical treatment, two valuable products are recovered from the spent fuel: plutonium and unused uranium. Approximately 3% of the fuel remains as high-level waste. After bituminization, concreting or vitrification, these highly radioactive materials are subject to long-term disposal.

Spent nuclear fuel contains approximately 1% plutonium. This is a very good nuclear fuel that does not need any enrichment process. Plutonium can be mixed with depleted uranium to form mixed oxide or MOX fuel, which is supplied as fresh fuel assemblies for loading into reactors. It can be used to load into reactors. Recovered uranium can be returned for additional enrichment or supplied as fresh fuel for operating reactors. A closed fuel cycle is a more efficient system for maximizing the use of uranium without additional mining (in terms of energy units, the savings are about 30%). And although the industry immediately approved this approach, such schemes for the processing of spent nuclear fuel have not yet become widespread.

One of the reasons for such an incomplete use of the possibilities of uranium is that most of the existing industrial reactors belong to the so-called "light water" LWR reactors. They are good in many ways, but they are not designed to squeeze all the energy out of the fuel to the last watt. However, there are other types of reactors - the so-called "fast" (fast neutron reactors), capable of "processing" spent fuel to extract much more energy.

TVS (fuel assembly)

Nuclear fuel- materials that are used in nuclear reactors to carry out a controlled nuclear fission chain reaction. Nuclear fuel is fundamentally different from other types of fuel used by mankind, it is extremely energy intensive, but also very dangerous for humans, which imposes many restrictions on its use for safety reasons. For this and many other reasons, nuclear fuel is much more difficult to use than any type of fossil fuel, and requires many special technical and organizational measures for its use, as well as highly qualified personnel dealing with it.

general information

A nuclear chain reaction is the fission of a nucleus into two parts, called fission fragments, with the simultaneous release of several (2-3) neutrons, which, in turn, can cause fission of the following nuclei. Such fission occurs when a neutron enters the nucleus of an atom of the original substance. The fission fragments formed during nuclear fission have a large kinetic energy. The deceleration of fission fragments in matter is accompanied by the release of a large amount of heat. Fission fragments are nuclei formed directly as a result of fission. Fission fragments and their radioactive decay products are commonly referred to as fission products. Nuclei that fission with neutrons of any energy are called nuclear fuel (as a rule, these are substances with an odd atomic number). There are nuclei that fission only by neutrons with energies above a certain threshold value (as a rule, these are elements with an even atomic number). Such nuclei are called raw materials, since when a neutron is captured by a threshold nucleus, nuclei of nuclear fuel are formed. The combination of nuclear fuel and raw material is called nuclear fuel. Below is the distribution of the fission energy of the 235 U nucleus between different fission products (in MeV):

Kinetic energy of fission fragments	162	81%
Kinetic energy of fission neutrons	5	2,5%
Energy of γ-radiation accompanying neutron capture	10	5%
Energy of γ-radiation of fission products	6	3%
Energy of β-radiation of fission products	5	2,5%
Energy Carried Away by Neutrinos	11	5,5%
Total fission energy	~200	100%

Since the neutrino energy is carried away irrevocably, only 188 MeV/atom = 30 pJ/atom = 18 TJ/mol = 76.6 TJ/kg is available for use (according to other data (see link) 205.2 - 8.6 = 196 .6 MeV/atom) .

Natural uranium consists of three isotopes: 238U (99.282%), 235U (0.712%) and 234U (0.006%). It is not always suitable as a nuclear fuel, especially if the structural materials and the moderator absorb neutrons extensively. In this case, nuclear fuel is made on the basis of enriched uranium. In thermal reactors, uranium with an enrichment of less than 6% is used, and in fast and intermediate neutron reactors, uranium enrichment exceeds 20%. Enriched uranium is obtained at special enrichment plants.

Classification

Nuclear fuel is divided into two types:

• Natural uranium, containing fissile nuclei 235 U, as well as raw materials 238 U, capable of forming plutonium 239 Pu when capturing a neutron;
• Secondary fuel that does not occur in nature, including 239 Pu obtained from fuel of the first type, as well as 233 U isotopes formed during the capture of neutrons by 232 Th thorium nuclei.

According to the chemical composition, nuclear fuel can be:

• Metallic , including alloys ;
• Oxide (for example, UO 2);
• Carbide (e.g. PuC 1-x)
• Mixed (PuO 2 + UO 2)

Theoretical aspects of application

Nuclear fuel is used in nuclear reactors in the form of pellets a few centimeters in size, where it is usually located in hermetically sealed fuel elements (TVELs), which in turn, for ease of use, are combined into several hundred into fuel assemblies (FAs).

Nuclear fuel is subject to high requirements for chemical compatibility with fuel rod cladding, it must have a sufficient melting and evaporation temperature, good thermal conductivity, a slight increase in volume during neutron irradiation, and manufacturability.

The use of metallic uranium, especially at temperatures above 500 °C, is difficult due to its swelling. After nuclear fission, two fission fragments are formed, the total volume of which is greater than the volume of a uranium (plutonium) atom. Part of the atoms - fission fragments are atoms of gases (krypton, xenon, etc.). Gas atoms accumulate in the pores of uranium and create an internal pressure that increases with increasing temperature. Due to a change in the volume of atoms in the process of fission and an increase in the internal pressure of gases, uranium and other nuclear fuels begin to swell. Swelling is understood as the relative change in the volume of nuclear fuel associated with nuclear fission.

Swelling depends on burnup and fuel element temperature. The number of fission fragments increases with burnup, and the internal pressure of the gas increases with burnup and temperature. The swelling of nuclear fuel can lead to the destruction of the fuel element cladding. Nuclear fuel is less prone to swelling if it has high mechanical properties. Metallic uranium just does not apply to such materials. Therefore, the use of metallic uranium as a nuclear fuel limits the burnup depth, which is one of the main characteristics of nuclear fuel.

The radiation resistance and mechanical properties of the fuel are improved after uranium alloying, during which small amounts of molybdenum, aluminum and other metals are added to uranium. Doping additives reduce the number of fission neutrons per neutron capture by nuclear fuel. Therefore, alloying additions to uranium tend to be chosen from materials that weakly absorb neutrons.

Good nuclear fuels include some of the refractory compounds of uranium: oxides, carbides, and intermetallic compounds. The most widely used ceramics - uranium dioxide UO 2 . Its melting point is 2800 °C, density is 10.2 g/cm³. Uranium dioxide has no phase transitions and is less prone to swelling than uranium alloys. This allows you to increase burnout up to several percent. Uranium dioxide does not interact with zirconium, niobium, stainless steel and other materials at high temperatures. The main disadvantage of ceramics is low thermal conductivity - 4.5 kJ/(m·K), which limits the specific power of the reactor in terms of melting temperature. Thus, the maximum heat flux density in VVER reactors for uranium dioxide does not exceed 1.4⋅10 3 kW/m², while the maximum temperature in fuel rods reaches 2200 °C. In addition, hot ceramics are very brittle and can crack.

Practical use

Receipt

uranium fuel

Uranium nuclear fuel is obtained by processing ores. The process takes place in several stages:

• For poor deposits: In modern industry, due to the lack of rich uranium ores (exceptions are Canadian and Australian deposits of the unconformity type, in which the concentration of uranium reaches 3%), the method of underground leaching of ores is used. This eliminates costly ore mining. Preliminary preparation goes directly underground. Through injection wells sulfuric acid is pumped underground over the deposit, sometimes with the addition of ferric salts (to oxidize uranium U (IV) to U (VI)), although ores often contain iron and pyrolusite, which facilitate oxidation. Through extraction wells a solution of sulfuric acid with uranium rises to the surface with special pumps. Then it goes directly to the sorption, hydrometallurgical extraction and simultaneous enrichment of uranium.
• For ore deposits: use ore concentration and radiometric ore concentration .
• Hydrometallurgical processing - crushing, leaching, sorption or extraction extraction of uranium to obtain purified uranium oxide (U 3 O 8), sodium diuranate (Na 2 U 2 O 7) or ammonium diuranate ((NH 4) 2 U 2 O 7) .
• Transfer of uranium from oxide to UF 4 tetrafluoride, or from oxides directly to obtain UF 6 hexafluoride, which is used to enrich uranium in the 235 isotope.
• Enrichment by gas thermal diffusion or centrifugation.
• UF 6 enriched in the 235 isotope is converted into UO 2 dioxide, from which fuel rod “pills” are made or other uranium compounds are obtained for the same purpose.