Nuclear power plant with fast neutron reactors. Fast reactor

In our country, the first estimates of the properties of the fast spectrum of neutrons as applied to nuclear reactors were made in 1946 on the initiative of I.V. Kurchatova. Since 1949, A.I. became the head of work on fast reactors. Leypunsky, under whose scientific leadership at approximately the same time the possibility of expanded reproduction of nuclear fuel and the use of liquid metal coolant in reactors with a fast neutron spectrum was shown by calculation. Extensive research to develop the physical and physical-technical foundations of fast reactors began at the Physics and Power Engineering Institute in Obninsk, and then in many other organizations.

To conduct research in physics and engineering problems fast neutron reactors at IPPE, critical assemblies (zero power reactors) and fast neutron research reactors (RR) were built and put into operation: BR-1(in 1955), BR-2(in 1956), BR-5(in 1959), BFS-1(in 1961), BFS-2(in 1969), BR-10(reconstruction of BR-5, in 1973).

As a result of the studies carried out at these first installations, the possibility of achieving a nuclear fuel breeding factor in fast reactors KV>1 was confirmed; uranium dioxide was recommended as the main nuclear fuel, and liquid sodium as the main coolant.

The first demonstration fast reactor was the current one BOR-60 research reactor.

• gaining experience in operating fast neutron reactors of higher power;
• verification of methods for calculating neutronic characteristics (critmass, heat release field, plutonium production and quality, reactivity coefficients);
• checking the reliability of equipment and fuel; desalting plant sea ​​water, checking security systems;
• problems with oil, with steam generators, with fuel rods, spent assembly drum (SAD), with the reloading system, with structural materials of fuel rods, fuel assemblies and their solutions;
• materials science research, research on the reproduction factor, testing of natural circulation, experiment with entering the boiling mode in a fuel assembly, experiments on the dynamics of the development of intercircuit leakage.

Fast reactor BN-600- operates as part of a 600 MW power unit - has been supplying electricity to the grid since 1980. It uses mainly uranium oxide fuel enriched to 17, 21 and 26%, and small quantity MOX fuel. This is an integral type reactor, intermediate sodium-sodium heat exchangers and main circulation pumps are located in the reactor vessel. The pressure of the sodium coolant in the housing is slightly (0.05 MPa) higher than atmospheric pressure, so the risk of rupture of the housing is eliminated. Steam generators installed outside the hull supply steam to three 200 MW turbine generators.

On June 27, 2014, the physical start-up of power unit No. 4 took place with reactor BN-800, on December 10, 2015, it was first included in the country’s unified energy system, and on October 31, 2016, it was put into commercial operation. The reactor began to operate using the so-called hybrid core, in which the main part (84%) consists of fuel assemblies with uranium fuel, and 16% - fuel assemblies with MOX fuel. The transfer of this reactor to full loading with MOX fuel is planned in 2019. A plant has been built to produce MOX fuel.

IN reactor BN-800 used as verified technical solutions, implemented in BN-600, as well as new ones that significantly increase the safety of the power plant, such as: zero sodium void reactivity effect, hydraulically weighted emergency protection rods that are triggered when coolant flow is reduced, passive emergency cooling systems, a special “trap” is provided under the core to collect and retain the melt and fragments of the core during its destruction as a result of a severe accident, the seismic resistance of the structure has been increased.

Fast reactors currently operating in the world

The Japanese government has decided to completely decommission the Monju Nuclear Power Plant, the country's only nuclear power plant with a fast neutron reactor.

The Nuclear Regulatory Agency (NRA) has delayed consideration of the restart of the JOYO fast sodium research reactor. The application for permission to relaunch JOYO was submitted to the regulator on March 30, 2017. The application does not contain an estimated restart date.

Thus, since 1972 (since the launch BN-350) in our country, fast reactors are used to generate electricity and desalinate water. Currently, Russia is the only country in the world whose nuclear energy structure includes fast neutron reactors. This was achieved due to the fact that only in our country all necessary steps development of BN technology - fast reactors with sodium coolant.

The unique Russian fast neutron reactor operating at the Beloyarsk Nuclear Power Plant was brought to a power of 880 megawatts, the Rosatom press service reports.

The reactor operates at power unit No. 4 of the Beloyarsk NPP and is currently undergoing routine testing of generating equipment. In accordance with the test program, the power unit ensures that electrical power is maintained at a level of at least 880 megawatts for 8 hours.

The reactor power is being increased in stages in order to eventually receive certification at the design power level of 885 megawatts based on test results. At the moment, the reactor is certified for a power of 874 megawatts.

Let us recall that the Beloyarsk NPP operates two fast neutron reactors. Since 1980, the BN-600 reactor has been operating here - for a long time it was the only reactor of this type in the world. But in 2015, the phased launch of the second BN-800 reactor began.

Why is it so important and considered historical event for the global nuclear industry?

Fast neutron reactors make it possible to implement a closed fuel cycle (it is not currently implemented in the BN-600). Since only uranium-238 is “burned,” after processing (removing fission products and adding new portions of uranium-238), the fuel can be reloaded into the reactor. And since the uranium-plutonium cycle produces more plutonium than decays, the excess fuel can be used for new reactors.

Moreover, this method can be used to process surplus weapons-grade plutonium, as well as plutonium and minor actinides (neptunium, americium, curium) extracted from spent fuel from conventional thermal reactors (minor actinides currently represent a very dangerous part of radioactive waste). At the same time, the amount of radioactive waste compared to thermal reactors is reduced by more than twenty times.

Why, despite all their advantages, have fast neutron reactors not become widespread? This is primarily due to the peculiarities of their design. As mentioned above, water cannot be used as a coolant, since it is a neutron moderator. Therefore, fast reactors mainly use metals in liquid state- from exotic lead-bismuth alloys to liquid sodium(the most common option for nuclear power plants).

“In fast neutron reactors, thermal and radiation loads are much higher than in thermal reactors,” explains “PM” chief engineer Beloyarsk NPP Mikhail Bakanov. - This leads to the need to use special structural materials for the reactor vessel and in-reactor systems. The housings of fuel rods and fuel assemblies are made not of zirconium alloys, as in thermal reactors, but of special alloyed chromium steels, which are less susceptible to radiation ‘swelling’. On the other hand, for example, the reactor vessel is not subject to loads associated with internal pressure - it is only slightly higher than atmospheric pressure.”

According to Mikhail Bakanov, in the first years of operation the main difficulties were associated with radiation swelling and cracking of the fuel. These problems, however, were soon resolved, new materials were developed - both for fuel and for fuel rod housings. But even now, campaigns are limited not so much by fuel burnup (which on the BN-600 reaches 11%), but by the resource life of the materials from which the fuel, fuel rods and fuel assemblies are made. Further operational problems were associated mainly with leaks of sodium in the secondary circuit, a chemically active and fire-hazardous metal that reacts violently to contact with air and water: “Only Russia and France have long-term experience in operating industrial fast neutron power reactors. Both we and the French specialists faced the same problems from the very beginning. We successfully solved them, having foreseen from the very beginning special means monitoring the tightness of circuits, localizing and suppressing sodium leaks. But the French project turned out to be less prepared for such troubles; as a result, the Phenix reactor was finally shut down in 2009.”

“The problems really were the same,” adds Nikolai Oshkanov, director of the Beloyarsk NPP, “but they were solved here and in France in various ways. For example, when the head of one of the assemblies on Phenix bent in order to grab and unload it, French specialists developed a complex and quite expensive system‘visions’ through a layer of sodium. And when we had the same problem, one of our engineers suggested using a video camera placed in the simplest design type of diving bell, - a pipe open at the bottom with argon blowing from above. Once the sodium melt was expelled, operators were able to engage the mechanism via video link and the bent assembly was successfully removed.”

The active zone of a fast neutron reactor is arranged like an onion, in layers

370 fuel assemblies form three zones with different enrichment of uranium-235 - 17, 21 and 26% (initially there were only two zones, but in order to equalize the energy release, three were made). They are surrounded by side screens (blankets), or breeding zones, where assemblies containing depleted or natural uranium, consisting mainly of the 238 isotope, are located. At the ends of the fuel rods above and below the core there are also tablets of depleted uranium, which form the end screens (zones reproduction).

Fuel assemblies (FA) are a set of fuel elements (fuel elements) assembled in one housing - special steel tubes filled with uranium oxide pellets with various enrichments. So that the fuel rods do not come into contact with each other, and the coolant can circulate between them, thin wire is wound onto the tubes. Sodium enters the fuel assembly through the lower throttling holes and exits through the windows in the upper part.

At the bottom of the fuel assembly there is a shank that is inserted into the commutator socket, at the top there is a head part, by which the assembly is grabbed during overload. Fuel assemblies of different enrichments have different mounting locations, so it is simply impossible to install the assembly in the wrong place.

To control the reactor, 19 compensating rods containing boron (a neutron absorber) to compensate for fuel burnout, 2 automatic control rods (to maintain a given power), and 6 active protection rods are used. Since uranium’s own neutron background is low, for controlled startup of the reactor (and control at low power levels) an “illumination” is used - a photoneutron source (gamma emitter plus beryllium).

Power units with fast neutron reactors can significantly expand fuel base nuclear energy and minimize radioactive waste through the organization of a closed nuclear fuel cycle. Only a few countries have such technologies, and the Russian Federation, according to experts, is the world leader in this field.

The BN-800 reactor (from “fast sodium”, with an electrical power of 880 megawatts) is a pilot industrial fast neutron reactor with a liquid metal coolant, sodium. It should become a prototype of commercial, more powerful power units with BN-1200 reactors.

sources

40 km from Yekaterinburg, in the middle of the most beautiful Ural forests, is the town of Zarechny. In 1964, the first Soviet industrial nuclear power plant, Beloyarskaya, was launched here (with an AMB-100 reactor with a capacity of 100 MW). Now the Beloyarsk NPP remains the only one in the world where an industrial fast neutron power reactor, the BN-600, operates.

Imagine a boiler that evaporates water, and the resulting steam spins a turbogenerator that generates electricity. Something like this in general outline and arranged nuclear power plant. Only the “boiler” is energy atomic decay. The designs of power reactors can be different, but according to the operating principle they can be divided into two groups - thermal neutron reactors and fast neutron reactors.

The basis of any reactor is the fission of heavy nuclei under the influence of neutrons. True, there are significant differences. In thermal reactors, uranium-235 is fissioned under the influence of low-energy thermal neutrons, which produces fission fragments and new neutrons that have high energy(so-called fast neutrons). The probability of a thermal neutron being absorbed by a uranium-235 nucleus (with subsequent fission) is much higher than a fast one, so the neutrons need to be slowed down. This is done with the help of moderators—substances that, when colliding with nuclei, neutrons lose energy. The fuel for thermal reactors is usually low-enriched uranium, graphite, light or heavy water are used as a moderator, and the coolant is plain water. Most operating nuclear power plants are constructed according to one of these schemes.

Fast neutrons produced as a result of forced nuclear fission can be used without any moderation. The scheme is as follows: fast neutrons produced during the fission of uranium-235 or plutonium-239 nuclei are absorbed by uranium-238 to form (after two beta decays) plutonium-239. Moreover, for every 100 fissioned uranium-235 or plutonium-239 nuclei, 120−140 plutonium-239 nuclei are formed. True, since the probability of nuclear fission by fast neutrons is less than by thermal ones, the fuel must be enriched to a greater extent than for thermal reactors. In addition, it is impossible to remove heat using water here (water is a moderator), so you have to use other coolants: usually these are liquid metals and alloys, from very exotic options such as mercury (such a coolant was used in the first American experimental reactor Clementine) or lead -bismuth alloys (used in some reactors for submarines- in particular, Soviet boats project 705) to liquid sodium (the most common option in industrial power reactors). Reactors operating according to this scheme are called fast neutron reactors. The idea of ​​such a reactor was proposed in 1942 by Enrico Fermi. Of course, the military showed the most ardent interest in this scheme: fast reactors during operation produce not only energy, but also plutonium for nuclear weapons. For this reason, fast neutron reactors are also called breeders (from the English breeder - producer).

What's inside him

The active zone of a fast neutron reactor is structured like an onion, in layers. 370 fuel assemblies form three zones with different enrichment of uranium-235 - 17, 21 and 26% (initially there were only two zones, but in order to equalize the energy release, three were made). They are surrounded by side screens (blankets), or breeding zones, where assemblies containing depleted or natural uranium, consisting mainly of the 238 isotope, are located. At the ends of the fuel rods above and below the core there are also tablets of depleted uranium, which form the end screens (zones reproduction). The BN-600 reactor is a multiplier (breeder), that is, for 100 uranium-235 nuclei split in the core, 120-140 plutonium nuclei are produced in the side and end screens, which makes it possible to expand the reproduction of nuclear fuel. Fuel assemblies (FA) are a set of fuel elements (fuel rods) assembled in one housing - special steel tubes filled with uranium oxide pellets with various enrichments. So that the fuel rods do not come into contact with each other and the coolant can circulate between them, thin wire is wound onto the tubes. Sodium enters the fuel assembly through the lower throttling holes and exits through the windows in the upper part. At the bottom of the fuel assembly there is a shank that is inserted into the commutator socket, at the top there is a head part, by which the assembly is grabbed during overload. Fuel assemblies of different enrichments have different mounting locations, so it is simply impossible to install the assembly in the wrong place. To control the reactor, 19 compensating rods containing boron (a neutron absorber) to compensate for fuel burnout, 2 automatic control rods (to maintain a given power), and 6 active protection rods are used. Since uranium’s own neutron background is low, for controlled startup of the reactor (and control at low power levels) an “illumination” is used - a photoneutron source (gamma emitter plus beryllium).

Zigzags of history

It is interesting that the history of world nuclear energy began precisely with the fast neutron reactor. On December 20, 1951, the world's first fast neutron power reactor, EBR-I (Experimental Breeder Reactor), with an electrical power of only 0.2 MW, came into operation in Idaho. Later, in 1963, a nuclear power plant with a Fermi fast neutron reactor was launched near Detroit - already with a capacity of about 100 MW (in 1966 there was a serious accident with the melting of part of the core, but without any consequences for environment or people).

In the USSR, since the late 1940s, Alexander Leypunsky has been working on this topic, under whose leadership the foundations of the theory of fast reactors were developed at the Obninsk Institute of Physics and Energy (FEI) and several experimental stands were built, which made it possible to study the physics of the process. As a result of the research, in 1972 the first Soviet fast neutron nuclear power plant came into operation in the city of Shevchenko (now Aktau, Kazakhstan) with a BN-350 reactor (originally designated BN-250). It not only generated electricity, but also used heat to desalinate water. Soon the French nuclear power plant with the fast reactor Phenix (1973) and the British one with the PFR (1974), both with a capacity of 250 MW, were launched.

However, in the 1970s, thermal neutron reactors began to dominate the nuclear power industry. This was due for various reasons. For example, the fact that fast reactors can produce plutonium, which means this can lead to a violation of the law on the non-proliferation of nuclear weapons. However, most likely the main factor was that thermal reactors were simpler and cheaper, their design was developed on military reactors for submarines, and uranium itself was very cheap. The industrial fast neutron power reactors that came into operation around the world after 1980 can be counted on the fingers of one hand: these are Superphenix (France, 1985−1997), Monju (Japan, 1994−1995) and BN-600 (Beloyarsk NPP, 1980) , which in present moment is the only operating industrial power reactor in the world.

They're coming back

However, at present, the attention of specialists and the public is again focused on nuclear power plants with fast neutron reactors. According to estimates made by the International Agency for atomic energy(IAEA) in 2005, the total volume of proven uranium reserves, the extraction costs of which do not exceed $130 per kilogram, is approximately 4.7 million tons. According to IAEA estimates, these reserves will last for 85 years (based on the demand for uranium for electricity production at 2004 levels). The content of the 235 isotope, which is “burned” in thermal reactors, in natural uranium is only 0.72%, the rest is uranium-238, “useless” for thermal reactors. However, if we switch to using fast neutron reactors capable of “burning” uranium-238, these same reserves will last for more than 2500 years!

Reactor assembly shop, where individual parts of the reactor are assembled from individual parts using the SKD method

Moreover, fast neutron reactors make it possible to implement a closed fuel cycle (it is not currently implemented in the BN-600). Since only uranium-238 is “burned,” after processing (removing fission products and adding new portions of uranium-238), the fuel can be reloaded into the reactor. And since the uranium-plutonium cycle produces more plutonium than decays, the excess fuel can be used for new reactors.

Moreover, this method can be used to process surplus weapons-grade plutonium, as well as plutonium and minor actinides (neptunium, americium, curium) extracted from spent fuel from conventional thermal reactors (minor actinides currently represent a very dangerous part of radioactive waste). At the same time, the amount of radioactive waste compared to thermal reactors is reduced by more than twenty times.

Reboot blindly

Unlike thermal reactors, in the BN-600 reactor the assemblies are located under a layer of liquid sodium, so the removal of spent assemblies and the installation of fresh ones in their place (this process is called reloading) occurs in a completely closed mode. In the upper part of the reactor there are large and small rotary plugs (eccentric relative to each other, that is, their axes of rotation do not coincide). A column with control and protection systems, as well as an overload mechanism with a collet-type gripper, is mounted on a small rotary plug. The rotary mechanism is equipped with a “hydraulic seal” made of a special low-melting alloy. IN in good condition it is solid, and to reboot it is heated to the melting point, while the reactor remains completely sealed, so that releases of radioactive gases are practically eliminated. The reloading process shuts down many steps. First, the gripper is brought to one of the assemblies located in the in-reactor storage of spent assemblies, removes it and transfers it to the unloading elevator. Then it is lifted into the transfer box and placed in the spent assemblies drum, from where, after being cleaned with steam (from sodium), it enters the spent fuel pool. At the next stage, the mechanism removes one of the core assemblies and moves it to the in-reactor storage facility. After this, the required one is removed from the fresh assembly drum (in which the fuel assemblies that came from the factory are pre-installed) and installed in the fresh assembly elevator, which delivers it to the reloading mechanism. Last stage— installation of fuel assemblies into a vacant cell. At the same time, for safety reasons, restrictions are imposed on the operation of the mechanism. certain restrictions: for example, two adjacent cells cannot be released simultaneously; in addition, during overload, all control and protection rods must be in the active zone. The process of reloading one assembly takes up to an hour, reloading a third of the core (about 120 fuel assemblies) takes about a week (in three shifts), this procedure is carried out every micro-campaign (160 effective days, in terms of full power). True, now the fuel burnup has increased, and only a quarter of the core is overloaded (approximately 90 fuel assemblies). In this case, the operator does not have direct visual feedback, and is guided only by the indicators of the column rotation angle sensors and grippers (positioning accuracy - less than 0.01 degrees), extraction and installation forces.

The reboot process includes many stages, is performed using a special mechanism and resembles a game of “15”. Ultimate Goal— fresh assemblies from the corresponding drum enter the desired slot, and spent ones into their own drum, from where, after being cleaned with steam (from sodium), they will enter the cooling pool.

Smooth only on paper

Why, despite all their advantages, have fast neutron reactors not become widespread? This is primarily due to the peculiarities of their design. As mentioned above, water cannot be used as a coolant, since it is a neutron moderator. Therefore, fast reactors mainly use metals in a liquid state - from exotic lead-bismuth alloys to liquid sodium (the most common option for nuclear power plants).

“In fast neutron reactors, thermal and radiation loads are much higher than in thermal reactors,” explains Mikhail Bakanov, chief engineer of the Beloyarsk NPP, to PM. “This leads to the need to use special structural materials for the reactor vessel and in-reactor systems. The housings of fuel rods and fuel assemblies are made not of zirconium alloys, as in thermal reactors, but of special alloyed chromium steels, which are less susceptible to radiation 'swelling'. On the other hand, for example, the reactor vessel is not subject to loads associated with internal pressure - it is only slightly above atmospheric."

According to Mikhail Bakanov, in the first years of operation the main difficulties were associated with radiation swelling and cracking of the fuel. These problems, however, were soon solved, new materials were developed - both for fuel and for fuel rod housings. But even now, campaigns are limited not so much by fuel burnup (which on the BN-600 reaches 11%), but by the resource life of the materials from which the fuel, fuel rods and fuel assemblies are made. Further operational problems were associated mainly with leaks of sodium in the secondary circuit, a chemically active and fire-hazardous metal that reacts violently to contact with air and water: “Only Russia and France have long-term experience in operating industrial fast neutron power reactors. Both we and the French specialists faced the same problems from the very beginning. We successfully solved them, from the very beginning providing special means for monitoring the tightness of the circuits, localizing and suppressing sodium leaks. But the French project turned out to be less prepared for such troubles; as a result, the Phenix reactor was finally shut down in 2009.”

“The problems were really the same,” adds Nikolai Oshkanov, director of the Beloyarsk NPP, “but they were solved here and in France in different ways. For example, when the head of one of the assemblies at Phenix bent in order to grab and unload it, French specialists developed a complex and rather expensive system for “seeing” through a layer of sodium. And when we had the same problem, one of our engineers suggested using a video camera, placed in a simple structure like a diving bell - a pipe open at the bottom with argon blown in from above. When the sodium melt was displaced, the operators, using video communication, were able to capture the mechanism, and the bent assembly was successfully removed.”

Fast future

“There would not be such interest in fast reactor technology in the world if it were not for the successful long-term operation of our BN-600,” says Nikolai Oshkanov. “The development of nuclear energy, in my opinion, is primarily associated with the serial production and operation of fast reactors . Only they make it possible to involve all natural uranium in the fuel cycle and thus increase efficiency, as well as reduce the amount of radioactive waste by tens of times. In this case, the future of nuclear energy will be truly bright.”

Slide 11. In the core of a fast neutron reactor, fuel rods with highly enriched 235U fuel are placed. The active zone is surrounded by a breeding zone consisting

from fuel elements containing fuel raw materials (depleted 228U or 232Th). Neutrons escaping from the core are captured in the breeding zone by nuclei of fuel raw materials, resulting in the formation of new nuclear fuel. The advantage of fast reactors is the possibility of organizing expanded reproduction of nuclear fuel in them, i.e. simultaneously with energy generation, produce new nuclear fuel instead of burnt-out nuclear fuel. Fast reactors do not require a moderator, and the coolant does not need to slow down the neutrons.

The main purpose of a fast neutron reactor is the production of weapons-grade plutonium (and some other fissile actinides), components atomic weapons. But such reactors are also used in the energy sector, in particular, to ensure the expanded reproduction of fissile plutonium 239Pu from 238U in order to burn all or a significant part of natural uranium, as well as existing reserves of depleted uranium. With the development of fast neutron reactor energy, the problem of self-sufficiency can be solved nuclear power fuel.

Slide 12. Breeder reactor, a nuclear reactor in which the “burning” of nuclear fuel is accompanied by the expanded reproduction of secondary fuel. In a breeder reactor, neutrons released during the fission process of nuclear fuel (for example, 235U) interact with the nuclei of the raw material placed in the reactor (for example, 238U), resulting in the formation of secondary nuclear fuel (239Pu). In a breeder-type reactor, the fuel being reproduced and burned are isotopes of the same chemical element (for example, 235U is burned, 233U is reproduced); in a reactor-converter type reactor, isotopes of different chemical elements(for example, 235U is burned, 239Pu is reproduced).

In fast reactors, the nuclear fuel is an enriched mixture containing at least 15% of the 235U isotope. Such a reactor provides expanded reproduction of nuclear fuel (in it, along with the disappearance of atoms capable of fission, some of them are regenerated (for example, the formation of 239Pu)). The main number of fissions is caused by fast neutrons, and each fission act is accompanied by the appearance of a large number of neutrons (compared to fission by thermal neutrons), which, when captured by 238U nuclei, transforms them (through two successive β-decays) into 239Pu nuclei, i.e. new nuclear fuel. This means that, for example, for 100 fissioned fuel nuclei (235U) in fast neutron reactors, 150 239Pu nuclei capable of fission are formed. (The reproduction factor of such reactors reaches 1.5, i.e., for 1 kg of 235U up to 1.5 kg of Pu is obtained). 239Pu can be used in a reactor as a fissile element.

From the point of view of world energy development, the advantage of a fast neutron reactor (BN) is that it allows the use of isotopes as fuel heavy elements, incapable of fission in thermal neutron reactors. The fuel cycle may involve reserves of 238U and 232Th, which in nature are much greater than 235U, the main fuel for thermal neutron reactors. The so-called “waste uranium” remaining after the enrichment of nuclear fuel with 235U can also be used. Note that plutonium is also produced in conventional reactors, but in much smaller quantities.

Slide 13. BN - fast neutron nuclear reactor. Vessel breeder reactor. The coolant of the primary and secondary circuits is usually sodium. The third circuit coolant is water and steam. Fast reactors do not have a moderator.

The advantages of fast reactors include greater degree fuel burnout (i.e. longer campaign period), and the disadvantages are high cost due to the inability to use the simplest coolant - water, structural complexity, high capital costs and high cost highly enriched fuel.

Highly enriched uranium is uranium with a mass content of the uranium-235 isotope equal to or more than 20%. To ensure a high concentration of nuclear fuel, it is necessary to achieve maximum heat release per unit volume of the core. The heat release of a fast neutron reactor is ten to fifteen times higher than the heat release of reactors with slow neutrons. Heat removal in such a reactor can only be accomplished using liquid metal coolants, such as sodium, potassium, or energy-intensive gas coolants that have the best thermal and thermophysical characteristics, such as helium and dissociating gases. Typically liquid metals are used, such as molten sodium (sodium melting point 98 °C). The disadvantages of sodium include its high chemical reactivity towards water, air and fire hazard. The temperature of the coolant at the inlet to the reactor is 370 °C, and at the outlet - 550, which is ten times higher than similar indicators, say, for VVER - there the water temperature at the inlet is 270 degrees, and at the outlet - 293.