Neutrons?
Neutrons are particles that are part of most atomic nuclei, along with protons. During the nuclear fission reaction, the uranium nucleus splits into two parts and in addition emits several neutrons. They can get into other atoms and trigger one or more fission reactions. If each neutron released during the decay of uranium nuclei hits neighboring atoms, an avalanche-like chain of reactions will begin with the release of more and more energy. If there are no deterrents, a nuclear explosion will occur.
But in nuclear reactor Some of the neutrons either come out or are absorbed by special absorbers. Therefore, the number of fission reactions remains the same all the time, exactly what is necessary to obtain energy. The energy from the radioactive decay reaction produces heat, which is then used to generate steam to drive a power plant's turbine.
The neutrons that keep the nuclear reaction constant can have different energies. Depending on the energy, they are called either thermal or fast (there are also cold ones, but those are not suitable for nuclear power plants). Most reactors in the world are based on the use of thermal neutrons, but the Beloyarsk NPP has a fast reactor. Why?
What are the advantages?
In a fast neutron reactor, part of the neutron energy goes, as in conventional reactors, to maintain the fission reaction of the main component of nuclear fuel, uranium-235. And part of the energy is absorbed by a shell made of uranium-238 or thorium-232. These elements are useless for conventional reactors. When neutrons hit their nuclei, they turn into isotopes suitable for use in nuclear power as fuel: plutonium-239 or uranium-233.
Enriched uranium. Unlike spent nuclear fuel, uranium is not nearly so radioactive that it needs to be handled only by robots. You can even hold it briefly with your hands wearing thick gloves. Photo: US Department of Energy
Thus, fast neutron reactors can be used not only to supply energy to cities and factories, but also to produce new nuclear fuel from relatively inexpensive raw materials. The following facts speak in favor of economic benefits: a kilogram of uranium smelted from ore costs about fifty dollars, contains only two grams of uranium-235, and the rest is uranium-238.
However, fast neutron reactors are practically not used in the world. BN-600 can be considered unique. Neither the Japanese Monju, nor the French Phoenix, nor a number of experimental reactors in the USA and Great Britain are currently operating: thermal neutron reactors turned out to be easier to construct and operate. There are a number of obstacles on the way to reactors that can combine energy production with nuclear fuel production. And judging by its successful operation for 35 years, the designers of the BN-600 were able to bypass at least some of the obstacles.
What is the problem?
In sodium. Any nuclear reactor must have several components and elements: fuel assemblies with nuclear fuel, elements for controlling the nuclear reaction, and a coolant that absorbs the heat generated in the device. The design of these components, the composition of the fuel and coolant may differ, but without them the reactor is impossible by definition.
In a fast neutron reactor, it is necessary to use a material as a coolant that does not retain neutrons, otherwise they will turn from fast to slow, thermal ones. At dawn nuclear energy the designers tried to use mercury, but it dissolved the pipes inside the reactor and began to leak out. The heated toxic metal, which also became radioactive under the influence of radiation, caused so much trouble that the mercury reactor project was quickly abandoned.
Pieces of sodium are usually stored under a layer of kerosene. Although this liquid is flammable, it does not react with sodium and does not release water vapor from the air to it. Photo: Superplus / Wikipedia
BN-600 uses liquid sodium. At first glance, sodium is little better than mercury: it is extremely chemically active, reacts violently with water (in other words, it explodes if thrown into water) and reacts even with substances contained in concrete. However, it does not interfere with neutrons, and with the proper level of construction work and subsequent maintenance, the risk of leakage is not that great. In addition, sodium, unlike water vapor, can be pumped at normal pressure. A jet of steam from a ruptured steam line under pressure of hundreds of atmospheres cuts metal, so in this sense sodium is safer. As for chemical activity, it can also be used for good. In the event of an accident, sodium reacts not only with concrete, but also with radioactive iodine. Sodium iodide no longer leaves the nuclear power plant building, while gaseous iodine accounted for almost half of the emissions during the accident at the nuclear power plant in Fukushima.
Soviet engineers who developed fast neutron reactors first built the experimental BR-2 (the same unsuccessful one, mercury), and then the experimental BR-5 and BOR-60 with sodium instead of mercury. The data obtained from them made it possible to design the first industrial “fast” reactor BN-350, which was used at a unique nuclear chemical and energy plant - a nuclear power plant combined with a seawater desalination plant. At the Beloyarsk NPP, the second reactor of the BN type - “fast, sodium” - was built.
Despite the experience accumulated by the time the BN-600 was launched, the first years were marred by a series of liquid sodium leaks. None of these incidents posed a radiation threat to the population or led to serious exposure of plant personnel, and since the early 1990s, sodium leaks have stopped altogether. To put this into global context, Japan's Monju suffered a serious leak of liquid sodium in 1995, which led to a fire and shutdown of the plant for 15 years. Only Soviet designers succeeded in translating the idea of a fast neutron reactor into an industrial rather than experimental device, whose experience allowed Russian nuclear scientists to develop and build the next generation reactor - BN-800.
BN-800 has already been built. On June 27, 2014, the reactor started operating at minimum power, and a power start-up is expected in 2015. Since starting a nuclear reactor is a very complex process, experts separate the physical start-up (the beginning of a self-sustaining chain reaction) and the energy start-up, during which the power unit begins to supply the first megawatts of electricity to the network.
Beloyarsk NPP, control panel. Photo from the official website: http://www.belnpp.rosenergoatom.ru
In the BN-800, the designers implemented a number of important improvements, including, for example, an emergency air cooling system for the reactor. Developers say its advantage is independence from energy sources. If, as in Fukushima, electricity disappears at a nuclear power plant, then the flow of the cooling reactor will still not disappear - circulation will be maintained naturally, due to convection, rising heated air. And if the core melts suddenly, the radioactive melt will not go outside, but into a special trap. Finally, protection against overheating is a large supply of sodium, which in the event of an accident can absorb the generated heat even if all cooling systems completely fail.
Following the BN-800, it is planned to build a BN-1200 reactor with even greater power. The developers expect that their brainchild will become a serial reactor and will be used not only at the Beloyarsk NPP, but also at other stations. However, these are just plans for now; for a large-scale transition to fast neutron reactors, a number of problems still need to be solved.
Beloyarsk NPP, construction site of a new power unit. Photo from the official website: http://www.belnpp.rosenergoatom.ru
What is the problem?
In economics and ecology of fuel. Fast neutron reactors operate on a mixture of enriched uranium oxide and plutonium oxide - this is the so-called mox fuel. Theoretically, it can be cheaper than conventional fuel due to the fact that it uses plutonium or uranium-233 from inexpensive uranium-238 or thorium irradiated in other reactors, but so far mox fuel is inferior in price to conventional fuel. It turns out to be a kind of vicious circle that is not so easy to break: it is necessary to fine-tune the technology for constructing reactors, the extraction of plutonium and uranium from the material irradiated in the reactor, and ensure control over the non-proliferation of high-level materials. Some ecologists, for example representatives of the non-profit center Bellona, point to the large volume of waste produced during the processing of irradiated material, because along with valuable isotopes in a fast neutron reactor, a significant amount of radionuclides is formed that need to be buried somewhere.
In other words, even the successful operation of a fast neutron reactor in itself does not guarantee a revolution in nuclear energy. It is a necessary, but not sufficient condition for moving from limited reserves of uranium-235 to much more accessible uranium-238 and thorium-232. Whether the technologists involved in the processes of nuclear fuel reprocessing and nuclear waste disposal will be able to cope with their tasks is a topic for a separate story.
Nuclear energy has always received increased attention due to its promise. In the world, about twenty percent of electricity is obtained using nuclear reactors, and in developed countries this figure for the product of nuclear energy is even higher - more than a third of all electricity. However, the main type of reactors remains thermal ones, such as LWR and VVER. Scientists believe that one of the main problems of these reactors in the near future will be a shortage of natural fuel, uranium, and its isotope 238, necessary for carrying out a fission chain reaction. Based on the possible depletion of resources of this natural fuel material for thermal reactors, restrictions are placed on the development of nuclear energy. The use of nuclear reactors using fast neutrons, in which fuel reproduction is possible, is considered more promising.
Development history
Based on the program of the Ministry of Atomic Industry of the Russian Federation at the beginning of the century, tasks were set to create and ensure the safe operation of nuclear energy complexes, modernized nuclear power plants of a new type. One of these facilities was the Beloyarsk nuclear power plant, located 50 kilometers near Sverdlovsk (Ekaterinburg). The decision to create it was made in 1957, and in 1964 the first unit was put into operation.
Two of its blocks operated thermal nuclear reactors, which by the 80-90s of the last century had exhausted their resources. At the third block, the BN-600 fast neutron reactor was tested for the first time in the world. During his work, the results planned by the developers were obtained. The safety of the process was also excellent. During the project period, which ended in 2010, no serious violations or deviations occurred. Its final term expires by 2025. It can already be said that fast neutron nuclear reactors, which include the BN-600 and its successor, the BN-800, have a great future.
Launch of the new BN-800
OKBM scientists Afrikantov from Gorky (present-day Nizhny Novgorod) prepared a project for the fourth power unit of the Beloyarsk NPP back in 1983. Due to the accident that occurred in Chernobyl in 1987 and the introduction of new safety standards in 1993, work was stopped and the launch was postponed indefinitely. Only in 1997, after receiving a license for the construction of unit No. 4 with a BN-800 reactor with a capacity of 880 MW from Gosatomnadzor, the process resumed.
On December 25, 2013, heating of the reactor began for the further entry of coolant. In June of the fourteenth, as planned according to plan, a mass sufficient to carry out a minimal chain reaction occurred. Then things stalled. MOX fuel, composed of fissile oxides of uranium and plutonium, similar to that used in Unit 3, was not ready. This is what the developers wanted to use in the new reactor. I had to combine and look for new options. As a result, in order not to postpone the launch of the power unit, they decided to use uranium fuel in part of the assembly. The launch of the BN-800 nuclear reactor and unit No. 4 took place on December 10, 2015.
Process description
During operation in a reactor with fast neutrons, secondary elements are formed as a result of the fission reaction, which, when absorbed by the uranium mass, form the newly created nuclear material plutonium-239, capable of continuing the process of further fission. The main advantage of this reaction is the production of neutrons from plutonium, which is used as fuel for nuclear reactors at nuclear power plants. Its presence makes it possible to reduce the production of uranium, the reserves of which are limited. From a kilogram of uranium-235 you can get a little more than a kilogram of plutonium-239, thereby ensuring fuel reproduction.
As a result, energy production in nuclear power units with minimal consumption of scarce uranium and no restrictions on production will increase hundreds of times. It is estimated that in this case, uranium reserves will last humanity for several tens of centuries. The optimal option in nuclear energy to maintain a balance in terms of minimum uranium consumption will be a ratio of 4 to 1, where for every four thermal reactors one operating on fast neutrons will be used.
BN-800 targets
During its operational life in power unit No. 4 of the Beloyarsk NPP, certain tasks were assigned to the nuclear reactor. The BN-800 reactor must operate on MOX fuel. A small hitch that occurred at the beginning of work did not change the plans of the creators. According to the director of the Beloyarsk NPP, Mr. Sidorov, the full transition to MOX fuel will be carried out in 2019. If this comes true, the local fast neutron nuclear reactor will become the first in the world to operate entirely with such fuel. It should become a prototype for future similar fast reactors with liquid metal coolant, more productive and safer. Based on this, the BN-800 is testing innovative equipment under operating conditions, checking the correct application of new technologies that affect the reliability and efficiency of the power unit.
class="eliadunit">
Checking work new system fuel cycle.
Tests for burning radioactive waste with a long lifespan.
Disposal of weapons-grade plutonium accumulated in large quantities.
The BN-800, just like its predecessor, the BN-600, should become a starting point for Russian developers to accumulate invaluable experience in the creation and operation of fast reactors.
Advantages of a fast neutron reactor
The use of BN-800 and similar nuclear reactors in nuclear power allows
Significantly increase the life of uranium resource reserves, which significantly increases the amount of energy received.
The ability to reduce the lifespan of radioactive fission products to a minimum (from several thousand years to three hundred).
Increase the safety of nuclear power plants. The use of a fast neutron reactor allows the possibility of core melting to be leveled to a minimum level, can significantly increase the level of self-protection of the facility, and eliminate plutonium release during processing. Reactors of this type with sodium coolant have increased level security.
On August 17, 2016, power unit No. 4 of the Beloyarsk NPP reached 100% power operation. Since December last year, the integrated Ural system has been receiving energy generated at a fast reactor.
class="eliadunit">After the launch and successful operation of the world's first nuclear power plant in 1955, on the initiative of I. Kurchatov, a decision was made to build an industrial nuclear power plant with a channel-type pressurized water reactor in the Urals. Features of this type of reactor include superheating of steam to high parameters directly in the core, which opened up the possibility of using serial turbine equipment.
In 1958, in the center of Russia, in one of the most picturesque corners of the Ural nature, the construction of the Beloyarsk Nuclear Power Plant began. For installers, this station began back in 1957, and since the topic of nuclear power plants was closed in those days, in correspondence and life it was called the Beloyarsk State District Power Plant. This station was started by employees of the Uralenergomontazh trust. Through their efforts, in 1959, a base with a workshop for the production of water and steam pipelines (1 circuit of the reactor) was created, three residential buildings were built in the village of Zarechny, and construction of the main building began.
In 1959, workers from the Tsentroenergomontazh trust appeared at the construction site and were tasked with installing the reactor. At the end of 1959, the site for the construction of the nuclear power plant was relocated from Dorogobuzh, Smolensk region, and installation work was headed by V. Nevsky, the future director of the Beloyarsk NPP. All work on the installation of thermal mechanical equipment was completely transferred to the Tsentroenergomontazh trust.
The intensive period of construction of the Beloyarsk NPP began in 1960. At this time, the installers, along with construction work, had to master new technologies for the installation of stainless pipelines, linings of special rooms and radioactive waste storage facilities, installation of reactor structures, graphite masonry, automatic welding, etc. We learned on the fly from specialists who had already taken part in the construction of nuclear facilities. Having moved from the technology of installation of thermal power plants to the installation of equipment for nuclear power plants, the workers of Tsentroenergomontazh successfully completed their tasks, and on April 26, 1964, the first power unit of the Beloyarsk NPP with the AMB-100 reactor supplied the first current to the Sverdlovsk energy system. This event, along with the commissioning of the 1st power unit of the Novovoronezh NPP, meant the birth of the country's large nuclear power industry.
The AMB-100 reactor was a further improvement in the reactor design of the World's First Nuclear Power Plant in Obninsk. It was a channel-type reactor with higher thermal characteristics of the core. Obtaining steam of high parameters due to nuclear overheating directly in the reactor was a big step forward in the development of nuclear energy. the reactor operated in one unit with a 100 MW turbogenerator.
Structurally, the reactor of the first power unit of the Beloyarsk NPP turned out to be interesting in that it was created virtually without a frame, i.e., the reactor did not have a heavy, multi-ton, durable body, like, say, a water-cooled water-cooled VVER reactor of similar power with a body 11-12 m long , with a diameter of 3-3.5 m, wall and bottom thickness of 100-150 mm or more. The possibility of constructing nuclear power plants with open-channel reactors turned out to be very tempting, since it freed heavy engineering plants from the need to manufacture steel products weighing 200-500 tons. But the implementation of nuclear overheating directly in the reactor turned out to be associated with well-known difficulties in regulating the process, especially in terms of monitoring its progress , with the requirement for precision operation of many instruments, the presence of a large number of pipes of various sizes under high pressure, etc.
The first unit of the Beloyarsk NPP reached its full design capacity, however, due to the relatively small installed capacity of the unit (100 MW), the complexity of its technological channels and, therefore, high cost, the cost of 1 kWh of electricity turned out to be significantly higher than that of thermal stations in the Urals.
The second unit of the Beloyarsk NPP with the AMB-200 reactor was built faster, without great stress in the work, since the construction and installation team was already prepared. The reactor installation has been significantly improved. It had a single-circuit cooling circuit, which simplified the technological design of the entire nuclear power plant. Just like in the first power unit, main feature AMB-200 reactor produces high-parameter steam directly into the turbine. On December 31, 1967, power unit No. 2 was connected to the network - this completed the construction of the 1st stage of the station.
A significant part of the history of operation of the 1st stage of the BNPP was filled with romance and drama, characteristic of everything new. This was especially true during the period of block development. It was believed that there should be no problems with this - there were prototypes from the AM “First in the World” reactor to industrial reactors for plutonium production, on which basic concepts, technologies, design solutions, many types of equipment and systems, and even a significant part of the technological regimes were tested . However, it turned out that the difference between the industrial nuclear power plant and its predecessors is so great and unique that new, previously unknown problems arose.
The largest and most obvious of them was the unsatisfactory reliability of the evaporation and superheating channels. After a short period of their operation, gas depressurization of fuel elements or coolant leaks appeared with unacceptable consequences for the graphite masonry of reactors, technological operating and repair modes, radiation exposure on personnel and the environment. According to the scientific canons and calculation standards of that time, this should not have happened. In-depth studies of this new phenomenon forced us to reconsider the established ideas about the fundamental laws of boiling water in pipes, since even with a low heat flux density, a previously unknown type of heat transfer crisis arose, which was discovered in 1979 by V.E. Doroshchuk (VTI) and subsequently called the “heat transfer crisis of the second kind.”
In 1968, a decision was made to build a third power unit with a fast neutron reactor at the Beloyarsk NPP - BN-600. Scientific guidance The creation of BN-600 was carried out by the Institute of Physics and Power Engineering, the design of the reactor plant was carried out by the Experimental Mechanical Engineering Design Bureau, and the general design of the unit was carried out by the Leningrad branch of Atomelectroproekt. The block was built by a general contractor - the Uralenergostroy trust.
When designing it, the operating experience of the BN-350 reactors in Shevchenko and the BOR-60 reactor was taken into account. For the BN-600, a more economical and structurally successful integral layout of the primary circuit was adopted, according to which the reactor core, pumps and intermediate heat exchangers are located in one housing. The reactor vessel, having a diameter of 12.8 m and a height of 12.5 m, was installed on roller supports fixed to the base plate of the reactor shaft. The mass of the assembled reactor was 3900 tons, and the total amount of sodium in the installation exceeded 1900 tons. Biological protection was made of steel cylindrical screens, steel blanks and pipes with graphite filler.
The quality requirements for installation and welding work for the BN-600 turned out to be an order of magnitude higher than those achieved previously, and the installation team had to urgently retrain personnel and master new technologies. So in 1972, when assembling a reactor vessel from austenitic steels, a betatron was used for the first time to control the transmission of large welds.
In addition, during the installation of internal devices of the BN-600 reactor, special requirements for cleanliness were imposed, and all parts brought in and removed from the intra-reactor space were recorded. This was due to the impossibility of further flushing the reactor and pipelines with sodium coolant.
Nikolai Muravyov played a major role in the development of reactor installation technology, who was invited to work from Nizhny Novgorod, where he previously worked in a design bureau. He was one of the developers of the BN-600 reactor project, and by that time he was already retired.
The installation team successfully completed the assigned tasks of installing the fast neutron unit. Filling the reactor with sodium showed that the cleanliness of the circuit was maintained even higher than required, since the pour point of sodium, which depends in the liquid metal on the presence of foreign contaminants and oxides, turned out to be lower than those achieved during the installation of the BN-350, BOR-60 reactors in the USSR and nuclear power plants " Phoenix" in France.
The success of the installation teams at the construction of the Beloyarsk NPP largely depended on the managers. First it was Pavel Ryabukha, then the young energetic Vladimir Nevsky came, then he was replaced by Vazgen Kazarov. V. Nevsky did a lot for the formation of a team of installers. In 1963, he was appointed director of the Beloyarsk Nuclear Power Plant, and later he headed Glavatomenergo, where he worked hard to develop the country’s nuclear power industry.
Finally, on April 8, 1980, the power start-up of power unit No. 3 of the Beloyarsk NPP with the BN-600 fast neutron reactor took place. Some design characteristics of the BN-600:
- electrical power – 600 MW;
- thermal power – 1470 MW;
- steam temperature – 505 o C;
- steam pressure – 13.7 MPa;
- gross thermodynamic efficiency – 40.59%.
Special attention should be paid to the experience of handling sodium as a coolant. It has good thermophysical and satisfactory nuclear physical properties, and is well compatible with stainless steels, uranium and plutonium dioxide. Finally, it is not scarce and relatively inexpensive. However, it is very chemically active, which is why its use required solving at least two serious problems: minimizing the likelihood of sodium leakage from circulation circuits and inter-circuit leaks in steam generators and ensuring effective localization and termination of sodium combustion in the event of a leak.
The first task was generally quite successfully solved at the stage of developing equipment and pipeline projects. The integral layout of the reactor turned out to be very successful, in which all the main equipment and pipelines of the 1st circuit with radioactive sodium were “hidden” inside the reactor vessel, and therefore its leakage, in principle, was possible only from a few auxiliary systems.
And although BN-600 is today the largest power unit with a fast neutron reactor in the world, Beloyarsk NPP is not one of the nuclear power plants with a large installed capacity. Its differences and advantages are determined by the novelty and uniqueness of production, its goals, technology and equipment. All reactor installations of the BelNPP were intended for pilot industrial confirmation or denial of technical ideas and solutions laid down by designers and constructors, research of technological regimes, structural materials, fuel elements, control and protective systems.
All three power units have no direct analogues either in our country or abroad. They embodied many of the ideas for the future development of nuclear energy:
- power units with industrial-scale channel water-graphite reactors were built and commissioned;
- serial turbo units with high parameters with thermal power cycle efficiency from 36 to 42% were used, which no nuclear power plant in the world has;
- fuel assemblies were used, the design of which excludes the possibility of fragmentation activity entering the coolant even when the fuel rods are destroyed;
- carbon steel is used in the primary circuit of the reactor of the 2nd unit;
- the technology for using and handling liquid metal coolant has been largely mastered;
The Beloyarsk NPP was the first nuclear power plant in Russia to face in practice the need to solve the problem of decommissioning spent reactor plants. The development of this area of activity, which is very relevant for the entire nuclear energy industry, had a long incubation period due to the lack of an organizational and regulatory document base and the unresolved issue of financial support.
The more than 50-year period of operation of the Beloyarsk NPP has three fairly distinct stages, each of which had its own areas of activity, specific difficulties in its implementation, successes and disappointments.
The first stage (from 1964 to the mid-70s) was entirely associated with the launch, development and achievement of the design level of power of the 1st stage power units, a lot of reconstruction work and solving problems associated with imperfect designs of units, technological regimes and ensuring sustainable operation of fuel channels. All this required enormous physical and intellectual efforts from the station staff, which, unfortunately, were not crowned with confidence in the correctness and prospects of choosing uranium-graphite reactors with nuclear superheating of steam for further development nuclear energy. However, the most significant part of the accumulated operating experience of the 1st stage was taken into account by designers and constructors when creating uranium-graphite reactors of the next generation.
The beginning of the 70s was associated with the choice of a new direction for the further development of the country's nuclear energy - fast neutron reactor plants with the subsequent prospect of building several power units with breeder reactors using mixed uranium-plutonium fuel. When determining the location for the construction of the first pilot industrial unit using fast neutrons, the choice fell on the Beloyarsk NPP. This choice was significantly influenced by the recognition of the ability of the construction teams, installers and plant personnel to properly build this unique power unit and subsequently ensure its reliable operation.
This decision marked the second stage in the development of the Beloyarsk NPP, which for the most part was completed with the decision of the State Commission to accept the completed construction of the power unit with the BN-600 reactor with an “excellent” rating, rarely used in practice.
Ensuring the quality of work at this stage was entrusted to the best specialists both from construction and installation contractors and from the station operating personnel. The plant personnel acquired extensive experience in setting up and mastering nuclear power plant equipment, which was actively and fruitfully used during commissioning work at the Chernobyl and Kursk nuclear power plants. Special mention should be made of the Bilibino NPP, where, in addition to commissioning work, an in-depth analysis of the project was carried out, on the basis of which a number of significant improvements were made.
With the commissioning of the third block, the third stage of the station’s existence began, which has been going on for more than 35 years. The goals of this stage were to achieve the design parameters of the unit, confirm in practice the viability of design solutions and gain operating experience for subsequent consideration in the design of a serial unit with a breeder reactor. All these goals have now been successfully achieved.
The safety concepts laid down in the unit design were generally confirmed. Since the boiling point of sodium is almost 300 o C higher than operating temperature, the BN-600 reactor operates almost without pressure in the reactor vessel, which can be made of highly plastic steel. This virtually eliminates the possibility of rapidly developing cracks. And the three-circuit scheme of heat transfer from the reactor core with an increase in pressure in each subsequent circuit completely eliminates the possibility of radioactive sodium from the 1st circuit getting into the second (non-radioactive) circuit, and even more so into the steam-water third circuit.
Confirmation of the achieved high level of safety and reliability of the BN-600 is the safety analysis performed after the accident at the Chernobyl nuclear power plant, which did not reveal the need for any urgent technical improvements. Statistics on the activation of emergency protections, emergency shutdowns, unplanned reductions in operating power and other failures show that the BN-6OO reactor is at least among the 25% of the best nuclear units in the world.
According to the results of the annual competition, Beloyarsk NPP in 1994, 1995, 1997 and 2001. was awarded the title “Best NPP in Russia”.
Power unit No. 4 with the fast neutron reactor BN-800 is in the pre-startup stage. The new 4th power unit with the BN-800 reactor with a capacity of 880 MW was brought to the minimum controlled power level on June 27, 2014. The power unit is designed to significantly expand the fuel base of nuclear energy and minimize radioactive waste through the organization of a closed nuclear fuel cycle.
The possibility of further expansion of the Beloyarsk NPP with power unit No. 5 with a fast reactor with a capacity of 1200 MW is being considered - the main commercial power unit for serial construction.
When we are told, for example, that “a power plant on solar panels with a capacity of 1200 MW has been built,” this does not mean at all that this solar power plant will provide the same amount of electricity as the VVER-1200 nuclear reactor provides. Solar panels cannot work at night - therefore, if averaged over the seasons, they are idle for half of the day, and this already reduces the capacity factor by half. Solar panels, even the newest varieties, work much worse in cloudy weather, and the average values here are also not encouraging - clouds with rain and snow, fogs reduce capacity by half. “SPP with a capacity of 1200 MW” sounds ringing, but we must keep in mind the figure of 25% - this capacity can be technologically used only by ¼.
Solar panels, unlike nuclear power plants, operate not for 60-80 years, but for 3-4 years, losing the possibility of conversion sunlight into electric current. You can, of course, talk about some kind of “cheaper generation”, but this is outright deceit. Solar power plants require large areas of territory; so far no one has really dealt with the problems of disposing of used solar panels anywhere. Recycling will require the development of quite serious technologies, which are unlikely to please the environment. If we talk about power plants using wind, then the words will have to be used almost the same, since in this case the capacity factor is about a quarter of the installed capacity. Sometimes instead of wind there is calm, sometimes the wind is so strong that it forces the “mills” to stop, as it threatens the integrity of their structure.
Weather vagaries of renewable energy sources
There is no escape from the second “Achilles heel” of renewable energy sources. Power plants based on them operate not when the electricity they generate is needed by consumers, but when the weather outside is sunny or the wind is of suitable strength. Yes, such power plants can generate electricity, but what if the power transmission networks are not able to receive it? The wind blew at night, you can turn on wind power plants, but at night you and I sleep, and the enterprises do not work. Yes, such traditional power plants based on renewable resources, such as hydroelectric power stations, are able to cope with this problem by increasing the idle discharge of water (“past the turbine”) or simply accumulating a supply of water in their reservoirs, but in the event of floods, it is not so easy for them. And for solar and wind power plants, energy storage technologies are not so developed as to “store” the generated electricity for the moment when grid consumption increases.
There is also the other side of the coin. Will an investor invest in the construction of, say, a gas power plant in a region where solar panels are installed in large quantities? How can you recoup the money invested if “your” power plant doesn’t work half the time? Payback period, bank interest... “Oh, why do I need this? headache!» - declares the cautious capitalist and builds nothing. And here we have a weather anomaly, it rained for a week with complete calm. And the cries of outraged consumers forced to run diesel generators on their front lawns fade into a rumble. You cannot force investors to build thermal power plants; without benefits and subsidies from the state, they will not take risks. And this, in any case, becomes an additional burden on state budgets, as well as in the case if the state, having not found accommodating investors, builds thermal power plants on its own.
We hear a lot about how many solar panels are used in Germany, right? But at the same time, the number of power plants operating on local brown coal in the country is growing, mercilessly emitting into the atmosphere the same “e-two” that must be combated in order to fulfill the terms of the 2015 Paris Agreement. “Brown power plants” are forced to build the federal government of Germany, the governing bodies of the federal states - they have no other choice, otherwise those same fans of “green energy” will take to the streets to protest due to the fact that there is no current in their sockets, which in the evenings you have to sit by a torch.
We exaggerate, of course, but only to make the absurdity of the situation more obvious. If the generation of electricity literally depends on the weather, then it turns out that it is technically impossible to satisfy basic electricity needs using the sun and wind. Yes, theoretically, it is possible to connect the whole of Europe with Africa with additional power lines (power lines) so that the current from the sunny Sahara comes to houses standing on the gloomy coast of the North Sea, but this costs absolutely incredible money, the payback period of which is close to infinity. Should there be a coal or gas powered one next to each solar power plant? Let us repeat, but the combustion of hydrocarbon energy resources at power plants does not make it possible to fully implement the provisions of the Paris Agreement on reducing CO 2 emissions.
Nuclear power plant as the basis of “green energy”
Dead end? For those countries that have decided to get rid of nuclear energy, this is it. Of course, they are looking for a way out of it. They are improving coal and gas combustion systems, abandoning fuel oil power plants, making efforts to increase the efficiency of furnaces, steam generators, and boilers, and increasing efforts to use energy-saving technologies. This is good, this is useful, this must be done. But Russia and its Rosatom They propose a much more radical option - to build a nuclear power plant.
Construction of a nuclear power plant, Photo: rusatom-overseas.com
Does this method seem paradoxical to you? Let's look at it from a logical point of view. Firstly, there are no CO 2 emissions from nuclear reactors as such - there are no chemical reactions, the flame does not roar wildly in them. Consequently, the fulfillment of the terms of the Paris Agreement “takes place.” The second point is the scale of electricity generation at nuclear power plants. In most cases, nuclear power plant sites have at least two, or even all four, reactors; their total installed capacity is enormous, and the capacity factor consistently exceeds 80%. This “breakthrough” of electricity is sufficient to satisfy the needs of not just one city, but an entire region. But nuclear reactors “don’t like” when their power is changed. Sorry, now there will be a few technical details to make it clearer what we mean.
Nuclear reactor control and protection systems
The principle of operation of a power reactor is not so complicated schematically. The energy of atomic nuclei is converted into thermal energy of the coolant, thermal energy is converted into mechanical energy of the electric generator rotor, which, in turn, is converted into electrical energy.
Atomic – thermal – mechanical – electrical, this is a kind of energy cycle.
Ultimately, the electrical power of the reactor depends on the power of the controlled, controlled atomic chain reaction of nuclear fuel fission. We emphasize – controlled and manageable. Unfortunately, we have known well since 1986 what happens if a chain reaction gets out of control and management.
How is the course of a chain reaction monitored and controlled, what needs to be done to ensure that the reaction does not immediately spread to the entire volume of uranium contained in the “nuclear cauldron”? Let us recall the school truisms without going into the scientific details of nuclear physics - this will be quite enough.
What is a chain reaction “on fingers”, if someone has forgotten: one neutron arrived, knocked out two neutrons, two neutrons knocked out four, and so on. If the number of these very free neutrons becomes too large, the fission reaction will spread throughout the entire volume of uranium, threatening to develop into a “big bang.” Yes, sure, nuclear explosion will not take place, it requires that the content of the uranium-235 isotope in the fuel exceed 60%, and in power reactors the fuel enrichment does not exceed 5%. But even without atomic explosion the problems will be in over your head. The coolant will overheat, its pressure in the pipelines will increase supercritically, after their rupture, the integrity of the fuel assemblies may be compromised and all radioactive substances will escape outside the reactor, insanely polluting the surrounding areas and burst into the atmosphere. However, the details of the Chernobyl nuclear power plant disaster are known to everyone, we will not repeat them.
Accident at the Chernobyl nuclear power plant, Photo: meduza.io
One of the main components of any nuclear reactor is the control and protection system. Free neutrons should not be more than a rigidly calculated value, but they should not be less than this value - this will lead to the attenuation of the chain reaction, the nuclear power plant will simply “stop.” Inside the reactor there must be a substance that absorbs excess neutrons, but in an amount that allows the chain reaction to continue. Nuclear physicists have long figured out which substance does this best - the boron-10 isotope, so the control and protection system is also simply called “boron”.
Rods with boron are included in the design of reactors with graphite and water moderator; for them there are the same technological channels as for fuel rods and fuel elements. Neutron counters in the reactor operate continuously, automatically giving commands to the system that controls the boron rods, which moves the rods, immersing them in or removing them from the reactor. At the beginning of the fuel session, there is a lot of uranium in the reactor - the boron rods are immersed deeper. Time passes, the uranium burns out, and the boron rods begin to be gradually removed - the number of free neutrons must remain constant. Yes, we note that there are also “emergency” boron rods “hanging” above the reactor. In case of violations that could potentially send the chain reaction out of control, they are plunged into the reactor instantly, killing the chain reaction in the bud. A pipeline has burst, a coolant leak has occurred - this is a risk of overheating, emergency boron rods are triggered instantly. Let's stop the reaction and slowly figure out what exactly happened and how to fix the problem, and the risk should be reduced to zero.
There are different neutrons, but we have the same boron
Simple logic, as you see, shows that increasing and decreasing the energy power of a nuclear reactor – “power maneuver”, as the power engineers say – is a very difficult job, which is based on nuclear physics and quantum mechanics. A little more “deep into the process”, not too far, don’t be afraid. In any fission reaction of uranium fuel, secondary free neutrons are formed - the same ones that in the school formula “knocked out two neutrons.” In a power reactor, two secondary neutrons are too many; for controllability and controllability of the reaction, a coefficient of 1.02 is needed. 100 neutrons arrived, 200 neutrons were knocked out, and out of these 200 secondary neutrons, 98 should “eat”, absorb that same boron-10. Boron suppresses excessive activity, we tell you that for sure.
But remember what happens if you feed a child a bucket of ice cream - he will happily eat the first 5-6 servings, and then go away because he “can’t fit in any more.” Humans are made of atoms, and therefore the character of atoms is no different from ours. Boron-10 can eat neutrons, but not an infinite number, the same “can’t fit anymore” will definitely come. The bearded men in white coats at the nuclear power plant suspect that many people realize that at heart nuclear scientists remain curious children, so they try to use as “mature” vocabulary as possible. Boron in their vocabulary is not “eaten by neutrons”, but “burned out” - this sounds much more respectable, you will agree. One way or another, every demand from the power grid to “turn down the reactor” leads to more intense burnout of the boron protection and control system and causes additional difficulties.
Model of a fast neutron reactor, Photo: topwar.ru
With a coefficient of 1.02, everything is also not so simple, since in addition to prompt secondary neutrons that appear immediately after the fission reaction, there are also delayed ones. After fission, a uranium atom falls apart, and neutrons also fly out of these fragments, but after a few microseconds. There are few of them compared to instant ones, only about 1%, but with a coefficient of 1.02 they are very important, because 1.02 is an increase of only 2%. Therefore, the calculation of the amount of boron must be carried out with pinpoint accuracy, constantly balancing on the fine line of “the reaction getting out of control - an unscheduled shutdown of the reactor.” Therefore, in response to every demand, “turn on the gas!” or “Slow down, why are you so fired up!” a chain reaction of the nuclear power plant duty shift begins, when each nuclear worker on its staff offers a larger number of idiomatic expressions...
And once again about nuclear power plants as the basis of “green energy”
Now let's return to where we left off - high power generation capacity, over a large territory served by nuclear power plants. The larger the territory, the more opportunities there are to place RES powered by RES. The more such ES, the higher the likelihood that peak consumption will coincide with the period of their greatest generation. This is where the electricity from solar panels will come from, this is where the wind energy will come from, this is where the tidal wave will successfully hit the side, and all together they will smooth out the peak load, allowing nuclear workers at the nuclear power plant to calmly drink tea, looking at the monotonously, without interruption, working neutron counters.
Renewable Energy, hsto.org
The calmer the situation at the nuclear power plant, the fatter the burghers can become, since they can continue to heat their sausages on the grill without any problems. As you can see, there is nothing paradoxical in the combination of renewable energy sources and nuclear generation as a base, everything is exactly the opposite - such a combination, if the world has seriously decided to fight CO 2 emissions, is the optimal way out of the situation, without in any way crossing out all the options modernizations and improvements of thermal power plants that we talked about.
Continuing the “kangaroo style”, we suggest “jumping” to the very first sentence of this article – about the finiteness of any traditional energy resources on planet Earth. Because of this, the main, strategic direction of energy development is the conquest of the thermonuclear reaction, but its technology is incredibly complex and requires coordinated, joint efforts of scientists and designers from all countries, serious investments and many years of hard work. How long it will take can now be guessed using coffee grounds or bird entrails, but you need to plan, of course, for the most pessimistic scenario. We need to look for fuel that can provide that same basic generation for as long as possible. There seems to be plenty of oil and gas, but the planet’s population is also growing, and more and more kingdom-states are striving for the same level of consumption as in the countries of the “golden billion”. According to geologists, there are 100-150 years of fossil hydrocarbon fuel left on Earth, unless consumption grows at a faster rate than at present. And it seems that it will turn out that way, since the population developing countries craves an increase in comfort level...
Fast reactors
The way out of this situation proposed by the Russian nuclear project is known; this is the closure of the nuclear fuel cycle through the involvement of nuclear breeder reactors and fast neutron reactors in the process. A breeder is a reactor in which, as a result of a fuel session, the output of nuclear fuel is more than what was initially loaded, a breeder reactor. Those who have not yet completely forgotten the school physics course may well ask the question: excuse me, but what about the law of conservation of mass? The answer is simple - no way, since in a nuclear reactor the processes are nuclear, and the law of conservation of mass does not apply in its classical form.
More than a hundred years ago, Albert Einstein linked mass and energy together in his special theory of relativity, and in nuclear reactors this theory is strictly practical. The total amount of energy is conserved, but in this case there is no question of conservation of the total amount of mass. A huge reserve of energy “sleeps” in the atoms of nuclear fuel, released as a result of the fission reaction; we use part of this reserve for our own benefit, and the other part miraculously transforms uranium-238 atoms into a mixture of atoms of plutonium isotopes. Fast neutron reactors, and only they, make it possible to convert the main component of uranium ore - uranium-238 - into a fuel resource. The reserves of uranium-235, depleted in content and not used in thermal nuclear reactors, accumulated during the operation of thermal neutron nuclear power plants amount to hundreds of thousands of tons, which no longer need to be extracted from mines, which no longer need to be “exfoliated” from waste rock - its there is an incredible amount of uranium at enrichment plants.
MOX fuel at your fingertips
Theoretically it’s understandable, but not completely, so let’s try it again “on our fingers”. The very name “MOX fuel” is just an English abbreviation written in letters of the Slavic alphabet, which is written as MOX. Explanation – Mixed-Oxide fuel, free translation – “fuel from mixed oxides”. Basically, this term refers to a mixture of plutonium oxide and uranium oxide, but this is only basically. Since our respected American partners were unable to master the technology for producing MOX fuel from weapons-grade plutonium, Russia also abandoned this option. But the plant we built was designed in advance to be universal - it is capable of producing MOX fuel from spent fuel from thermal reactors. If anyone has read the articles Geoenergetics.ru in this regard, he remembers that the isotopes of plutonium 239, 240 and 241 in spent fuel are already “mixed” - there are 1/3 of them each, so in MOX fuel created from spent fuel there is a mix of plutonium, a kind of mix inside a mix .
The second part of the main mix is depleted uranium. To exaggerate: we take a mix of plutonium oxide extracted from spent nuclear fuel using the PUREX process, add ownerless uranium-238 and get MOX fuel. In this case, uranium-238 does not participate in the chain reaction; only the mixed plutonium isotopes “burn.” But uranium-238 is not just “present” - occasionally, reluctantly, from time to time it takes in one neutron, turning into plutonium-239. Some of this new plutonium “burns up” right away, while some simply does not have time to do this before the end of the fuel session. That, in fact, is the whole secret.
The numbers are arbitrary, taken out of thin air, just for clarity. The initial composition of MOX fuel is 100 kilograms of plutonium oxide and 900 kilograms of uranium-238. While the plutonium was “burning”, 300 kilos of uranium-238 turned into additional plutonium, of which 150 kilos immediately “burned”, and 150 kilos did not have time. They pulled out the fuel assembly and “shaken out” the plutonium from it, but it turned out to be 50 kilos more than it was originally. Well, or the same thing, but with wood: you threw 2 logs into the firebox, your stove heated all night, and in the morning you pulled out... three logs. From 900 kg of useless uranium-238, which does not participate in the chain reaction, when used as part of MOX fuel, we obtained 150 kilograms of fuel, which immediately “burned out” for our benefit, and 150 kilograms were left for further use. And there is 300 kilos less of this waste, useless uranium-238, which is also not bad.
The actual ratios of depleted uranium-238 and plutonium in MOX fuel are, of course, different, since with 7% plutonium in MOX fuel the mixture behaves almost the same as conventional uranium fuel with about 5% enrichment in uranium-235. But the numbers we came up with show main principle MOX fuel - useless uranium-238 is converted into nuclear fuel, its huge reserves become an energy resource. According to rough estimates, if we assume that on Earth we stop using hydrocarbon fuels to produce electricity and switch only to the use of uranium-238, it will last us for 2,500 - 3,000 years. Quite a decent amount of time to master the technology of controlled thermonuclear fusion.
MOX fuel allows us to simultaneously solve another problem - to reduce the reserves of spent fuel accumulated in all member countries of the “nuclear club”, and to reduce the amount of radioactive waste accumulated in spent fuel. The point here is not about some miraculous properties of MOX fuel, everything is more prosaic. If the SNF is not used, but is attempted to be sent to eternal geological burial, then all the high-level waste that it contains will have to be sent to burial along with it. But the use of technologies for reprocessing spent nuclear fuel in order to extract plutonium from it willy-nilly forces us to reduce the volume of this radioactive waste. In the struggle for the use of plutonium, we are simply forced to destroy radioactive waste, but at the same time the process of such destruction becomes much less expensive - after all, plutonium is used.
MOX fuel is an expensive pleasure that needs to be made cheap
At the same time, the production of MOX fuel in Russia began quite recently, even with the newest, most technologically advanced fast neutron reactor - BN-800, the transition to 100% use of MOX fuel occurs online, and is also not yet completed. It is quite natural that currently the production of MOX fuel is more expensive than the production of traditional uranium fuel. Reducing the cost of production, as in any other industry, is possible, first of all, through mass, “conveyor” production.
Consequently, in order for closing the nuclear fuel cycle to be feasible from an economic point of view, Russia needs a larger number of fast neutron reactors; this should become a strategic line for the development of nuclear energy. More reactors – good and different!
At the same time, it is necessary not to lose sight of the second possibility of using MOX fuel - as fuel for VVER reactors. Fast neutron reactors create such an additional amount of plutonium that they themselves cannot really use - they simply don’t need so much, there is enough plutonium for VVER reactors. We have already written above that MOX fuel, in which 93% depleted uranium-238 accounts for 7% plutonium, behaves almost the same as conventional uranium fuel. But the use of MOX fuel in thermal reactors leads to a decrease in the efficiency of the neutron absorbers used in VVERs. The reason for this is that boron-10 absorbs fast neutrons much worse - these are its physical properties, which we cannot influence in any way. The same problem arises with emergency boron rods, the purpose of which is to instantly stop the chain reaction in case of emergency situations.
A reasonable solution is to reduce the amount of MOX fuel in VVER to 30-50%, which is already being implemented in some light water reactors in France, Japan and other countries. But even in this case, it may be necessary to modernize the boron system and carry out all the necessary safety justifications, cooperation with the IAEA supervisory authorities to obtain licenses for the use of MOX fuel in thermal reactors. Or, in short, the number of boron rods will have to be increased, both those that are intended for control and those that are “stored” in case of emergency. But only the development of these technologies will make it possible to move on to mass production of this type of fuel and to reduce the cost of its production. At the same time, this will make it possible to more actively solve the problem of reducing the amount of spent nuclear fuel and more actively use depleted uranium reserves.
The prospects are close, but the road is not easy
The development of this technology in combination with the construction of breeder reactors for energetic plutonium - fast neutron reactors - will allow Russia not only to close the nuclear fuel cycle, but also to make it economically attractive. There are also great prospects for the use of SNUP fuel (mixed nitride uranium-plutonium fuel). Experimental fuel assemblies, irradiated at the BN-600 reactor in 2016, have already proven their effectiveness both during reactor tests and based on the results of post-reactor studies. The results obtained provide for the continuation of work to justify the use of SNUP fuel in the creation of the BREST-300 reactor plant and on-site modules for the production of SNUP fuel at the experimental demonstration complex being built in Seversk. BREST-300 will allow us to continue developing the technologies necessary to completely close the nuclear fuel cycle, provide a more complete solution to the problems of spent nuclear fuel and radioactive waste, and implement the ideology of “returning to nature as much radioactivity as was extracted.” The BREST-300 reactor, like the BN reactors, is a fast neutron reactor, which only emphasizes the correctness of the strategic direction of nuclear energy development - a combination of pressurized water reactors and fast neutron reactors.
Mastering the technology of 100% use of MOX fuel on BN-800 also provides the opportunity to create BN-1200 reactors - not only more powerful, but also more economically profitable. The decision to create the BN-1200 reactor in Russia has been made, which means that the pace of research work by nuclear specialists will only have to increase, and the creation of the MBIR, scheduled for 2020, can significantly help in solving all problems, in mastering the technology of complete fuel closure nuclear cycle. Russia was and remains the only country that has created fast neutron power reactors, ensuring our world leadership in this most important area of nuclear energy.
Of course, everything that has been said is just a first acquaintance with the features of fast neutron reactors, but we will try to continue, since this topic is important and, as it seems to us, quite interesting.
In contact with
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 a nuclear power plant was built. Only the “boiler” is the energy of 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 by low-energy thermal neutrons, producing fission fragments and new high-energy neutrons (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 other coolants have to be used: 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 Project 705 submarines) 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 for expanded 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, was launched 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 to 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 is currently 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 Atomic Energy Agency (IAEA) in 2005, the total proven reserves of uranium, the cost of extraction of which does 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 its normal state it is solid, but 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 supplies it to the reloading mechanism. The last stage is the installation of fuel assemblies into the vacated cell. At the same time, certain restrictions are imposed on the operation of the mechanism for safety reasons: for example, it is impossible to simultaneously release two adjacent cells, 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 performed every micro-campaign (160 effective days, calculated at 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”. The ultimate goal is to get fresh assemblies from the corresponding drum into the desired slot, and spent ones into their own drum, from where, after being cleaned with steam (from sodium), they will fall into 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 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 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 burnout (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 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.”
