Features of nuclear fuel and its use in nuclear energy. The operating principle of a nuclear power plant. Help What does spent nuclear fuel look like?

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 electrical energy 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 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 energy 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 producing 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. They obtain energy from nuclear fuel, and then it is transferred to another working fluid (water, metal or organic liquid, gas) in the form of heat; then it is converted into electricity according to the same scheme as in conventional ones.

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

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 the fuel elements, heats up and through the pipeline 4 enters the steam generator.

Rice. 1. Nuclear reactor

The reactor is housed 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)- the main part of the reactor. A nuclear reaction directly occurs 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. On the outside, the fuel elements are covered with a corrosion-resistant, thin metal shell. 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 body of the engine (turbine) of power plants is carried out according to single-circuit, double-circuit and three-circuit schemes (Fig. 2).

Rice. 2. Nuclear power plant
a – according to a single-circuit scheme; b – according to a double-circuit scheme; c – according to a 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) located inside the primary 2 and secondary 3 biological protection. If the nuclear power plant is built according to a single-circuit thermal circuit, steam from the reactor through the pressure regulator 4 enters the turbine 5 . The turbine shaft is connected to the electric 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 the regenerative heater 10 , and then it enters the reactor.

In a dual-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 (secondary circuit feedwater). In water-cooled reactors, the coolant in the steam generator is cooled by approximately 15...40 o C and then by a circulation pump 12 is sent back to the reactor.

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

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 circuit can be used in boiling reactors and in reactors with gas coolant. The most widespread double-circuit circuit when using water, gas and organic liquids as a coolant. 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. A nuclear reaction of one gram of fissile material (uranium-235) releases energy equivalent to 22×10 3 kW × h (19×10 6 cal). To obtain this amount of energy, it is necessary to burn 1900 kg of oil.

Uranium-235 is readily available and its energy reserves are approximately the same as those of fossil fuels. However, if nuclear fuel is used at such low efficiency as currently available, the available uranium sources will be depleted within 50-100 years. At the same time, nuclear fuel “deposits” are practically inexhaustible - this is uranium dissolved in sea water. There is hundreds of times more of it in the ocean than on land. The cost of obtaining one kilogram of uranium dioxide from seawater is about $60-80, and in the future it will drop to $30, and the cost of uranium dioxide mined 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 approximately two times lower than that of fossil coal. At coal-fired power plants, the share of fuel falls 50-70% of the cost of electricity, and at nuclear power plants - 15-30%. A modern thermal power plant with a capacity of 2.3 million kW (for example, Samara State District Power Plant) consumes about 18 tons of coal (6 trains) or 12 thousand tons of fuel oil (4 trains) every day. Nuclear, of the same power, consumes only 11 kg of nuclear fuel per day, and 4 tons during the year. However, a nuclear power plant is more expensive than a thermal power plant 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 the construction of nuclear power plants due to:

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

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

• increase the burnup of nuclear fuel;
• raise the plutonium breeding rate.

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 to reproduce nuclear fuel, creating fast neutron reactors. When operating a fast neutron reactor, it is possible to simultaneously obtain approximately 1.7 kg of plutonium-239 per 1 kg of burned uranium-235. In this way, the low thermal efficiency of nuclear power plants can be covered.

Fast neutron reactors are tens of times more efficient (in terms of using nuclear fuel) than fuel neutron reactors. They do not contain a moderator and use highly enriched nuclear fuel. Neutrons escaping from the core are absorbed not by structural materials, but by uranium-238 or thorium-232 located around them.

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. Converting uranium-238 into plutonium-239 in reactors will increase nuclear fuel resources by about 100 times, and thorium-232 into uranium-233 by 200 times.

In Fig. Figure 3 shows a diagram of a nuclear power plant using fast neutrons.

Distinctive features of a fast neutron nuclear power plant are:

1. changing the criticality of a nuclear reactor is carried out by reflecting part of the fission neutrons of nuclear fuel from the periphery back into the core using reflectors 3 ;
2. reflectors 3 can rotate, changing the neutron leakage and, therefore, the intensity of fission reactions;
3. nuclear fuel is reproduced;
4. Excess thermal energy is removed from the reactor using a radiator refrigerator 6 .

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

Converters of thermal energy into electrical energy

Based on 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 unit is usually connected to the reactor, in which the working fluid can be hydrogen, helium, or a helium-xenon mixture. The efficiency of converting heat supplied directly to the turbogenerator into electricity is quite high - converter efficiency η = 0,7-0,75.

The diagram of a nuclear power plant with a dynamic gas turbine (machine) converter is shown in Fig. 4.

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

Rice. 4. Diagram of a nuclear power plant with a gas turbine converter:
1 – reactor; 2 – circuit with liquid metal coolant; 3 – heat exchanger for supplying heat to the gas; 4 – turbine; 5 – electric generator; 6 – compressor; 7 – refrigerator-emitter; 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 consists in the fact that in a closed circuit consisting of dissimilar materials, a thermo-emf occurs 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 that have a higher efficiency, while the temperature of the hot junction must be raised to 1400 K and higher.

Thermionic converters (TEC) make it possible to generate 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 – electricity consumer; 4 – dielectric; 5 – conductor; 6 – working fluid (gas).

Rice. 6. Scheme of operation of a thermoelectric generator

Rice. 7. Scheme of operation of the thermionic converter

To maintain the emission current, heat is supplied to the cathode Q 1 . The electrons emitted by the cathode, having overcome the vacuum gap, reach the anode and are absorbed by it. When electrons “condense” at the anode, energy is released equal to the work function of electrons with the opposite sign. If we provide a continuous supply of heat to the cathode and remove it from the anode, then through the load R direct current will flow. Electron emission occurs efficiently at cathode temperatures above 2200 K.

Safety and reliability of nuclear power plants

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

Radiation safety is ensured by:

1. creation of reliable structures and devices for biological protection of personnel from radiation;
2. purification of air and water leaving the premises of the nuclear power plant;
3. extraction and reliable localization of radioactive contamination;
4. daily radiation monitoring of nuclear power plant premises and individual radiation monitoring of personnel.

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

1. high security zone;
2. restricted area;
3. normal mode zone.

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

During the operation of nuclear power plants, solid, liquid and gaseous radioactive waste is generated. They must be disposed of in a manner that does not create environmental pollution.

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

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

Core of a nuclear power reactor (A.Z.ENR)- this is the part of its volume in which the conditions are structurally organized for the implementation of a continuous self-sustaining chain reaction of nuclear fuel fission and balanced removal of the heat generated in it for the purpose of its subsequent use.

Having thought about the meaning of this definition in relation to the core of a thermal nuclear reactor, one can understand that the fundamental components of such a core are nuclear fuel, moderator, coolant and other structural materials. The latter are objectively necessary, since nuclear fuel and moderator in the core and the core itself zone should be fixedly fixed in the reactor, representing, if possible, a dismountable technological unit.

Nuclear fuel is usually understood as the totality of all fissile nuclides in the core. Most of the thermal nuclear reactors used in nuclear power plants at the initial stage of operation operate on pure uranium fuel, but during the campaign they reproduce a significant amount of secondary nuclear fuel - plutonium-239, which immediately after its formation is included in the process of neutron multiplication in the reactor . Therefore, the fuel in such nuclear reactors at any arbitrary moment in the campaign should be considered a combination of three fissile components: 235 U, 238 U and 239 Pu. Uranium-235 and plutonium-239 are fissioned by neutrons of any energy in the reactor spectrum, and 238 U, as already noted, is fissioned only by fast above-threshold (with E > 1.1 MeV) neutrons.

The main characteristic of uranium nuclear fuel is its initial enrichment (x), which is understood as the share (or percentage) of uranium-235 nuclei among all uranium nuclei. And since more than 99.99% of uranium consists of two isotopes - 235 U and 238 U, the enrichment value is:
x = N 5 /N U = N 5 /(N 5 +N 8) (4.1.1)
Natural uranium metal contains approximately 0.71% 235 U nuclei, and more than 99.28% is 238 U. Other isotopes of uranium (233 U, 234 U, 236 U and 237 U) are present in natural uranium in such small quantities that they may not be taken into account.

In nuclear power plant reactors, uranium enriched to 1.8 ÷ 5.2% is used; in reactors of marine transport nuclear power plants, the initial enrichment of nuclear fuel is 20 ÷ 45%. The use of low-enrichment fuel at nuclear power plants is explained by economic considerations: the technology for producing enriched fuel is complex, energy-intensive, requires complex and bulky equipment, and therefore is an expensive technology.

Uranium metal is thermally unstable, subject to allotropic transformations at relatively low temperatures and chemically unstable, and therefore unacceptable as fuel for power reactors. Therefore, uranium in reactors is used not in pure metallic form, but in the form of chemical (or metallurgical) compounds with other chemical elements. These connections are called fuel compositions.

The most common fuel compositions in reactor technology are:
UO 2, U 3 O 8, UC, UC 2, UN, U 3 Si, (UAl 3) Si, UBe 13.

The other chemical element(s) of the fuel composition is called fuel diluent. In the first two of the listed fuel compositions, the diluent is oxygen, in the second two - carbon, in the subsequent ones, respectively, nitrogen, silicon, aluminum with silicon and beryllium.
The basic requirements for a diluent are the same as for a moderator in a reactor: it must have a high microsection for elastic scattering and a possibly lower microsection for the absorption of thermal and resonant neutrons.

The most common fuel composition in nuclear power reactors is uranium dioxide(UO 2), and its diluent - oxygen - fully meets all the mentioned requirements .

Melting point of dioxide (2800 o C) and its high thermal stability allow you to have high temperature fuel with a permissible operating temperature of up to 2200 o C.

The use of nuclear fuel in reactors for energy production has a number of features due to physical properties and the nature of the processes occurring. These features determine the specifics of nuclear energy, technology requirements, special operating conditions, economic indicators and impact on the environment.

First of all, we note the high calorific value of nuclear fuel. During the combustion (oxidation), for example, of carbon in the reaction C + O 2 → CO 2, 4 eV of energy is released for each interaction, and the resulting carbon monoxide leads to a greenhouse effect with global consequences for the planet. The fission of one nuclear fuel atom releases approximately 200 MeV of energy. The energy release in these two processes differs by 50 million times. In terms of unit mass, the energy releases differ by a factor of 2.5 million.

High caloric content causes a sharp reduction in both the mass and physical volumes of nuclear fuel required to produce a given amount of energy. Thus, storage and transportation of feedstock (uranium concentrate) and finished nuclear fuel require relatively low costs. The consequence of this is the independence of the location of nuclear power plants from the areas of fuel production and production, which significantly influences the choice of economically advantageous location of productive forces. We can say that the use of nuclear fuel can correct the “injustice” of nature in the extremely uneven geographical distribution of energy resources. Difficulties associated with seasonal climatic conditions of fuel delivery and supply, which constantly arise in the East and the Far North, are eliminated. The high energy intensity of nuclear fuel determines the relatively small number of workers involved in the extraction, production and delivery of fuel to the consumer per unit of energy produced compared to the extraction and transportation of organic fuel, which ultimately ensures high labor productivity in nuclear energy.

An important feature of nuclear fuel is the fundamental impossibility of its complete combustion. To operate a reactor at a given power for a given time, the fuel load must be above a critical mass. This excess provides a margin of reactivity that is necessary for a given or calculated amount of fuel separated per unit volume or mass, i.e. to achieve a given burnout depth. After reaching this burnup, when the reactivity reserve is exhausted, it is necessary to replace the spent fuel with new one. The unloaded fuel contains a significant amount of fissionable and fertile materials and, after being purified from fission products, can be returned to the fuel cycle. It follows from this that nuclear fuel must circulate repeatedly through reactors and nuclear industry enterprises: radiochemical plants and factories for the production of fuel rods and fuel assemblies(TVS). By recycling (reusing) uranium and plutonium, the need for natural uranium and fuel enrichment capacity is significantly reduced. Note that the amount of nuclear fuel to be processed in the fuel cycle for a nuclear power plant with an electrical power of 1 GW is 20-30 tons/year for VVER-1000 and approximately 50 tons/year for RBMK-1000.

The requirement to constantly contain a large mass of fuel in the reactor core, designed for a long period of operation to ensure a given burnup, causes significant one-time costs for paying for the first fuel load and subsequent batches prepared for loading. This is a very significant and fundamental difference in the conditions for using nuclear fuel in power plants compared to organic fuel.

The accumulation of radioactive fission products in the fuel during their subsequent decay after the termination of the chain reaction leads to residual heat release, which decreases with time approximately according to a power law:

N(t) = 0,07N[t -0,2 – (t+ ) -0.2 ], (2.1)

Where N- reactor power before shutdown, N(t) is the heat release power after the reactor is shut down,  is the time the reactor operates at power N to the stop, t- time after stopping. From expression (2.1) it follows that immediately after shutdown, the heat release in the core is 7% of the rated power. Residual energy release, activity of the coolant and elements of the reactor core, the need to take into account hypothetical emergency situations impose special requirements on the design, construction and operation of nuclear power plants, reactor protection and control systems. These requirements have no analogues in thermal power engineering using fossil fuels. Satisfying the safety requirements of nuclear power plants causes an increase in capital costs by 1.5-2 times compared to traditional thermal power plants.

2.2. Burn-up is a measure of energy production

nuclear fuel

The energy characteristic of any fuel is its calorific value, i.e. heat release per unit mass. The energy characteristic of nuclear fuel is specific energy production - thermal energy that can be released per unit mass of nuclear fuel with a given isotopic composition over the entire period of stay in the reactor. Specific energy production nuclear fuel (B) is usually measured in megawatt days per ton (MW day/t) or megawatt days per kilogram (MW day/kg).

The release of thermal energy in a reactor is the result of nuclear fission and can be expressed in terms of the number of nuclei or the mass of split fuel divided by their total number. This mass unit of burnup ( burn-up depth In 1) it can be expressed as a percentage, kg/t, g/kg, etc. The value B 1 also denotes the amount of fission products accumulated in fuel rods. Specific energy production and burnup of nuclear fuel are equivalent quantities having different dimensions. They are the most important parameters characterizing the use of nuclear fuel in reactors. The burnup depth has a great influence on the technical and economic indicators of not only nuclear power plants, but also the entire fuel cycle.

Let us determine the relationship between B and B 1 for uranium dioxide - the fuel of modern power reactors. The number of uranium nuclei in a gram of uranium dioxide is equal to Avogadro's number divided by the molecular weight: 6.022·10 23 /270 = 2.32·10 21 1/g. The energy released during one fission event is 3.2·10 -11 J. The number of fissions required to produce 1 MW·day (8.64·10 10 J) is 2.7·10 21 . Thus, to obtain energy of 1 MW day, it is necessary to ensure the fission of 1.16 g of uranium dioxide. Denoting this quantity by k, let’s write down the relationship between energy and mass burnup units:

B 1 = k V. (2.2)

If in a ton of uranium dioxide 1% of uranium atoms (2.32 10 25) are separated, then the energy production will be 2.32 10 25 / 2.7 10 21 = 8593 MW day/t. The burnout of 1% of heavy atoms corresponds for uranium dioxide to 2.44·10 20 divisions/cm 3 .

If we take into account only the weight of uranium, then k= 1.05. In this case, a burnup of 1% corresponds to a uranium energy production of 9520 MW day/t. In further calculations related to thermal neutron reactors, we will take k= 1.05. However, the burnup depth does not completely determine the consumption of fissile nuclides in the reactor core. Along with nuclear fission, the reaction of radiation capture and transformation of fissile nuclides into non-fissile ones takes place. For 235 U, the probability of capturing a neutron without fission and producing the 236 U isotope is approximately 0.15. This means the loss of a fissile isotope without the release of energy. For 239 Pu, transformation into the non-fissile isotope 240 Pu as a result of radiative capture has a probability of 0.26. The presence of radiation capture competing with the fission process leads to an ineffective increase in the consumption of fissile nuclides. In thermal neutron reactors, when producing 1 MW day of thermal energy, not 1.05 g, but 1.2-1.22 g of 235 U are consumed, including 0.15-0.17 g without releasing energy, but with At 1% burnup, the energy production of uranium is 8300 MW day/t. All this is taken into account when calculating the core and when determining the required enrichment of the fuel by fissile isotope.

Nuclear fuel usually means the totality of all fissile nuclides in the core. Most of the thermal ENRs used in NPP power units at the initial stage of operation operate on pure uranium fuel, but during the campaign they reproduce a significant amount secondary nuclear fuel- plutonium-239, which immediately after its formation is included in the process of neutron multiplication in the reactor. Therefore, the fuel in such nuclear reactors at any the moment of the campaign should be considered, at a minimum, a combination of three fissile components: 235 U, 238 U and 239 Pu. Uranium-235 and plutonium-239 are fissile by neutrons of any energy in the reactor spectrum, and 238 U, as already noted, only fast suprathreshold(With E > 1.1 MeV)neutrons.

The main characteristic of uranium nuclear fuel is its initial enrichment (x), by which we mean the share (or percentage) of uranium-235 nuclei among all uranium nuclei. And since more than 99.99% of uranium consists of two isotopes - 235 U and 238 U, the enrichment value is:

Natural uranium metal contains approximately 0.714% 235 U nuclei, and more than 99.286% is 238 U (other uranium isotopes: 233 U, 234 U, 236 U and 237 U are present in natural uranium in such small quantities that they may not be accepted in attention).

If the fuel is not fresh (irradiated - SNF), then it is characterized by one more parameter - burn-up depth.

Nuclear fuel is an expensive thing. Mining uranium ore, obtaining natural metallic uranium, enriching it with the 235 U isotope, manufacturing a fuel composition, sintering it into tablets and their finishing, manufacturing fuel rods and fuel assemblies - all these are very complex technological processes that require large material and energy costs. It is clear that throwing a fairly large amount of unburned nuclear fuel into a radioactive waste cemetery would be very unwise.

Spent (irradiated) fuel is sent to regeneration, where fuel components are separated through a chain of complex technological operations from the fission products accumulated during operation, re-enriched with the 235 U isotope and reincorporated into the fuel cycle. Note that the regeneration of nuclear fuel is no less complex and expensive than the production of “fresh” fuel.

That is why it is very important that during the campaign, as much of the loaded fuel as possible is burned out, and as little of it as possible remains for regeneration. The measure for assessing the efficiency of fuel use in power reactors are two main characteristics.

A) Fuel burnout - this is the share (or percentage) of the burned main fuel (235 U) of its initial amount.

The degree of burnout is indicated by the letter z and in accordance with the definition is equal to:

Using basic substitutions, it is easy to show that the degree of burnout at any point in the campaign t- a value directly proportional to the amount of energy production W(t), if we do not take into account that part of the generated energy that is obtained as a result of fission of plutonium nuclei.

From (15.3.1) it follows that

The efficiency of using the main fuel in the reactor during a core campaign can be judged by the figures for the maximum degree of burnup (that is, the degree of burnup at the end of the campaign).

For reactors of the RBMK-1000 type z max = 0.35 ¸ 0.37, and for pressurized water reactors (VVER-440, VVER-1000) z max = 0.30 ¸ 0.33.

In practice, the degree of burnout can also be measured in %.

b) Burnout depth is the energy production at the moment of the campaign per unit mass of the initially loaded uranium.

Here we are talking about everything uranium(235 U + 238 U), loaded into the active zone before the start of the campaign. If we designate the burnout depth as b, then in accordance with the definition

Burnout depth is usually measured in MW day/t, MW day/kg

or GW day/t.

The following figures give an idea of ​​the fuel burnout depth:

* for reactors of the RBMK-1000 type b max => 20 MW. day/kg;

* for VVER-type reactors 1000 b max => 40 ¸ 50 MW. day/kg.

In nuclear power plant reactors, low-enrichment uranium is used (enriched to 1.8 ¸ 5.2%), in reactors of marine transport nuclear power plants, the initial enrichment of nuclear fuel is 21 ¸ 45%, and in installations with liquid metal reactors, nuclear fuel enriched up to 90% is used. The use of low-enriched fuel at nuclear power plants is explained by economic considerations: enriched fuel production technology It is complex, energy-intensive, requires complex and bulky equipment, and therefore is an expensive technology.

Uranium metal is thermally unstable, subject to phase transformations at relatively low temperatures and chemically unstable, and therefore unacceptable as fuel for power reactors. Therefore, uranium in reactors is not used in pure metallic form, but in the form of chemical (or metallurgical) compounds with other chemical elements. These compounds are called fuel compositions.

The most common fuel compositions in reactor technology are:

UO 2, U 3 O 8, UC, UC 2, UN, U 3 Si, (UAl 3) Si, UBe 13. (Cu-UO 2)

The other chemical element(s) of the fuel composition is called fuel diluent. In the first two of the listed fuel compositions, the diluent is oxygen, in the second two it is carbon, in the subsequent ones it is nitrogen, silicon, aluminum with silicon and beryllium, respectively.

The basic requirements for a diluent are the same as for a moderator in a reactor: it must have a high microsection for elastic scattering and a possibly lower microsection for the absorption of thermal and resonant neutrons.

The most common fuel composition in nuclear power reactors is uranium dioxide(UO 2), and its diluent - oxygen - fully meets all the mentioned requirements .

Melting point of dioxide (2800 o C) and its high thermal stability allow you to have high temperature fuel with a permissible operating temperature of up to 2200 o C.

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 to go through a long path of 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. The International Atomic Energy Agency's Red Book report shows that uranium production and demand are rising despite the Fukushima nuclear accident, which has left many wondering about the prospects for nuclear power. Over the past few years alone, proven uranium reserves have increased by 7%, which is associated with the discovery of new deposits. The largest producers remain Kazakhstan, Canada and Australia; they mine up to 63% of the world's uranium. In addition, metal reserves are available in Australia, Brazil, China, Malawi, Russia, Niger, USA, Ukraine, China and other countries. Previously, 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 deposits are close to the surface of the earth. With the open 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 depth, in which case the underground mining method is used. A mine is dug up to two kilometers deep, the rock is extracted by drilling in horizontal drifts, and transported upward in freight elevators.

The mixture that is transported upward in this way has many components. The rock must be crushed, diluted with water and the excess removed. Next, sulfuric acid is added to the mixture to carry out the leaching process. During this reaction, chemists obtain a yellow precipitate of uranium salts. Finally, uranium with impurities is purified in a refining facility. Only after this is uranium oxide produced, which is traded on the stock exchange.

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

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

Nuclear fuel production

It is impossible to imagine the production of nuclear fuel without gas centrifuges, which are used to produce enriched uranium. After reaching the required concentration, the uranium dioxide is pressed into so-called tablets. They are created using lubricants that are removed during firing in kilns. The firing temperature reaches 1000 degrees. After this, the tablets are checked to ensure they meet the stated requirements. Surface quality, moisture content, and the ratio of oxygen and uranium are important.

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 Mashinostroitelny Zavod in the Moscow region, the Novosibirsk Chemical Concentrates Plant in Novosibirsk, the Moscow Polymetals Plant and others.

Each batch of fuel assemblies is created for a specific type of reactor. European fuel assemblies are made in the shape of a square, while Russian ones have a hexagonal cross-section. Reactors of the VVER-440 and VVER-1000 types are widely used in the Russian Federation. 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 installations 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 core of the RBMK-1000 reactor, over 200 thousand components made of zirconium alloys, as well as 14 million sintered uranium dioxide pellets, are needed. Sometimes the cost of manufacturing a fuel assembly can exceed the cost of the fuel contained in the elements, which is why it is so important to ensure high energy efficiency per kilogram of uranium.

Costs of production processes in %

Separately, it is worth mentioning fuel assemblies for research reactors. They are designed in such a way as to make observation and study of the neutron generation process as comfortable as possible. Such fuel rods for experiments in the fields of nuclear physics, isotope production, and radiation medicine are produced in Russia by the Novosibirsk Chemical Concentrates Plant. FAs 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 company works on enriching raw materials, assembling 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.

Uranium pellets, which are placed in 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 are used. Each container contains two assemblies. Transportation of non-irradiated (fresh) fuel does not pose a radiation hazard, since the radiation does not extend beyond the zirconium tubes into which the pressed uranium pellets are placed.

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

Fuel in the reactor core

A unit of nuclear fuel - a TVEL - is capable of releasing enormous amounts of energy over a long period of time. Neither coal nor gas can compare with such volumes. The fuel life cycle 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, personnel assemble the fuel assemblies for loading into the core (the working area of ​​the reactor where the decay reaction occurs). As a rule, the fuel is partially reloaded.

Full fuel is added to the core only at the time of the first startup of the reactor. This is due to the fact that the fuel rods in the reactor burn out unevenly, since the neutron flux varies in intensity in different zones of the reactor. Thanks to metering devices, station personnel have the opportunity to monitor the degree of burnout of each unit of fuel in real time and make replacements. Sometimes, instead of loading new fuel assemblies, assemblies are moved among themselves. In the center of the active zone, burnout occurs most intensely.

FA after a nuclear power plant

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

Recently, physicists have begun to use radioactive isotopes accumulated in spent nuclear fuel in industry and medicine. After the fuel has completed its campaign (the time the assembly is in the reactor core under operating conditions at rated power), it is sent to the cooling pool, then to storage directly in the reactor compartment, and after that for reprocessing or disposal. The cooling pool is designed to remove heat and protect against ionizing radiation, since the fuel assembly remains dangerous after removal from the reactor.

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

The fuel rods are dissolved in acid, after which researchers separate the 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.

1% plutonium can be recovered from spent nuclear fuel. 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 approximately 3% cheaper, but this technology requires large investments in the construction of industrial units, so it has not yet become widespread in the world. However, the Rosatom fuel company does not stop research in this direction. Pronedra recently wrote that the Russian Federation is working on a fuel capable of recycling isotopes of americium, curium and neptunium in the reactor core, which are included in the same 3% of highly radioactive waste.

Nuclear fuel producers: rating

1. The French company Areva until recently provided 31% of the global market for fuel assemblies. The company produces nuclear fuel and assembles components for nuclear power plants. In 2017, Areva underwent a qualitative renovation, 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 is actively developing the market in Eastern Europe, supplying fuel assemblies to Ukrainian nuclear power plants. Together with Toshiba, it provides 26% of the global nuclear fuel production market.
3. The fuel company TVEL of the state corporation Rosatom (Russia) is in third place. TVEL provides 17% of the global 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 of nuclear plants of Western design.
4. Japan Nuclear Fuel Limited, according to the latest data, provides 16% of the world market and supplies fuel assemblies to most nuclear reactors in Japan itself.
5. Mitsubishi Heavy Industries is a Japanese giant that produces 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 and research activities together with Areva. This company was chosen by the Japanese government to develop new reactors.