Clean nuclear fuel. How nuclear fuel is produced (9 photos). The principle of operation of a nuclear power plant
Nuclear fuel is usually understood as the set of all fissile nuclides in the core. Most of the thermal ENR used in NPP power units at the initial stage of operation operate at pure uranium fuel, but during the campaign they reproduce a significant amount secondary nuclear fuel- plutonium-239, which immediately after its formation is included in the process of neutron multiplication in the reactor. Therefore, the fuel in such nuclear reactors at any the moment of the campaign should be considered at least a set of three fissile components: 235 U, 238 U and 239 Pu. Uranium-235 and plutonium-239 are divided by neutrons of any energies of the reactor spectrum, and 238 U, as already noted, only fast suprathreshold(with E > 1.1 MeV)neutrons.
The main characteristic of uranium nuclear fuel is its initial enrichment (x), which is understood as the proportion (or percentage) of uranium-235 nuclei among all uranium nuclei. And since more than 99.99% of uranium consists of two isotopes - 235 U and 238 U, the enrichment value is:
Natural uranium metal contains approximately 0.714% of 235 U nuclei, and more than 99.286% is 238 U (other isotopes of uranium: 233 U, 234 U, 236 U and 237 U are present in natural uranium in such insignificant quantities that they may not be accepted in Attention).
If the fuel is not fresh (irradiated - SNF), then it is characterized by one more parameter - burnout depth.
Nuclear fuel is an expensive thing. Mining uranium ore, obtaining natural uranium metal, enriching it with the 235 U isotope, making a fuel composition, sintering it into pellets and finishing them, manufacturing fuel rods and fuel assemblies - all these are very complex technological processes that require large material and energy costs. It is clear that throwing a rather large amount of unburned nuclear fuel into a radioactive waste cemetery would be very unwise.
Spent (IRRADIATED) fuel is directed to regeneration where the fuel components are separated from the fission products accumulated during the operation through a chain of complex technological operations, re-enriched with the 235 U isotope and re-included in the fuel cycle. Note that the regeneration of nuclear fuel is no less complicated and expensive than the manufacture of “fresh” fuel.
That is why it is very important that during the campaign as much of the loaded fuel as possible burns out, and as little as possible remains for regeneration. There are two main characteristics that measure the efficiency of fuel use in power reactors.
a) Fuel burnup is the fraction (or percentage) of the burnt out main fuel (235 U) from its initial amount.
The burnout rate is indicated by the letter z and in accordance with the definition is equal to:
Using elementary substitutions, it is easy to show that the degree of burnout at any moment of the campaign t- a value directly proportional to the value of energy production W (t), if we do not take into account that part of the generated energy, which is obtained as a result of fission of plutonium nuclei.
From (15.3.1) it follows that
The efficiency of using the main fuel in the reactor during the core run can be judged by the figures for the maximum burnup (i.e., the burnup at the end of the run).
For RBMK-1000 reactors z max = 0.35 ¸ 0.37, and for water-cooled reactors (VVER-440, VVER-1000) z max = 0.30 ¸ 0.33.
In practice, the degree of burnout can be measured in%.
b) Burnout depth is the energy production at the moment of the campaign, per unit mass of the initially loaded uranium.
Here it comes about all uranium(235 U + 238 U) loaded into the core before the start of the campaign. If we denote the value of the burn-up depth through b, then in accordance with the definition
The burnout depth is usually measured in MW day / t, MW day / kg
or GW day / t.
The following figures give an idea of the depth of fuel burnup:
* for RBMK-1000 reactors b max => 20 MW. day / kg;
* for VVER-type reactors 1000 b max => 40 ¸ 50 MW. day / kg.
NPP reactors use low enrichment uranium (enriched to 1.8 5.2%), in reactors of marine transport nuclear power plants, the initial enrichment of nuclear fuel is 21 ¸ 45%, and plants with liquid metal reactors use nuclear fuel with an enrichment of up to 90%. The use of fuel with low enrichment at nuclear power plants is explained by economic considerations: the technology of production of enriched fuel is complex, energy-intensive, requires complex and bulky equipment, and therefore is an expensive technology.
Uranium metal is thermally unstable, subject to phase transformations at relatively low temperatures and is chemically unstable, and therefore unacceptable as fuel power reactors... Therefore, uranium in reactors is not used in a purely metallic form, but in the form of chemical (or metallurgical) compounds with other chemical elements. These compounds are called fuel compositions.
The most common fuel compositions in reactor technology:
UO 2, U 3 O 8, UC, UC 2, UN, U 3 Si, (UAl 3) Si, UBe 13. (Cu-UO 2)
Another (other) chemical element of the fuel composition is called fuel thinner. In the first two of the listed fuel compositions oxygen is the liquefier, in the second two - carbon, in the subsequent ones, respectively, nitrogen, silicon, aluminum with silicon and beryllium.
The basic requirements for a liquefier are the same as for a moderator in a reactor: it must have a high elastic scattering microsection and possibly a lower microsection for the absorption of thermal and resonance neutrons.
The most common fuel composition in power reactors of nuclear power plants is uranium dioxide(UO 2), and its thinner - oxygen - fully meets all the above requirements .
Melting point of dioxide (2800 o C) and its high thermal stability make it possible to have high temperature fuel with a permissible operating temperature up to 2200 o C.
A nuclear power plant (NPP) is a complex of technical structures designed to generate electrical energy by using the energy released during a controlled nuclear reaction.
Uranium is used as a common fuel for nuclear power plants. The fission reaction is carried out in the main block of a nuclear power plant - a nuclear reactor.
The reactor is mounted in a steel case, designed for high pressure - up to 1.6 x 107 Pa, or 160 atmospheres.
The main parts of VVER-1000 are:
1. The active zone, where the nuclear fuel is located, a chain reaction of nuclear fission proceeds and energy is released.
2. A neutron reflector surrounding the core.
3. Heat carrier.
4. Protection control system (CPS).
5. Radiation protection.
Heat in the reactor is released due to the chain reaction of nuclear fuel fission under the influence of thermal neutrons. In this case, nuclear fission products are formed, among which there are both solids and gases - xenon, krypton. Fission products are very radioactive, so the fuel (uranium dioxide tablets) is placed in sealed zirconium tubes - fuel rods (fuel elements). These tubes are combined in several pieces side by side to form a single fuel assembly. To control and protect the nuclear reactor, control rods are used, which can be moved along the entire height of the core. The rods are made from substances that strongly absorb neutrons, such as boron or cadmium. With deep insertion of the rods, a chain reaction becomes impossible, since neutrons are strongly absorbed and removed from the reaction zone. The rods are moved remotely from the control panel. With a small movement of the rods, the chain process will either develop or damp. In this way, the power of the reactor is regulated.
The station scheme is double-circuit. The first, radioactive, loop consists of one VVER-1000 reactor and four circulation cooling loops. The second circuit, non-radioactive, includes a steam generator and water supply unit and one turbine unit with a capacity of 1030 MW. The primary coolant is non-boiling high-purity water under a pressure of 16 MPa with the addition of boric acid solution - a strong neutron absorber, which is used to regulate the reactor power.
1. The main circulation pumps pump water through the reactor core, where it is heated to a temperature of 320 degrees due to the heat released during a nuclear reaction.
2. The heated coolant gives up its heat to the water of the secondary circuit (working fluid), evaporating it in the steam generator.
3. The cooled coolant enters the reactor again.
4. The steam generator produces saturated steam at a pressure of 6.4 MPa, which is fed to the steam turbine.
5. The turbine drives the rotor of the generator.
6. Waste steam is condensed in the condenser and fed back to the steam generator by the condensate pump. To maintain a constant pressure in the circuit, a steam volume compensator is installed.
7. The heat of condensation of steam is removed from the condenser by circulating water, which is supplied by a feed pump from the cooler pond.
8. Both the first and the second loop of the reactor are hermetically sealed. This ensures the safety of the reactor for personnel and the public.
If it is impossible to use a large amount of water for steam condensation, instead of using a reservoir, the water can be cooled in special cooling towers (cooling towers).
The safety and environmental friendliness of the reactor operation is ensured by strict compliance with the regulations (operating rules) and a large number of control equipment. All of it is designed for thoughtful and effective management reactor.
Emergency protection of a nuclear reactor - a set of devices designed to quickly stop a nuclear chain reaction in the reactor core.
Active emergency protection is automatically triggered when one of the parameters of a nuclear reactor reaches a value that can lead to an accident. Such parameters can be: temperature, pressure and flow rate of the coolant, level and rate of power increase.
Actuating elements of emergency protection are, in most cases, rods with a substance that absorbs neutrons well (boron or cadmium). Sometimes, to shut down the reactor, a liquid absorber is injected into the coolant circuit.
In addition to active protection, many modern designs also include passive protection elements. For example, modern versions of VVER reactors include the Emergency Core Cooling System (ECCS) - special tanks with boric acid located above the reactor. In the event of a maximum design basis accident (rupture of the primary cooling loop of the reactor), the contents of these tanks by gravity flow inside the reactor core and the nuclear chain reaction is extinguished by a large amount of boron-containing material that absorbs neutrons well.
According to the "Rules for Nuclear Safety of Reactor Installations of Nuclear Power Plants", at least one of the envisaged shutdown systems of the reactor must perform the function of emergency protection (EP). Emergency protection must have at least two independent groups of working bodies. On a signal from the AZ, the working bodies of the AZ should be activated from any working or intermediate positions.
AZ equipment should consist of at least two independent sets.
Each set of core protection equipment must be designed in such a way that in the range of variation of the neutron flux density from 7% to 120% of the nominal, protection is provided:
1. By the density of the neutron flux - at least three independent channels;
2. By the rate of increase of the neutron flux density - at least three independent channels.
Each set of reactor protection equipment must be designed in such a way that emergency protection is provided by at least three independent channels for each technological parameter for which protection is required over the entire range of changes in technological parameters set in the design of the reactor plant (RP).
The control commands of each set for the AZ actuators must be transmitted through at least two channels. When deactivating one channel in one of the AZ equipment sets without deactivating this kit an alarm should be generated automatically from operation for this channel.
Emergency protection must be triggered at least in the following cases:
1. Upon reaching the core setpoint for the neutron flux density.
2. When the core setpoint is reached by the rate of increase in the neutron flux density.
3. When the voltage disappears in any set of AZ equipment and power supply buses of the CPS that is not taken out of operation.
4. In case of failure of any two of the three channels of protection in terms of the neutron flux density or the rate of rise of the neutron flux in any set of core protection equipment that has not been taken out of operation.
5. When the AZ settings are reached by the technological parameters, according to which it is necessary to carry out the protection.
6. When initiating the activation of the AZ from the key from the block control point (BPU) or reserve control center (RPU).
The material was prepared by the Internet edition of www.rian.ru based on information from RIA Novosti and open sources
Spent nuclear fuel is uranium that has worked in a nuclear reactor and contains radioactive fission products. Therefore, it is also called irradiated or burnt out nuclear fuel.
How is spent nuclear fuel different from radioactive waste (RW)? First of all, the fact that SNF is a valuable product containing 2 useful components - unburnt uranium and transuranic elements. In addition, among the fission products there are radionuclides (radioactive isotopes) that can be successfully used in industry, medicine, and also in scientific research.
Once removed from the reactor, spent nuclear fuel (SNF) retains radioactivity and releases heat. Therefore, for some time, such fuel is kept in pools under water to remove heat and protect it from ionizing radiation. The next step could be:
- final disposal - completion of an open fuel cycle as is done in the USA, Canada and Sweden.
- reprocessing of spent nuclear fuel for further use - closed fuel cycle. The closed fuel cycle route was chosen by Russia, Great Britain, France and Japan.
Spent nuclear fuel is initially stored directly in the reactor compartment. Then it is moved to another location in special "dry storage" warehouses. In the closed fuel cycle for modern light water reactors, fuel travels exactly the same path. Starting from uranium mines and factories, uranium goes through all stages of conversion and enrichment to make reactor fuel.After the fuel is removed from the reactor, the fuel rods are processed in refineries where they are crushed and dissolved in acid. After special chemical treatment, two valuable products are isolated from the spent fuel: plutonium and unused uranium. In this case, approximately 3% of the fuel remains as high-level waste. After bituminization, concreting or vitrification, these highly radioactive materials are subject to long-term disposal.
Spent nuclear fuel contains approximately 1% plutonium. This is a very good nuclear fuel that does not need any enrichment process. Plutonium can be mixed with depleted uranium to produce mixed oxide fuel or MOX fuel, which is supplied as fresh fuel assemblies for loading into reactors. It can be used for loading into reactors. The recovered uranium can be returned for additional enrichment or supplied as fresh fuel for operating reactors. A closed fuel cycle is a more efficient system for maximum use of uranium without additional mining at mines (in energy units, the savings are about 30%). And although the industry immediately approved of this approach, such schemes for reprocessing spent nuclear fuel have not yet become widespread.
One of the reasons for such an incomplete use of the potential of uranium is that most of the existing industrial reactors belong to the so-called "light water" LWR reactors. They are good in many ways, but they are not designed to squeeze all the energy out of the fuel to the last watt. However, there are other types of reactors - the so-called "fast" (fast neutron reactors), capable of "reprocessing" spent fuel with the extraction of much more energy.
Nuclear Power Plants - NPP Are thermal power plants. At nuclear power plants, the energy of controlled nuclear reactions is used as a source. The unit capacity of NPP power units reaches 1.5 GW.
Nuclear Power Plants - Nuclear Power Plants - Fuels
As a common fuel for nuclear power plants, it is used U- uranium. The fission reaction is carried out in the main block of a nuclear power plant - a nuclear reactor. In a chain reaction of fission of nuclear matter, a significant amount of thermal energy is released, which is used to generate electricity.
Nuclear power plants - NPP - principle of operation
Fission of uranium nuclei produces fast neutrons. The fission rate is a chain reaction, at nuclear power plants it is regulated by moderators: heavy water or graphite. Neutrons contain a lot of thermal energy. Energy enters the steam generator through the coolant. High pressure steam is directed to turbine generators. The resulting electricity goes to transformers and then to switchgear. Part of the electricity is directed to meet the own needs of a nuclear power plant (NPP). The circulation of the coolant at nuclear power plants is provided by pumps: main and condensate. Excess heat from the nuclear power plant is directed to cooling towers.
Russian nuclear power plants - nuclear power plants - types of nuclear reactors:
- RBMK - high-power channel reactor,
- VVER - pressurized water power reactor,
- BN - fast neutron reactor.
Nuclear power plants - nuclear power plants - ecology
Nuclear Power Plants - Nuclear power plants do not emit flue gases into the atmosphere. There is no waste in the form of ash and slag at the NPP. Problems in nuclear power plants are excess heat and storage of radioactive waste. To protect people and the atmosphere from radioactive emissions at nuclear power plants, special measures are taken:
- improving the reliability of NPP equipment,
- duplication of vulnerable systems,
- high requirements for the qualifications of personnel,
- protection and protection from external influences.
Nuclear power plants are surrounded by a sanitary protection zone.
Why uranium?
Humanity has tied itself hand and foot with electric wires. Appliances, industrial equipment, street lighting, trolleybuses, metro, electric trains - all these benefits of civilization work from electrical network; they become meaningless "pieces of iron" if the current is lost for some reason. However, people are already so accustomed to the constancy of the power supply that any disconnection causes discontent and even discomfort. And really, what should a person do when all devices, including the most beloved ones - a TV, a computer and a refrigerator, went out at once? It is especially difficult to endure "separation" in the evening, when you so want after work or school, as they say, to extend the daylight hours. Will the tablet save or the phone, but after all, their charge is not eternal. It is even worse to be in a "prison cell", which, at the will of the blackout, can turn into an elevator car or a subway car.
Why all this conversation? And to the fact that "electrified" humanity needs stable and powerful sources of energy - first of all, electricity. With a shortage of it, disconnections from the network will become annoyingly frequent, and the standard of living will decrease. To prevent an unpleasant scenario from becoming a reality, it is necessary to build more and more power plants: global energy consumption is growing, and the existing power units are gradually aging.
But what can he offer to solve the problem modern energy mainly burning coal and gas? Of course, new gas installations destroying valuable chemical raw materials, or coal blocks that smoke the sky. By the way, emissions from thermal power plants are a well-known environmental problem, but fossil fuel extraction companies also cause harm to the environment. But its consumption is enormous. For example, to ensure the operation of a conventional refrigerator, about a hundred kilograms of coal or hundreds of cubic meters of natural gas will have to be burned during the year. And that's just one household appliance, of which there are many.
By the way, how much nuclear fuel is needed for the said refrigerator to work for a whole year? It's hard to believe, but ... just one gram!
The colossal energy intensity of nuclear fuel made from enriched uranium makes it a worthy competitor to coal and gas. Indeed, a nuclear power plant consumes a hundred thousand times less fuel than a thermal one. This means that mining for uranium mining is much smaller, which is important for the environment. Plus - there are no emissions of greenhouse and toxic gases.
Power unit nuclear power plant with a capacity of one thousand megawatts per year will consume only three dozen tons of nuclear fuel, and a thermal station of the same capacity will require about three million tons of coal or three billion cubic meters of gas. In other words, to obtain the same amount of electricity, either several wagons with nuclear fuel per year will be required, or several trains with coal ... per day.
And what about renewable energy sources? They are, of course, good, but still need improvement so far. Take, for example, the area occupied by the station. In the case of wind turbines and solar panels, it is two orders of magnitude higher than that of conventional power plants. For example, if a nuclear power plant (NPP) fits into an area of a couple of square kilometers, then a wind farm or a solar field of the same capacity will occupy several hundred square kilometers. Simply put, the ratio of areas is like that of a small village and a very large city. In the desert, this indicator may not be important, but in the area of agriculture or forestry - even more so.
It should be mentioned that nuclear fuel is always ready to work, regardless of the season, day or weather whims, while the sun, in principle, does not shine at night, and the wind blows when it pleases. Moreover, in some areas, renewable energy will not be profitable at all due to low solar energy flow or low average wind speed. For nuclear power plants, such problems simply do not exist.
These advantages of nuclear energy have determined the outstanding role of uranium - as a nuclear fuel - for modern civilization.
Who got how much?
In one old Soviet cartoon, the animals were solving an important task - they were sharing an orange. As a result, everyone except the wolf was given a tasty juicy wedge; the gray had to be content with the peel. In other words, he did not get a valuable resource. From this point of view, it is interesting to know how things are with uranium: do all the countries of the world have its reserves, or are there deprived ones?
In fact, there is a lot of uranium on Earth, and this metal can be found almost everywhere: in the crust of our planet, in the oceans, even in the human body. The problem lies in its "dispersion", "smearing" over terrestrial rocks, which results in a low concentration of uranium, which is often insufficient for the organization of economically profitable industrial mining. However, in some places there are accumulations with a high uranium content - deposits. They are unevenly distributed, respectively, and uranium reserves differ by country. Most of the deposits of this element "floated away" with Australia; besides, Kazakhstan, Russia, Canada and the countries of South Africa were lucky. However, this picture is not frozen, the state of affairs is constantly changing due to the exploration of new deposits and the exhaustion of old ones.
Distribution of proven uranium reserves by country (for reserves with production costs< $130/кг)
A huge amount of uranium is dissolved in the waters of the World Ocean: over four billion tons. It would seem that the ideal "deposit" - I do not want to mine. Scientists have developed special sorbents for extracting uranium from seawater back in the eighties of the last century. Why isn't this excellent method used universally? The problem is that the concentration of the metal is too low: only about three milligrams can be extracted from a ton of water! It is clear that such uranium will be too expensive. It is estimated that a kilogram will cost a couple of thousand dollars, which is much more expensive than the "land" analogue. But scientists are not upset and are inventing more and more effective sorbents. So, perhaps, this mining method will soon become competitive.
To date, the total amount of explored uranium reserves with a production cost of less than $ 130 per kilogram exceeds 5.9 million tons. Is this a lot? Quite enough: if the total capacity of nuclear power plants remains at the current level, then uranium will be enough for a hundred years. For comparison, proven oil and gas reserves can be depleted in just thirty to sixty years.
The top ten countries in terms of uranium reserves on their territory (for reserves with the cost of production< $130/кг)
However, one should not forget that, according to forecasts, nuclear energy will develop, so it is worth thinking about how to expand its resource base right now.
One of the ways to solve this problem is to find and timely develop new deposits. Judging by the available information, there should be no problems with this: only in the last few years new deposits have been found in some countries of Africa, South America, as well as in Sweden. True, it is impossible to say with certainty how profitable the extraction of the discovered reserves will be. It may happen that due to the low content of uranium in the ore and the difficulty of developing deposits, some of them will have to be left “for later”. The fact is that the prices for this metal are now quite low. From an economic point of view, this is not surprising. First, there are still deposits of relatively easily recoverable, and, therefore, cheap uranium in the world - it enters the market and “knocks down” the price. Secondly, after the Fukushima accident, some countries adjusted their plans for the construction of new nuclear power units, and Japan stopped all its nuclear power plants altogether - a drop in demand occurred, which made uranium even cheaper. But this is not for long. China and India have already entered the game, planning a large-scale construction of a nuclear power plant on their territory. Other Asian countries, as well as African and South American countries have less ambitious projects. Even Japan, apparently, will not be able to part with its nuclear energy. Therefore, demand will gradually recover, and coupled with the depletion of inexpensive deposits, this will lead to an increase in uranium prices. Analysts believe that the wait will not be long - just a few years. Then it will be possible to think about the development of the deposits left "for later".
It is interesting that the lists of countries with the largest reserves of uranium and states with the most developed nuclear energy practically do not coincide. Australia contains a third of the world's uranium "wealth", but there is not a single nuclear power plant on the green continent. Kazakhstan, the world leader in the production of this metal, is still preparing for the construction of several nuclear power units. For economic and other reasons, African countries are far from joining the world "atomic" family. The only nuclear power plant on this continent is located in South Africa, which recently announced its desire to further develop nuclear power... However, so far even South Africa has taken a time out.
What is left for the "atomic" giants - the USA, France, Japan - and China and India, which are advancing on their heels, if their needs are great, and the cat cries of its own reserves? Of course, try to gain control over uranium deposits and enterprises in other countries. This task is of a strategic nature, and, in solving it, the states enter into tough battles. Overbought large companies, political maneuvers are being undertaken, underground schemes are being implemented with bribery of the right people or judicial wars. In Africa, this struggle can and does - and is already pouring out - into civil wars and revolutions, secretly supported by the leading states seeking to redistribute their zones of influence.
In this regard, Russia is lucky: our nuclear power plants have quite decent reserves of uranium at their disposal, which are mined in the Trans-Baikal Territory, the Kurgan Region and the Republic of Buryatia. In addition, active exploration work is being organized. It is assumed that deposits in the Transbaikal region, Western Siberia, the Republic of Karelia, the Republic of Kalmykia and the Rostov region have great potential.
In addition, Rosatom also owns foreign assets - large stakes in uranium mining enterprises in Kazakhstan, the USA, Australia, and is also working on promising projects in southern Africa. As a result, among the world's leading companies engaged in the production of uranium, Rosatom confidently holds the third place after Kazatomprom (Kazakhstan) and Cameco (Canada).
By studying the chemical composition of meteorites, some of which are of Martian origin, scientists have discovered uranium. True, its content turned out to be much lower than in terrestrial rocks. Yeah, now it's clear why the Martians often visit us on their flying saucers.
But seriously, it is believed that uranium is present in all objects in the solar system. For example, in 2009 it was discovered in the lunar soil. Immediately, fantastic ideas arose, such as mining uranium on a satellite and then sending it to Earth. Another option is to "feed" the reactors of the lunar colonies huddled in the vicinity of the deposits. The deposits, however, have not yet been searched; and from an economic point of view, such production still seems unrealizable. But in the future - who knows ...
If you suffer for a long time, the fuel will turn out
Having uranium ore reserves is just one component of success. Unlike firewood or coal, which do not require particularly complex preparation before entering the furnace, the ore cannot be simply cut into pieces and thrown into the reactor. To explain why, it is necessary to mention a number of features inherent in uranium.
From a chemical point of view, this element is highly active, in other words, it tends to form various compounds; therefore, looking for nuggets in nature, like gold ones, is a completely hopeless business. What, then, is called uranium ore? A rock containing very small amounts of uranium minerals. It is often added: small, but sufficient for industrial mining to be approved by economists. For example, today it is considered expedient to develop ore, a ton of which contains only a few kilograms or even hundreds of grams of uranium. The rest is empty, unnecessary rock, from which uranium minerals are to be isolated. But even they cannot yet be loaded into a nuclear reactor. The fact is that these minerals are most often oxides or insoluble uranium salts in the company of other elements. Some of them may be of value to the industry, and the organization of their associated production can improve economic indicators... But even if there is no such need, uranium must still be purified from impurities. Otherwise, nuclear fuel made from "dirty" uranium can cause reactor malfunctions or even an accident.
However, purified uranium cannot be called nuclear fuel with complete certainty. The catch lies in its isotopic composition: there are only seven atoms of uranium-235 per thousand uranium atoms in nature, which is necessary for a chain fission reaction to proceed. The rest are uranium-238, which practically does not fission, and even absorbs neutrons. However, a natural uranium reactor can be launched, provided that a very effective moderator such as expensive heavy water or the purest graphite is used. Only they allow the neutrons, formed during the fission of the uranium-235 nucleus, to slow down so quickly that they have time to get into other uranium-235 nuclei and cause their fission, and not be ingloriously captured by uranium-238. But for a number of reasons, the overwhelming majority of reactors in the world use a different approach: natural uranium is enriched in a fissile isotope. In other words, the content of uranium-235 atoms is artificially increased from seven to several tens per thousand. Due to this, neutrons more often stumble upon them, and it becomes possible to use cheaper, albeit less effective moderators, for example, ordinary water.
Is enriched uranium a final product? Again, no, because power reactors provide for the transfer of "nuclear" heat to the coolant that washes the fuel - most often water. Due to the accumulation of fission products, the fuel - as it is in an operating reactor - becomes highly radioactive. Under no circumstances should it be allowed to dissolve in water. To do this, uranium is converted into a chemically stable state, and is also isolated from the coolant by covering it with a metal sheath. The result is a complex technical device containing enriched uranium compounds inside, which can be safely called nuclear fuel.
The aforementioned operations - uranium mining, its purification and enrichment, as well as the manufacture of nuclear fuel - are the initial stages of the so-called nuclear fuel cycle. It is necessary to get acquainted with each of them in more detail.
The half-life of uranium-238 is 4.5 billion years, while uranium-235 is only 700 million years. It turns out that the fissile isotope decays several times faster than the main one. If you think about it, this means that in the past, the content of uranium-235 in the natural mixture of isotopes was higher than now. For example, one billion years ago, out of a thousand uranium atoms, sixteen had a nucleus with 235 nucleons, two billion years ago their number was thirty-seven, and three billion years before today - as many as eighty! In fact, ore in those distant times contained uranium, which we now call enriched uranium. And it could well have happened that in some field a natural nuclear reactor would have started working by itself!
Scientists are confident that this is exactly what happened with several super-uranium-rich deposits of the Oklo deposit, located in the territory of modern Gabon. 1.8 billion years ago, a nuclear chain reaction spontaneously started in them. It was initiated by neutrons generated during spontaneous fission, and then a high concentration of uranium-235 and the presence of water in the ore - a neutron moderator - worked. In a word, the reaction became self-sustaining and proceeded, then intensifying, then dying out, for several hundred thousand years. Then the reactors "went out" - apparently due to a change in the water regime.
Today it is the only known natural nuclear reactor. Moreover, at present such processes cannot start in any field. The reason is quite understandable - there is too little uranium-235 left.
Try to dig
Uranium ores rarely come to the surface. Most often they occur at a depth of fifty meters to two kilometers.
Shallow deposits are developed in an open-pit or, as it is also called, a quarry method. Hard rocks are drilled and blasted, and then, with the help of loaders, they are loaded into dump trucks and taken out of the quarry. Loose rocks are developed and loaded into dump trucks using conventional or bucket wheel excavators, bulldozers are widely used. The power and size of this technique amaze the imagination: for example, the already mentioned dump trucks have a carrying capacity of a hundred or more tons! Unfortunately, the scale of the quarry itself is large, the depth of which can reach three hundred meters. After the completion of the work, he gapes huge pit in the earth's surface, and next to it rise dumps of rock that covered the uranium deposits. In principle, the quarry can be covered with these dumps by planting grass and trees on top; but it will be prohibitively expensive. Therefore, the pits are gradually filled with water, and lakes are formed that are not subject to economic use due to the increased content of uranium in the water. Problems related to groundwater pollution can also arise, so uranium pits require special attention.
However, the open development of uranium is gradually becoming a thing of the past for a completely banal reason - the deposits close to the surface are practically over. Now you have to deal with deeply buried ores. Traditionally, they are developed by underground (mine) method. Just do not imagine the harsh bearded men with pickaxes, crawling along the workings and chopping ore. The work of the miners is now largely mechanized. V rock containing uranium, boreholes are drilled - special deep holes into which explosives are laid. After the explosion, the crushed ore is taken by a loader with a ladle and runs along winding narrow galleries to the trolleys. The filled trolleys are carried to the vertical shaft of the mine by a small electric locomotive, and then with the help of a cage - a kind of elevator - the ore is lifted to the surface.
Underground mining has a number of features. First, it can be profitable only in the case of high-quality ores with a high uranium content, which lie no deeper than two kilometers. Otherwise, the costs of mining, mining and further processing of ore will make uranium practically "gold". Secondly, the underground kingdom of uranium mines is a closed space in which radioactive dust and no less radioactive radon gas soar. Therefore, without powerful ventilation and special means miners cannot do with protection such as respirators.
In both open-pit and mine mining, ore is recovered in the form of rather large lumps. When scooping them up with the bucket of an excavator or LHD, the operator does not know if he is taking ore rich in uranium minerals, waste rock, or something in between. After all, the deposit is not very homogeneous in its composition, and the use of powerful machines does not allow working subtly and gracefully. But to send for further processing pieces in which there is almost no uranium, at least, it is unreasonable! Therefore, the ore is sorted using the main property of uranium, by which it is not difficult to detect it - radioactivity. Special detectors of ionizing radiation allow, both during loading and already in the transport container, to divide the ore into several grades according to the intensity of the radiation emitted by it. Waste rock is sent to dumps. Rich ore - to the hydrometallurgical plant. But ore with a small but noticeable amount of uranium is sorted again, more carefully. First, it is crushed, divided by size, after which the pieces are dumped onto a moving conveyor belt. Above it is an ionizing radiation sensor, the signal from which is sent to automated system control of the gates located at the end of the belt. The sensor is configured so that it reacts to a radioactive piece of ore passing under it containing uranium minerals. Then the shutter turns and the ore falls into a special ore bunker, from where it is transported to the hydrometallurgical plant. In turn, the waste rock does not "disturb" the sensor and the damper in any way, and falls into another box - into the dump.
Simplified scheme of radiometric sorting of ore (modern complexes are much more complicated)
The described scheme is approximate, principled: nothing prevents the sorting of ore at enterprises by other known methods. However, practice has shown that they are poorly suited for uranium ores. Therefore, radiometric sorting - with radiation detectors - has gradually become the mainstream technology.
In reality, when sorting ore, a certain middle category is also distinguished, which, in terms of uranium content, cannot be attributed to either rich ore or waste rock. In other words, sending it to the hydrometallurgical plant is costly (a waste of time and reagents), but it is a pity to send it to the dumps. Such poor ore is piled up in large heaps and poured with sulfuric acid on outdoors gradually dissolving uranium. The resulting solution is pumped over for further processing.
At the hydrometallurgical plant, the rich ore is to be ground even more, almost to the state of dust, and then dissolved.
Ore is crushed in various mills - for example, drum-ball mills: the crushed material and metal balls such as cannonballs are poured into the rotating hollow drum. As they spin, the balls hit the pieces of ore, grinding them and grinding them into powder.
The crushed ore is "opened", that is, partially dissolved by treatment with sulfuric or nitric acid, or a mixture of them. The result is a uranium solution containing many impurities. Sometimes, if the uranium ore contains a lot of natural carbonates, the acid is not used. Otherwise, a reaction resembling extinguishing soda with vinegar will occur - with intense release of carbon dioxide, and the reagent will be wasted. How to be? It turns out that such minerals can be "opened" with a soda solution. As a result, a uranium solution will also be obtained, which will go for further processing.
But the remains of undissolved ore have to be sent to special tailing dumps, which are not the most environmentally friendly facilities. It is worth recalling the waste rock separated during the sorting process: it is piled into dumps. Both tailings and dumps contain small amounts of uranium, making them potentially hazardous. In this regard, the question arises: is it possible to organize production in such a way as to cause minimal harm to nature and ensure the safety of workers?
It is possible, and it has been practiced for a long time. The production method in question is called borehole in situ leaching. Its essence is that the field is "pierced" by many wells. Some of them, called injection, are fed sulfuric acid, which goes down to depth, passes through the ore and dissolves uranium. Then the solution of the valuable metal is taken from the bowels through other - pumping - wells.
So what happens: no dumps, no tailing dumps, no dust, no holes or unexpected sinkholes in the ground, and in the end - the same uranium solution? Yes. Moreover, the method of borehole in situ leaching is used to develop very poor ores that are economically unprofitable to be mined in an open-pit or mine way. But with such a set of advantages, there must be disadvantages! Well, first of all, drilling wells deeper than eight hundred meters is not rational from a cost point of view. Second, the method does not work in dense, non-porous ores. Thirdly, sulfuric acid still disturbs the composition and behavior of groundwater in the field, although these disturbances "dissolve" over time by themselves. It is much more dangerous if the solution spills over the surface or penetrates in a roundabout way - along cracks and faults - into the groundwater. Therefore, the process is closely monitored by drilling control wells.
Borehole underground leaching
To avoid the aforementioned problems, a "mine" version of underground leaching was invented: blocks of ore in mine workings are crushed by explosions, and then poured from above with a leaching solution (sulfuric acid), taking the uranium solution from below - through the drainage system.
In any case, underground leaching is currently the safest way to extract uranium for the environment. This is one of the reasons for the downright explosive growth of its popularity. If in 2000 only fifteen percent of uranium was mined by underground leaching, today this figure is practically close to fifty percent!
Underground leaching is becoming the leading technology for uranium mining
Usually, uranium deposits are searched using ionizing radiation sensors; more precisely, gamma radiation. First, an airplane equipped with such sensors flies over the terrain. He can only fix a radiation anomaly - a slightly increased background over the field. Then a helicopter is launched into action, which more slowly and more accurately "outlines" the boundaries of the promising area. In the end, prospectors come to this territory with measuring instruments and drills. Based on the results of their work, a map of the occurrence of uranium ores will be drawn and the cost of mining will be calculated.
However, uranium ore deposits can signal themselves in other ways. For example, changing appearance plants growing above them: willow-tea petals, usually pink, turn white; blueberries turn green or turn white. The deep roots of the juniper growing above the deposit absorb uranium well, and it accumulates in the branches and needles. By turning them into ash and checking for uranium content, one can understand whether it is worth mining the main metal of nuclear energy in this area.
Cleanliness is the key to health (nuclear reactor)
The uranium solution obtained during the "opening" of the ore or in the process of underground leaching is not very pure. In other words, in addition to uranium, it contains a bunch of chemical elements found in the earth's crust: sodium and potassium, calcium and magnesium, iron, nickel and copper - and many others. Do not be surprised at the formation of such a thick "compote", because sulfuric acid is highly reactive and dissolves many natural substances; it's good that not all the ore is whole. But for the manufacture of nuclear fuel, the most pure uranium is needed. If, among the uranium atoms, there are impurity atoms here and there, the reactor may not start up or, even worse, break down. The reasons for such problems will be discussed very soon, but for now we can set the task: to purify uranium. And it is also desirable to receive it in a solid form, convenient for transportation. Indeed, solutions for transportation are not suitable: they too "like" to spill or seep through leaks.
In industry, this problem is solved in several steps. First, the solution is concentrated by passing it through special materials that collect uranium - sorbents. The first opportunity for cleaning appears: the sorbents are selected in such a way that other elements hardly "sit down" on them and remain in solution. Then the uranium is washed off the sorbent, for example, with the same sulfuric acid. This procedure may seem pointless if you do not explain that the acid for "flushing" is needed much less compared to the volume of the original solution. This is how they kill two birds with one stone: they increase the concentration of uranium and partially remove unnecessary impurities.
The second stage of purification is associated with obtaining solid uranium compounds. They are precipitated from a concentrated solution by adding well-known "medical" reagents: ammonia, hydrogen peroxide, and alkalis or carbonates. It should be noted that uranium does not precipitate as a metal; it is generally not easy to obtain in metallic form due to its high chemical activity - this has already been mentioned. Under the action of the above-mentioned regents, various sparingly soluble uranium compounds descend to the bottom of the apparatus. Dried and crushed, they are a yellow powder, which is often called "yellow cake" due to its apparent resemblance to a cake. By calcining it at a high temperature, a less beautiful mixture of uranium oxides is obtained - dirty green or even black.
Yellowcake can be sent to uranium enrichment plants
Yellowcake or a mixture of uranium oxides is practically harmless from a radiation point of view. Therefore, for transportation, they are loaded into two-hundred-liter metal barrels or special containers. Being at a distance of one meter from such a container is not half as "harmful" as flying in an airplane, being exposed to cosmic radiation. But most people are not afraid to fly! So, there is no reason to be afraid of barrels with yellow cake.
When precipitating uranium compounds, they try to conduct the process so that most of the impurities remain in solution. But some of them still manage to "break through." It is especially bad if the products contain elements that strongly absorb neutrons - boron, cadmium, rare earth metals. Even in microconcentrations, they are able to interfere with the fission chain reaction. Having made fuel from contaminated uranium, it will be possible for a long time to guess why the reactor does not want to work normally.
In addition, undesirable impurities include elements that reduce the plasticity of nuclear fuel and cause it to swell and expand with increasing temperature. These include the commonly found naturally occurring silicon and phosphorus, as well as tungsten and molybdenum. By the way, plasticity is usually called the ability of a material to change its shape and size without collapsing. This is very important for fuel, which heats itself up from the inside due to a nuclear chain reaction occurring in it, and, therefore, undergoes thermal deformations. The high temperature should not lead to excessive expansion of the uranium fuel, otherwise it will rupture the protective shell and come into contact with the coolant. The consequence of such "communication" can be the dissolution of radioactive uranium fission products in a hot coolant (most often - water) with their subsequent spreading through all pipelines and apparatus. Probably, there is no need to explain that this threatens the deterioration of the radiation situation at the power unit: the doses received service personnel will grow significantly.
As the saying goes, it is better to overdo it than to miss it. Therefore, a third - final - stage of purification, called refining, is also required. Uranium compounds delivered in barrels or containers are dissolved in acid, now in nitric acid. The resulting solution is brought into contact with an extractant - a liquid organic substance that absorbs uranium, but not impurities. So, undesirable elements remain in solution, and uranium goes into "organic matter". As a result of a series of subsequent operations, it is again brought into the form of oxides that already have the necessary "reactor" purity.
Now everything is fine, and you can proceed to the next stage - an artificial increase in the concentration of uranium-235.
Secrets of enrichment
At the beginning of the chapter it was already mentioned that in the natural mixture of uranium isotopes there is very little fissile uranium-235 and too much "lazy" uranium-238: there are about nine hundred ninety-three atoms of the second for seven atoms of the first. For most reactors currently in operation, this is not suitable. They need fuel, in which several tens of thousands of uranium atoms belong to the isotope-235, and not just a few, as in natural uranium. And to create a bomb, almost pure uranium-235 is absolutely necessary.
It is very difficult to solve the problem of uranium enrichment, that is, increasing the content of the fissile isotope. It would seem, how so? After all, chemistry has the widest range of methods for isolating substances from mixtures. It is possible to "pick out" only a few hundred grams of uranium from a ton of ore! Is it really impossible to do the same with isotopes: somehow separate one from the other? The problem is that the chemical properties of all isotopes of a certain element are the same, because they are determined by the number of electrons, and not by the composition of the nucleus. In other words, it is impossible to carry out such a reaction in which uranium-235, for example, would remain in solution, and uranium-238 precipitated. For any manipulation, they both behave in the same way. In the same way, it will not be possible to chemically separate the isotopes of carbon or potassium - in general, any element.
There is such a parameter - the degree of enrichment, which is the percentage (percentage) of uranium-235 in the total mass of uranium. For example, the degree of enrichment of natural uranium, in which there are seven fissile atoms for every thousand atoms, is 0.7%. In the case of nuclear fuel for nuclear power plants, this figure has to be raised to 3-5%, and for the production of the filling of an atomic bomb - up to 90% and more.
How to be? It is necessary to find such properties by which the isotopes - at least minimally - would differ from each other. The first thing that comes to mind is the mass of an atom. Indeed, the nucleus of uranium-238 is three neutrons more than that of uranium-235; hence, the "lazy" isotope weighs a little more. And since mass is a measure of inertia, and it manifests itself in motion, the main methods of uranium enrichment are associated with differences in the movement of its isotopes under specially created conditions.
Historically, the first enrichment technology was electromagnetic isotope separation. From the name it is clear that electric and magnetic fields are somehow involved in the process. Indeed, in this method, the previously obtained uranium ions are accelerated by an electric field and launched into a magnetic field. Since the ions have a charge, in a magnetic field they begin to "carry", twist along an arc of a certain radius. For example, we can recall the separation of uranium rays in a magnetic field into three streams - an effect discovered by Rutherford. Electrically charged alpha and beta particles deviate from a straight path, but gamma rays do not. In this case, the radius of the arc along which a charged particle moves in a magnetic field depends on its mass: the more it weighs, the slower it turns. It can be compared to trying to fit into a sharp turn of two reckless drivers, one of whom is driving a car and the other is a truck. It is clear that it is much easier for a passenger car to make a maneuver, while a truck may well skid. Something similar happens in a magnetic field with rapidly moving uranium-235 and uranium-238 ions. The latter are slightly heavier, have greater inertia, and the radius of their rotation is slightly higher: due to this, the flow of uranium ions is divided into two. Figuratively speaking, you can put two boxes, in one of which to collect the fissile isotope, uranium-235, and in the second - "unnecessary" uranium-238.
In a magnetic field, the trajectory of charged particles is curved, and the stronger, the lighter the particle
The principle of the method of electromagnetic isotope separation: lighter uranium-235 ions move in a magnetic field along a trajectory of a smaller radius compared to uranium-238 ions
Electromagnetic separation is good in almost all respects, except for performance, which, as usual, limits its industrial application. This is why the American Y-12 plant in Oak Ridge, which produced enriched uranium for the "Kid" bomb dropped on Hiroshima, using electromagnetic separation technology, closed back in 1946. It is necessary to clarify that at Y-12 uranium, previously enriched by other, more productive methods, was brought to a high degree of enrichment. Their improvement has driven the last nail into the coffin lid of the electromagnetic isotope separation technology - it is no longer used in industry.
Interestingly, electromagnetic separation is a versatile method for separating small amounts of any isotopes in pure form. Therefore, our analogue of Y-12 - plant 418, now known as the "Electrokhimpribor Combine" (Lesnoy, Sverdlovsk region), has technologies for producing more than two hundred isotopes of forty-seven chemical elements from lithium to lead. These are not just impressive numbers - the plant's products are really needed by scientists, doctors, industrialists ... By the way, it is produced at the SU-20 facility, the same one that produced weapons-grade uranium with a degree of enrichment close to 90% in the early 1950s.
The first post-war decades became a time of active accumulation of nuclear weapons arsenals. The solution to this problem had the highest priority, so they did not pay much attention to expenses - it was important to launch the massive enrichment of uranium. The emphasis was placed on gas diffusion, an extremely energy-intensive, but at the same time efficient enrichment technology. Its roots lie in the field of gas theory, which states that at a certain temperature, the average speed of a gas molecule is inversely proportional to its mass: the heavier it is, the slower it moves. This difference is especially noticeable when moving along thin "tubes", the diameter of which is comparable to the size of the molecule. An illustrative, although not accurate, example is the launch of paper boats in a stream: a small boat, carried away by a stream of water, will move quickly; but if you fold a large vessel out of paper the size of a stream bed, it will go slower, constantly touching the banks. Returning to uranium, we can say that the target isotope with 235 nucleons in the nucleus will move along the "tube" faster than uranium-238. At the exit from it, a gas enriched in a fissile isotope will be obtained. The only question is how to turn uranium into gas and where to get such a thin "tube".
"Gasification" of uranium is a mandatory requirement of technology based on the theory of gases. There is nothing you can do about it. But after all, all uranium compounds are solids, which are difficult to melt, let alone evaporate. Although, if you think about it, there is one very successful compound - uranium hexafluoride, in which uranium is surrounded by six fluorine atoms. It readily turns into gas already at 56 ° C, and bypassing the liquid state. In physics, such a process is usually called sublimation or sublimation. This phenomenon has long been known, and there is nothing surprising in it. Sublimation, for example, is used by country housewives who dry clothes in the cold - ice evaporates in dry air, simply passing through the liquid state.
So you can imagine a uranium hexafluoride molecule
It turns out that uranium hexafluoride is very convenient from a technological point of view. At normal temperatures, it is solid and can be transported in special containers. It transforms into gas at a low temperature. Well, under a certain pressure, heated hexafluoride becomes a liquid that can be pumped through pipelines.
Another fortunate circumstance is that natural fluorine consists of only one isotope - fluorine-19. This means that the difference in the masses of molecules of uranium-235 hexafluoride and uranium-238 hexafluoride is determined exclusively by uranium isotopes. Otherwise, the separation would be too difficult or even impossible, since fluorine would have an excessive effect on the mass of the molecules.
The production of uranium hexafluoride in Russia is carried out by conversion - fluorination of various uranium compounds, for example, yellow cake or a mixture of oxides received from uranium mining enterprises. Molecular fluorine for these purposes is obtained from the natural mineral fluorite. It is treated with sulfuric acid to form hydrofluoric (hydrofluoric) acid, the electrolysis of which gives fluorine.
It is interesting that fluorination is at the same time the fourth stage of uranium purification, since the fluorides of most harmful impurities are not highly volatile: uranium in the form of hexafluoride "flies away" from them into the gas phase.
Uranium hexafluoride has one big drawback: it is corrosive and toxic. First, when it comes into contact with water or moisture in the air, poisonous hydrofluoric acid is released. Secondly, uranium itself is a general cellular poison that affects all organs. (Interestingly, its toxicity has chemical nature, and is practically unrelated to radioactivity). Therefore, uranium hexafluoride, which combines two hazards at once, should be transported and stored in special metal containers and under vigilant supervision. At the same time, the safety of the population and the environment is ensured.
So there is gas; and what about the thin "tubes"? Porous partitions - plates pierced by many very small pores - turned out to be a suitable solution. The diameter of the latter should be on the order of ten nanometers for the molecules to pass through them almost one by one. The need to manufacture partitions with pores of such a small size caused certain difficulties, but nevertheless the problem was solved using special approaches - sintering nickel or selective dissolution of one of the metals that make up the bimetallic alloy.
If a box with such a porous partition is made and uranium hexafluoride is pumped into it, molecules with a light isotope will pass through the partition a little faster. In other words, behind it, uranium hexafluoride will be slightly enriched in a fissile isotope. If you send gas to the next box of the same, the degree of enrichment will increase, and so on. True, to obtain a high degree of enrichment, cascades of thousands (!) Of boxes installed one after another, called steps, are needed. And how to make uranium go along these steps? Only by pumping it with the help of many compressors. Hence the disadvantages of the method: huge energy consumption, the need to build millions of square meters of production space - the length of the workshop can reach one kilometer - and the use of expensive materials. True, all this is covered by a really high performance. That is why the gaseous diffusion enrichment technology has long remained the main one for such atomic giants as the United States, France and China, which later joined them. Only in recent years have they begun an active transition to more economical gas centrifugation technology.
Scheme of operation of the gaseous diffusion stage
In the 1960s, the Angarsk Electrolysis Chemical Plant (Irkutsk Region, Russia), which was engaged in uranium enrichment using gaseous diffusion technology, consumed about one percent (!) Of all electricity produced in the Soviet Union. The energy was supplied to it by the Bratsk and Irkutsk HPPs. In fact, it was the largest consumer of electricity in the USSR.
In general, the first experience showed that gas diffusion can solve the problem, but at too high a cost. The Soviet Union, embroiled in an arms race, needed a more efficient and less energy-consuming uranium enrichment technology. It was not so easy for a war-weakened state to keep up with the United States with its powerful economic and energy potential. Affected, among other things, by the lack of capacity for the production of electricity in the European part of the country: this is why enrichment plants were built in Siberia, where they could receive power from large hydroelectric power plants. Still, gaseous diffusion plants consumed too much energy, not allowing an increase in the production of enriched uranium. Therefore, the USSR had to become a pioneer in the industrial application of an alternative technology - gas centrifuge.
Gas centrifugation consists in spinning a drum filled with gaseous uranium hexafluoride at a high speed. Under the action of centrifugal force, the heavier uranium-238 hexafluoride is "squeezed" to the wall of the drum, and uranium-235 hexafluoride, a lighter compound, remains at its axis. With the help of special tubes, it is possible to pick up slightly enriched uranium from the center of the drum, and slightly depleted uranium from the periphery.
Gas centrifuge operation diagram
Technically speaking, the drum just mentioned is the rotating part (rotor) of a gas centrifuge. It spins non-stop in an evacuated casing and rests with a needle on a thrust bearing made of a very durable material - corundum. The choice of material is not surprising, since the rotor speed can exceed 1,500 revolutions per second - a hundred times faster than the drum of the washing machine. A fragile substance will not withstand such an impact. Additionally, so that the thrust bearing does not wear out and does not collapse, the rotor is suspended in a magnetic field so that it barely presses on the corundum with its needle. This technique, as well as the high precision in the manufacture of centrifuge parts, allows it to rotate quickly, but almost silently.
As in the case of gas diffusion, one centrifuge is not a warrior in the field. To achieve the required degree of enrichment and productivity, they are combined into huge cascades of tens of thousands (!) Machines. Simplistically, each centrifuge is connected to its two neighbors. Uranium hexafluoride with a reduced uranium-235 content, taken from the wall in the upper part of the rotor, is sent to the previous centrifuge; and gas slightly enriched in uranium-235, which is taken from the axis of rotation in the lower part of the rotor, goes to the next machine. Thus, more and more enriched uranium is fed to each subsequent stage until a product of the required quality is obtained.
Cascades of gas centrifuges receding into the distance
Today centrifugal separation is the main method of uranium enrichment, since this technology requires about fifty times less electricity than gaseous diffusion. In addition, centrifuges are less bulky than diffusion units, making it easier to scale up production. The centrifugation method is used in Russia, Great Britain, Germany, the Netherlands, Japan, China, India, Pakistan, Iran; the transition to gas centrifuge technology in France and the USA is practically completed. In other words, there is no more room for gas diffusion.
Thanks to a long history of use and improvement, Russian gas centrifuges are the best in the world. For half a century, nine generations of high-speed cars have already changed, which gradually became more powerful and reliable. Thanks to this, the USSR successfully withstood the "nuclear race" with the United States, and when the most important task was solved, free capacities appeared. As a result, our country has become a world leader not only in the development and production of gas centrifuges, but also in the provision of commercial services for uranium enrichment.
Our gas centrifuges:
Traditionally, they have a height of half a meter to one meter, a diameter of ten to twenty centimeters;
They are arranged one above the other in three to seven tiers in order to save space;
They can work non-stop for up to thirty years, the record is thirty-two years.
The rotational speed of the rotor of a gas centrifuge is such that after a power outage it will rotate by inertia for about two months!
The boom in gas centrifuge technology is associated with the active development of nuclear power. Nuclear plants are commercial enterprises profit-oriented and therefore need cheap fuel and hence cheap enrichment technologies. This requirement gradually buried gas diffusion.
But gas centrifugation shouldn't rest on its laurels either. V recent times more and more often one can hear about laser enrichment - a method that has been known for more than forty years. It turns out that with the help of a finely tuned laser, uranium-235 compounds can be selectively ionized, that is, converted into charged particles. In this case, uranium-238 compounds are not ionized, remaining uncharged. The resulting ions can be easily separated from neutral molecules by chemical or physical means, for example, by attracting them with a magnet or a charged plate (collector).
Possible scheme of operation of the uranium laser enrichment facility
Apparently, laser enrichment is a very effective technology, but its economic indicators are still a mystery. All previous attempts to move from a laboratory version to industrial use "crashed on the rocks" of insufficient productivity and short equipment service life. Currently, a new attempt to create such a production is being made in the United States. But even if it turns out to be successful, the question remains economic efficiency... The enrichment services market will accept a new technology only if it is significantly cheaper than the existing one. But gas centrifuges have not yet reached the ceiling of their capabilities. Therefore, the immediate prospects for laser enrichment remain very vague.
There are a number of other methods of uranium enrichment: thermal diffusion, aerodynamic separation, ionic process, but they are practically not used.
When it comes to uranium enrichment technologies, it is imperative to remember that they open the way not only to nuclear fuel, but also to the bomb. The creation of ever more efficient and compact production facilities entails the threat of the proliferation of nuclear weapons. In principle, the development of technology can lead to a situation where the bomb will be manufactured by states with, to put it mildly, unstable regimes or even large terrorist organizations. And if a gaseous diffusion or gas centrifuge plant is difficult to build unnoticed, and to launch them will require the import of large volumes of characteristic materials and equipment, then laser enrichment practically guarantees secrecy. In general, the risk to the existing fragile world is increasing.
Uranium enrichment plants produce an enriched uranium product (UUU) - uranium hexafluoride with the required degree of enrichment. It is placed in special containers and sent to nuclear fuel factories. But at the same time, depleted uranium hexafluoride (DUHF) with an enrichment degree of 0.3% is formed at enrichment enterprises, which is lower than that of natural uranium. In other words, it is practically pure uranium-238. Where does it come from? Basically, the beneficiation process resembles the separation of valuable minerals from waste rock. DUHF is a kind of waste rock from which uranium-235 was withdrawn, although not completely. (100% separation of the fissile isotope from uranium-238 is economically disadvantageous). How much depleted uranium hexafluoride is formed? This depends on the degree of uranium enrichment required. For example, if it is 4.3%, as in the fuel of VVER reactors, then out of ten kilograms of uranium hexafluoride, which has a natural isotopic composition (0.7% uranium-235), only one kilogram of OUP and nine kilograms of DUHF are obtained. In a word, quite a lot. Over the entire period of operation of the enrichment plants at their sites, more than one and a half million tons of DUHF have been accumulated in special containers, of which about seven hundred thousand tons are in Russia. There is a different attitude to this substance in the world, but the prevailing opinion about DUHF as a valuable strategic raw material (see Chapter 7).
To fabricate - in the good sense of the word
The manufacture (fabrication) of nuclear fuel begins with the chemical conversion of an enriched uranium product into uranium dioxide. This process can be carried out in two main ways. The first of them is called "wet" technology and consists in dissolving hexafluoride in water, precipitation of poorly soluble compounds under the action of alkali and their calcination in a hydrogen atmosphere. The second technology - "dry" - is more preferable, since it does not produce liquid radioactive waste: uranium hexafluoride is burned in a hydrogen flame.
In both cases, uranium dioxide powder is obtained, which is pressed into small tablets and sintered in ovens at a temperature of about 1750 o C to give them strength, since the tablets have to "work" under conditions of high temperature and radiation. Then the tablets are processed on grinding machines using diamond tools. This step is necessary because the tablet size and surface quality must be very precise. Flaws in the manufacture of a separate tablet can lead to damage to the fuel in the reactor during its thermal expansion and, as a consequence, to a deterioration of the radiation situation at the nuclear power plant. Therefore, all uranium dioxide tablets undergo careful control and then go to a special box, where the machine places them in tubes made of zirconium with a small admixture of niobium.
A tube filled with pellets is called a fuel element or, in short, a fuel element. Then, to remove corrosive gases, the fuel element is evacuated, that is, the air is “sucked” from the tube, filled with an inert gas - the purest helium - and welded. The last stage of the nuclear fuel fabrication process is the assembly of fuel elements into a fuel assembly (FA) using spacer grids. They are needed so that the structure is strong and the fuel rods do not touch each other. Otherwise, at the point of contact, the shell may burn out, while the fuel will be exposed and come into contact with water, which is completely undesirable.
The sequence of operations in the production of nuclear fuel
Spacer grilles
So, a fuel assembly is a "bundle" of zirconium fuel rods, inside which there is a nuclear fuel - uranium dioxide enriched in a fissile isotope. It is necessary to clarify this choice of materials. In a nuclear reactor, the fuel assembly is in conditions of high temperature and a powerful flow of ionizing radiation, and is also washed from the outside by very hot pressurized water. Therefore, the elements of nuclear fuel must have chemical and radiation resistance, conduct heat well and expand very weakly when heated, otherwise a crack may appear in the cladding of the fuel element. Uranium dioxide and zirconium meet these requirements. However, it should be recalled once again that uranium dioxide pellets are inside the fuel rods and come into contact with water only through the fuel rod cladding, but not directly. Direct interaction with the coolant is extremely undesirable and occurs only when the zirconium shells are destroyed - for example, when cracks appear in them. In this case, the radioactive fission products of uranium contained in nuclear fuel begin to dissolve in water, which leads to an increase in its radioactivity and deterioration of the radiation situation at the nuclear power plant. For this reason, the fabrication of nuclear fuel is a complex and high-precision job that requires precision and constant monitoring.
From a radiation point of view, the production of nuclear fuel does not pose a particular danger. The risk is even less than in ore mining, since the purification process removes all associated radioactive substances from uranium.
However, when working with enriched uranium, critical mass can accumulate and, as a result, a self-sustaining chain reaction can arise, which was already discussed in Chapter 2. This can occur as a result of an error, violation of the rules of work, or even by accident. A total of sixty such accidents have been registered in the world, of which thirty-three in the USA and nineteen in the USSR / Russia. Here are two examples of domestic incidents.
July 14, 1961, Siberian Chemical Combine (concentration production). The formation of a critical mass as a result of the accumulation of highly enriched uranium hexafluoride (22.6%) in the oil in the expansion tank of the vacuum pump. As a result of the burst of radiation that accompanied the chain reaction that had arisen, the operator received a significant dose of radiation and suffered radiation sickness, albeit in a relatively mild form.
May 15, 1997 Novosibirsk plant of chemical concentrates (nuclear fuel production). The formation of a critical mass as a result of the accumulation of a deposit of highly enriched (90%) uranium at the bottom of two adjacent containers for collecting solutions due to their deformation. Fortunately, the radiation doses were negligible.
What's the conclusion? Enriched uranium must be handled with extreme caution, observing all safety requirements and, as they say, "including the head", that is, calculating the possible risks in advance.
In conclusion, we can give the approximate parameters of fuel assemblies used at Russian NPPs with VVER-1000 reactors.
The fuel pellet is a cylinder with a height of 9 to 12 millimeters and a diameter of 7.6 millimeters. It consists of uranium dioxide, the enrichment of which ranges from 3.3 to 5.0%.
The pellets are placed in a fuel element made of zirconium containing 1% niobium, about four meters long and 9.1 mm in diameter. The wall thickness of the fuel element is only 0.65 mm, therefore, with this length, it requires extremely careful handling. The fuel element is not completely filled with pellets: the height of the pellet layer is about 3.5 meters, and their total weight is about 1.6 kilograms, with 62 grams of uranium-235.
The fuel assembly (FA) is assembled from 312 fuel rods using 12-15 spacer grids. The height of the fuel assembly reaches almost 4.6 meters, and its weight is 760 kg. At the same time, the mass of uranium dioxide is about half a ton, the rest falls on zirconium and other metals. Seen from above, the assembly is a hexagon with a face size of 235 millimeters. Each assembly has 19 channels for reactor control rods containing boron carbide, an element that absorbs neutrons well.
The reactor holds 163 fuel assemblies, which corresponds to 80 tons of uranium dioxide, which is enough for 4 years of reactor operation.
Fuel assemblies for various types of reactors
Options are possible
So, the most common fuel for nuclear power plants is pelleted uranium dioxide, in which uranium is enriched in a fissile isotope (uranium-235). However, there are other types of nuclear fuel.
After uranium dioxide, the most common is a mixed oxide fuel known as MOX fuel. Currently, the main production is MOX fuel, which is a mixture of uranium and plutonium-239 oxides. This fuel makes it possible to use the excess amount of weapons-grade plutonium-239 accumulated during the "nuclear race" to generate electricity.
Uranium metal can also be used as a nuclear fuel. Its advantages are high thermal conductivity and maximum concentration of fissile nuclei - there are simply no other elements in the fuel. At the same time, uranium as a metal has the worst radiation, chemical and heat resistance in comparison with dioxide, therefore it is extremely rarely used in its pure form. To improve the parameters of metallic fuel, a little molybdenum, aluminum, silicon, and zirconium are added to uranium. Today, uranium metal and its alloys are used only in research reactors.
Instead of uranium dioxide, it is possible to use uranium nitride, that is, its combination with nitrogen. Nitride fuel has a higher thermal conductivity compared to dioxide fuel, and a comparable melting point (2855 ° C). Uranium nitride is considered a promising fuel for advanced reactors. In our country, the closest attention is paid to nitride fuel, since it is planned to use it in the next generation of fast neutron reactors.
Uranium is capable of forming compounds with carbon - carbides. The possibility of using carbides as fuel for reactors was intensively studied in the sixties and seventies of the last century. However, in the last period to this type fuel re-emerged interest associated with the development of plate fuel elements and micro-fuel elements. The positive features of carbides are good thermal conductivity, high melting point, high hardness, chemical and thermal stability, as well as compatibility with ceramic coatings, which is especially important for microfuel. Uranium carbide fuel may prove to be the best option for certain types of next generation reactors, in particular for gas-cooled fast reactors.
But still, the overwhelming majority of reactors on Earth still operate on nuclear fuel made from uranium dioxide. The power of tradition, so to speak.
Russian fuel cycle
Now, having familiarized yourself with the peculiarities of the mining and processing industries, it is worth taking a quick look at the history and current state of our domestic fuel cycle. We need to start, of course, with uranium mining.
At first, uranium ores were of interest to domestic scientists only as a source of radium. In 1900, Professor I.A. Antipov made at a meeting of the Petersburg Mineralogical Society a report on the discovery of the uranium mineral in samples brought from Fergana, from the Tyuya-Muyun mountain range. Later, this mineral was named tyuyamunite. In 1904, exploration work began at this deposit, in 1908 a pilot plant for the processing of uranium ore was built in St. Petersburg, and in 1913 an international joint-stock company for the extraction of Tyuyamuyun radium.
When the First began World War, work at the mine practically stopped, and only in 1922 an expedition of eight specialists was sent to Tyuya-Muyun. In the same 1922, in difficult post-revolutionary conditions, surrounded by bands of Basmachi, it was possible to re-establish industrial ore mining. It lasted until 1936, when abundant underground waters at a depth of two hundred meters interrupted the development of the field. However, this problem did not become critical, since the extraction of radium was established at the "Water field" on the Ukhta River - radioactive metal was extracted from underground salt waters. Few people were interested in uranium in itself in those years, since it was practically not used in industry.
A new surge of interest in uranium deposits occurred in the early 1940s, when the USSR faced the need to respond to the nuclear threat posed by the United States - that is, when the need arose to create domestic nuclear weapons.
Uranium for the first Soviet atomic bomb was literally bit by bit collected throughout the country and beyond. In 1943, uranium mining began at the tiny, by modern standards, Taboshar mine in Tajikistan, with a capacity of only 4 tons of uranium salts per year. Moreover, according to the memoirs of P.Ya. Antropov, the first minister of geology of the USSR, “uranium ore for processing along the mountain paths of the Pamirs was carried in sacks on donkeys and camels. Then there were no roads or proper equipment. "
In 1944-1945, as Europe was liberated from the Nazis, the USSR gained access to uranium ore from the Goten deposit in Bulgaria, the Jachymov mines in Czechoslovakia, and the mines of German Saxony. In addition, in 1946, the Tyuya-Muyun mine was re-launched, but it did not make a special contribution to the common cause.
In the 1950s, the Lermontov production association "Almaz" began mining uranium at the mines in the Beshtau and Byk mountains (Stavropol Territory). At the same time, the development of deposits in South Kazakhstan and Central Asia began.
After 1991, most of the developed fields ended up outside the borders of Russia, in independent states. From that moment on, the main production of uranium has been carried out by the mine method at the Priargunsky industrial mining and chemical association (Trans-Baikal Territory). In addition, two enterprises using in-situ leaching technology are gradually gaining strength - Khiagda (Republic of Buryatia) and Dalur (Kurgan Region). Production facilities in Yakutia are being designed. There are also promising regions for production - Transbaikal, West Siberian, North European ...
In terms of explored uranium reserves, Russia ranks third in the world.
Russian uranium mining enterprises are managed by the ARMZ Uranium Holding Company (www.armz.ru), owned by Rosatom, but the State Corporation also has foreign assets controlled by the international company Yurium One Inc. (www.uranium1.com). Thanks to the activities of these two organizations, Rosatom has become the third largest producer of uranium compounds in the world.
Situation in the global natural uranium production market (2014)
The baton from the mining enterprises is taken over by a whole complex of industries for refining, conversion and enrichment of uranium, as well as for the fabrication of nuclear fuel. Most of them come from the time period and the fifties of the last century - the time of active accumulation of nuclear weapons. Today they work for a purely peaceful industry - nuclear energy, and provide their services to foreign companies.
There are four enrichment plants in Russia, some of them also carry out operations for the final purification (refining) and fluorination (conversion) of uranium compounds.
The first gaseous diffusion uranium enrichment plant D-1 in Sverdlovsk-44 was put into operation in November 1949. At first, its products had to be additionally enriched at the SU-20 unit of the future Elektrokhimpribor plant in Sverdlovsk-45 (Lesnoy), but after a couple of years D-1 began to cope on its own and began to grow. And since 1967, the replacement of diffusion cascades with centrifuge cascades began. Today, on the site of the dismantled D-1, there is the world's largest uranium enrichment enterprise - the Ural Electrochemical Plant (Novouralsk, Sverdlovsk Region).
In 1953, the future Siberian Chemical Combine (Seversk, Tomsk Region) began operation in Tomsk-7, which, since 1973, began to gradually switch to gas centrifuge technology. The first enriched uranium from the Angarsk Electrolysis Chemical Plant (Angarsk, Irkutsk Region) was obtained in 1957, and the replacement of diffusion apparatus with centrifuges started in 1985. Finally, 1962 became the year of the launch of the Electrochemical Plant in Krasnoyarsk-45 (now Zelenogorsk, Krasnoyarsk Territory). A couple of years later, the first centrifuges were also installed there.
This brief information, of course, does not reflect the realities of that difficult era. Although from the secret, "numbered" names of closed cities and the vague names of factories, one can understand that the Soviet Union carefully kept its secrets of enrichment. Nevertheless, the locations of the main production facilities became known to American intelligence. But the active transition to gas centrifuge technology, she, as they say, missed. Perhaps this was the reason for some complacency of our competitors: not knowing that a more productive and efficient technology was being introduced in the USSR, the States adhered to the initially chosen method - gas diffusion. It is obvious that the current situation played into the hands of the Soviet Union and made it possible to quickly achieve nuclear parity. At the same time, the pioneering developments of Soviet scientists and engineers to create high-performance gas centrifuges did not go to waste, leading Russia to a leading position in the world market for uranium enrichment and centrifuge production.
The enriched uranium product from four plants goes to the Machine-Building Plant (Elektrostal, Moscow Region) and the Novosibirsk Chemical Concentrates Plant (Novosibirsk, the eponymous region), where a full cycle of nuclear fuel production is performed. Zirconium for fuel rods and other structural materials for fuel assemblies is supplied by the Chepetsk Mechanical Plant (Glazov, Udmurt Republic), the only enterprise in Russia and the third in the world to manufacture zirconium products.
The manufactured fuel assemblies are supplied to Russian and foreign nuclear power plants, and are also used in reactors for other purposes.
Enterprises for refining, conversion and enrichment of uranium, fabrication of nuclear fuel, production of gas centrifuges, as well as design and research organizations are united under the TVEL Fuel Company of Rosatom (www.tvel.ru).
As a result of many years of successful work of this company and its member enterprises, Rosatom confidently tops the list of the largest service providers in the field of uranium enrichment (36% of the world market).
There is a nuclear fuel bank in Angarsk - a guarantee stock that can be purchased by a country deprived, for whatever reason, of the opportunity to buy uranium on the free market. From this reserve it will be able to make fresh nuclear fuel and ensure the uninterrupted operation of its nuclear power.
The share of Rosatom in the world nuclear fuel market is 17%, due to which TVEL fuel is loaded into every sixth power reactor on Earth. Deliveries go to Hungary, Slovakia, Czech Republic, Bulgaria, Ukraine, Armenia, Finland, India and China.
Above - the world market for uranium enrichment (2015), below - the world market for fuel fabrication (2015)
Open or closed?
It can be noted that this chapter did not address the issues of nuclear fuel production for research reactors, as well as reactors installed on nuclear submarines and icebreakers. The entire discussion was devoted to nuclear fuel used in nuclear power plants. However, this was not done by accident. The point is that there are no fundamental differences between the sequence of fuel production for nuclear power plants and, for example, nuclear submarines. Of course, there may be deviations in technology associated with the specifics of ship and research reactors. For example, the former should be small in size and, at the same time, quite powerful - this is a completely natural requirement for an icebreaker and, moreover, for a maneuverable nuclear submarine. The necessary indicators can be achieved by increasing the enrichment of uranium, that is, by increasing the concentration of fissile nuclei - then less fuel will be needed. This is exactly what they do: the degree of enrichment of uranium used as fuel for ship reactors is in the region of 40% (depending on the project, it can vary from 20 to 90%). In research reactors, a common requirement is to achieve maximum neutron flux power, and the number of neutrons in a reactor is also directly related to the number of fissile nuclei. Therefore, in installations intended for scientific research, highly enriched uranium with a much higher content of uranium-235 is sometimes used than in the fuel of nuclear power reactors. But the enrichment technology does not change from this.
The design of the reactor can determine the chemical composition of the fuel and the material from which the fuel element is made. Currently, the main chemical form of fuel is uranium dioxide. As for the fuel elements, they are predominantly zirconium, but, for example, stainless steel fuel elements are produced for the BN-600 fast neutron reactor. This is due to the use of liquid sodium in BN reactors as a coolant, in which zirconium degrades (corrodes) faster than stainless steel... Nevertheless, the essence of the nuclear fuel fabrication process remains the same - uranium dioxide powder is synthesized from the enriched uranium product, which is pressed into pellets and sintered, the pellets are placed in fuel rods, and the fuel rods are assembled into fuel assemblies (FA).
Moreover, if we consider the nuclear fuel cycles of different countries, it turns out, for example, that in Russia uranium compounds are directly fluorinated with molecular fluorine during conversion, and abroad they are first treated with hydrofluoric acid and only then with fluorine. The difference can be found in chemical composition solutions for "opening" ore, sorbents and extractants; the parameters of the processes may differ ... But the scheme of the nuclear fuel cycle does not change from this. The fundamental difference lies only between its open (open) and closed (closed) versions: in the first case, after “work” at a nuclear power plant, the fuel is simply isolated from the environment in a deep repository, and in the latter, it is processed with the extraction of valuable components (see chapter 7). Russia is one of the few countries implementing a closed cycle.
An example of a closed fuel cycle indicating the role of TVEL Fuel Company of Rosatom
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