Nuclear reactor definition. Nuclear reactor. What is a nuclear reactor

Chapter VIII The principle of operation and capabilities of a nuclear reactor I. The design of a nuclear reactor A nuclear reactor consists of the following five main elements: 1) nuclear fuel; 2) neutron moderator; 3) control system; 4) cooling system; 5) protective

Chapter 11 INTERNAL DEVICE OF DIELECTRIC §1. Molecular dipoles§2. Electronic polarization §3. polar molecules; orientational polarization§4. electric fields in the voids of the dielectric §5. Dielectric constant of liquids; Clausius formula - Mossotti§6.

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Industrial nuclear reactors were originally developed only in countries with nuclear weapons. The USA, USSR, Great Britain and France actively explored different versions of nuclear reactors. However, subsequently, three main types of reactors began to dominate in the nuclear power industry, differing mainly in fuel, coolant used to maintain the required core temperature, and moderator used to reduce the speed of neutrons released during the decay process and necessary to maintain the chain reaction.

Among them, the first (and most common) type is the enriched uranium reactor, in which both the coolant and the moderator are ordinary or “light” water (light water reactor). There are two main varieties of a light water reactor: a reactor in which the steam that rotates the turbines is generated directly in the core (boiling water reactor), and a reactor in which steam is generated in an external, or second, circuit connected to the primary circuit by heat exchangers and steam generators (VVER , see below). The development of a light water reactor began as early as the programs of the US armed forces. Thus, in the 1950s, the General Electric and Westinghouse companies developed light water reactors for submarines and aircraft carriers of the US Navy. These firms were also involved in the implementation of military programs for the development of technologies for the regeneration and enrichment of nuclear fuel. In the same decade, the graphite-moderated boiling water reactor was developed in the Soviet Union.

The second type of reactor that I found practical use, is a gas-cooled reactor (with a graphite moderator). Its creation was also closely associated with early nuclear weapons development programs. In the late 1940s and early 1950s, Great Britain and France, striving to create their own atomic bombs, focused on the development of gas-cooled reactors, which produce weapons-grade plutonium quite efficiently and, moreover, can operate on natural uranium.

The third type of reactor that has had commercial success is a reactor in which both the coolant and the moderator are heavy water, and the fuel is also natural uranium. At the beginning of the nuclear age, the potential benefits of a heavy water reactor were explored in a number of countries. However, then the production of such reactors was concentrated mainly in Canada, in part because of its vast reserves of uranium.

There are currently five types of nuclear reactors in the world. These are the VVER reactor (Water-Water Power Reactor), RBMK (High Power Channel Reactor), heavy water reactor, ball bed reactor with gas circuit, fast neutron reactor. Each type of reactor has design features that distinguish it from others, although, of course, individual design elements can be borrowed from other types. VVER were built mainly on the territory former USSR and in Eastern Europe, there are many RBMK-type reactors in Russia, countries Western Europe and South-East Asia, heavy water reactors were mainly built in America.

VVER. VVER reactors are the most common type of reactors in Russia. Very attractive are the low cost of the moderator coolant used in them and the relative safety of operation, despite the need to use enriched uranium in these reactors. From the very name of the VVER reactor it follows that both the moderator and the coolant are ordinary light water. Uranium enriched to 4.5% is used as fuel.

RBMK. RBMK is built on a slightly different principle than VVER. First of all, boiling occurs in its core - a steam-water mixture comes from the reactor, which, passing through the separators, is divided into water returning to the reactor inlet, and steam, which goes directly to the turbine. The electricity generated by the turbine is spent, as in the VVER reactor, also for the operation of circulation pumps. Its schematic diagram is in Fig.4.

The electrical power of the RBMK is 1000 MW. NPPs with RBMK reactors make up a significant share in the nuclear power industry. So, they are equipped with Leningrad, Kursk, Chernobyl, Smolensk, Ignalina nuclear power plants.

When comparing various types of nuclear reactors, it is worth stopping at the two most common types of these devices in our country and in the world: VVER and RBMK. The most fundamental differences are: VVER - pressure vessel reactor (pressure is maintained by the reactor pressure vessel); RBMK - channel reactor (pressure is maintained independently in each channel); in VVER, the coolant and moderator are the same water (an additional moderator is not introduced), in RBMK, the moderator is graphite, and the coolant is water; in VVER, steam is generated in the second vessel of the steam generator; in RBMK, steam is generated directly in the reactor core (boiling water reactor) and goes directly to the turbine - there is no second circuit. Due to the different structure of the active zones, the operating parameters of these reactors are also different. For the safety of the reactor, such a parameter as reactivity factor- it can be figuratively represented as a value showing how changes in one or another parameter of the reactor will affect the intensity of the chain reaction in it. If this coefficient is positive, then with an increase in the parameter by which the coefficient is given, the chain reaction in the reactor, in the absence of any other influences, will increase and at the end it will become possible to switch to an uncontrollable and cascade increasing reaction - the reactor will accelerate. During the acceleration of the reactor, intense heat release occurs, leading to the melting of the heat emitters, the flow of their melt into the lower part of the core, which can lead to the destruction of the reactor vessel and the release of radioactive substances into the environment.

Table 13 shows the reactivity indicators for RBMK and VVER.

In a VVER reactor, when steam appears in the core or when the coolant temperature rises, leading to a decrease in its density, the number of collisions of neutrons with atoms of the coolant molecules decreases, the moderation of neutrons decreases, as a result of which all of them leave the core without reacting with other nuclei. The reactor stops.

To sum up, the RBMK reactor requires less fuel enrichment, has a better ability to produce fissile material (plutonium), has a continuous operating cycle, but is more potentially dangerous in operation. The degree of this danger depends on the quality of the emergency protection systems and the qualifications of the operating personnel. In addition, due to the lack of a secondary circuit, RBMK has more radiation emissions into the atmosphere during operation.

heavy water reactor. In Canada and America, the developers of nuclear reactors, in solving the problem of maintaining a chain reaction in a reactor, preferred to use heavy water as a moderator. Heavy water has very low neutron absorption and very high moderating properties, exceeding those of graphite. As a result, heavy water reactors operate on unenriched fuel, which makes it possible not to build complex and dangerous enterprises for uranium enrichment.

Ball bed reactor. In a spherical-filled reactor, the active zone has the shape of a ball, into which fuel elements, also spherical, are filled. Each element is a graphite sphere in which particles of uranium oxide are interspersed. Gas is pumped through the reactor - carbon dioxide CO2 is most often used. The gas is supplied to the core under pressure and subsequently enters the heat exchanger. The reactor is controlled by absorber rods inserted into the core.

Fast neutron reactor. A fast neutron reactor is very different from all other types of reactors. Its main purpose is to ensure the expanded breeding of fissile plutonium from uranium-238 with the aim of burning all or a significant part of natural uranium, as well as existing stocks of depleted uranium. With the development of power engineering of fast neutron reactors, the problem of self-sufficiency can be solved nuclear power fuel.

There is no moderator in a fast neutron reactor. In this regard, not uranium-235 is used as fuel, but plutonium and uranium-238, which can be fissile from fast neutrons. Plutonium is needed to provide sufficient neutron flux density, which uranium-238 alone cannot provide. The heat release of a fast neutron reactor is ten to fifteen times greater than the heat release of slow neutron reactors, and therefore, instead of water (which simply cannot cope with such an amount of energy for transfer), sodium melt is used (its inlet temperature is 370 degrees, and at the outlet - 550, At present, fast neutron reactors are not widely used, mainly due to the complexity of the design and the problem of obtaining sufficiently stable materials for structural parts.In Russia, there is only one reactor of this type (at Beloyarsk NPP).It is believed that such reactors have a great future.

To summarize, it is worth saying the following. VVER reactors are quite safe to operate, but require highly enriched uranium. RBMK reactors are safe only with proper operation and well-designed protection systems, but they are capable of using low-enriched fuel or even spent fuel from VVERs. Heavy water reactors are good for everyone, but it is painfully expensive to produce heavy water. The technology for the production of reactors with pebble bed is not yet well developed, although this type of reactor should be recognized as the most suitable for wide application, in particular, due to the absence of catastrophic consequences in a reactor runaway accident. Fast neutron reactors are the future of fuel production for nuclear energy, these reactors use nuclear fuel most efficiently, but their design is very complex and still unreliable.

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

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

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

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

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

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

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

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

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

What is a nuclear reactor made of?

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

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

Fuel for nuclear power plants

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

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

Enriched uranium

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

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

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

uranium powder

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

Uranium tablets

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

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

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

What is TVEL and TVS?

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

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

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

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

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

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

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

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

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

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

Nuclear power plant: principle of operation

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

IT IS IMPORTANT TO KNOW:

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

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

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

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

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

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

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

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

Types of nuclear reactors

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

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

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

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

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

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

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

A few facts about nuclear reactors...

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

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

Pressurized water reactor

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

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

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

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

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

Nuclear power plant safety

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

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

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

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

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

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

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

The nuclear reactor works smoothly and accurately. Otherwise, as you know, there will be trouble. But what's going on inside? Let's try to formulate the principle of operation of a nuclear (atomic) reactor briefly, clearly, with stops.

In fact, the same process is going on there as in a nuclear explosion. Only now the explosion occurs very quickly, and in the reactor all this stretches for a long time. In the end, everything remains safe and sound, and we get energy. Not so much that everything around immediately smashed, but quite enough to provide electricity to the city.

how a reactor worksNPP cooling towers
Before you understand how a controlled nuclear reaction works, you need to know what a nuclear reaction is in general.

A nuclear reaction is a process of transformation (fission) atomic nuclei when interacting with elementary particles and gamma rays.

Nuclear reactions can take place both with absorption and with the release of energy. Second reactions are used in the reactor.

A nuclear reactor is a device whose purpose is to maintain a controlled nuclear reaction with the release of energy.

Often a nuclear reactor is also called a nuclear reactor. Note that there is no fundamental difference here, but from the point of view of science, it is more correct to use the word "nuclear". There are now many types of nuclear reactors. These are huge industrial reactors designed to generate energy at power plants, nuclear submarine reactors, small experimental reactors used in scientific experiments. There are even reactors used to desalinate seawater.

The history of the creation of a nuclear reactor

The first nuclear reactor was launched in the not so distant 1942. It happened in the USA under the leadership of Fermi. This reactor was called the "Chicago woodpile".

In 1946, the first Soviet reactor started up under the leadership of Kurchatov. The body of this reactor was a ball seven meters in diameter. The first reactors did not have a cooling system, and their power was minimal. By the way, the Soviet reactor had an average power of 20 watts, while the American one had only 1 watt. For comparison: the average power of modern power reactors is 5 Gigawatts. Less than ten years after the launch of the first reactor, the world's first industrial nuclear power plant was opened in the city of Obninsk.

The principle of operation of a nuclear (atomic) reactor

Any nuclear reactor has several parts: core with fuel and moderator, neutron reflector, coolant, control and protection system. The isotopes of uranium (235, 238, 233), plutonium (239) and thorium (232) are most often used as fuel in reactors. The active zone is a boiler through which flows ordinary water(coolant). Among other coolants, “heavy water” and liquid graphite are less commonly used. If we talk about the operation of a nuclear power plant, then a nuclear reactor is used to generate heat. Electricity itself is generated in the same way as in other types of power plants - steam rotates a turbine, and the energy of movement is converted into electrical energy.

Below is a diagram of the operation of a nuclear reactor.

scheme of operation of a nuclear reactorScheme of a nuclear reactor at a nuclear power plant

As we have already said, the decay of a heavy uranium nucleus produces lighter elements and a few neutrons. The resulting neutrons collide with other nuclei, also causing them to fission. In this case, the number of neutrons grows like an avalanche.

Here it is necessary to mention the neutron multiplication factor. So, if this coefficient exceeds a value equal to one, a nuclear explosion occurs. If the value is less than one, there are too few neutrons and the reaction dies out. But if you maintain the value of the coefficient equal to one, the reaction will proceed for a long time and stably.

The question is how to do it? In the reactor, the fuel is in the so-called fuel elements (TVELs). These are rods that contain nuclear fuel in the form of small pellets. The fuel rods are connected into hexagonal cassettes, of which there can be hundreds in the reactor. Cassettes with fuel rods are located vertically, while each fuel rod has a system that allows you to adjust the depth of its immersion in the core. In addition to the cassettes themselves, there are control rods and emergency protection rods among them. The rods are made of a material that absorbs neutrons well. Thus, the control rods can be lowered to different depths in the core, thereby adjusting the neutron multiplication factor. The emergency rods are designed to shut down the reactor in the event of an emergency.

How is a nuclear reactor started?

We figured out the very principle of operation, but how to start and make the reactor function? Roughly speaking, here it is - a piece of uranium, but after all, a chain reaction does not start in it by itself. The fact is that in nuclear physics there is the concept of critical mass.

Nuclear fuelNuclear fuel

Critical mass is the mass of fissile material necessary to start a nuclear chain reaction.

With the help of fuel elements and control rods, a critical mass of nuclear fuel is first created in the reactor, and then the reactor is brought to the optimal power level in several stages.

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In this article, we have tried to give you general idea on the design and principle of operation of a nuclear (atomic) reactor. If you still have questions on the topic or the university asked a problem in nuclear physics - please contact the specialists of our company. We, as usual, are ready to help you solve any pressing issue of your studies. In the meantime, we are doing this, your attention is another educational video!

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Device and principle of operation

Power release mechanism

The transformation of a substance is accompanied by the release of free energy only if the substance has a reserve of energies. The latter means that the microparticles of the substance are in a state with a rest energy greater than in another possible state, the transition to which exists. Spontaneous transition is always prevented by an energy barrier, to overcome which the microparticle must receive some amount of energy from the outside - the energy of excitation. The exoenergetic reaction consists in the fact that in the transformation following the excitation, more energy is released than is required to excite the process. There are two ways to overcome the energy barrier: either due to the kinetic energy of the colliding particles, or due to the binding energy of the acceding particle.

If we keep in mind the macroscopic scales of the energy release, then the kinetic energy necessary for the excitation of reactions must have all, or at first at least some of the particles of the substance. This can only be achieved by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the value of the energy threshold that limits the course of the process. In the case of molecular transformations, that is chemical reactions, such an increase is usually hundreds of kelvins, in the case nuclear reactions is a minimum of 10 7 due to the very high height of the Coulomb barriers of the colliding nuclei. Thermal excitation of nuclear reactions has been carried out in practice only in the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

Excitation by the joining particles does not require a large kinetic energy, and, therefore, does not depend on the temperature of the medium, since it occurs due to unused bonds inherent in the particles of attractive forces. But on the other hand, the particles themselves are necessary to excite the reactions. And if again we have in mind not a separate act of reaction, but the production of energy on a macroscopic scale, then this is possible only when a chain reaction occurs. The latter arises when the particles that excite the reaction reappear as products of an exoenergetic reaction.

Design

Any nuclear reactor consists of the following parts:

• Core with nuclear fuel and moderator;
• Neutron reflector that surrounds the core;
• Chain reaction regulation system, including emergency protection;
• Radiation protection;
• Remote control system.

Physical principles of operation

See also main articles:

The current state of a nuclear reactor can be characterized by the effective neutron multiplication factor k or reactivity ρ , which are related by the following relationship:

These values ​​are characterized by the following values:

• k> 1 - the chain reaction increases in time, the reactor is in supercritical state, its reactivity ρ > 0;
• k < 1 - реакция затухает, реактор - subcritical, ρ < 0;
• k = 1, ρ = 0 - the number of nuclear fissions is constant, the reactor is in a stable critical condition.

Nuclear reactor criticality condition:

The conversion of the multiplication factor to unity is achieved by balancing the multiplication of neutrons with their losses. There are actually two reasons for losses: capture without fission and leakage of neutrons outside the breeding medium.

Obviously, k< k 0 , поскольку в конечном объёме вследствие утечки потери нейтронов обязательно больше, чем в бесконечном. Поэтому, если в веществе данного состава k 0 < 1, то цепная самоподдерживающаяся реакция невозможна как в бесконечном, так и в любом конечном объёме. Таким образом, k 0 определяет принципиальную способность среды размножать нейтроны.

k 0 for thermal reactors can be determined by the so-called "formula of 4 factors":

• η is the neutron yield per two absorptions.

The volumes of modern power reactors can reach hundreds of m³ and are determined mainly not by the conditions of criticality, but by the possibilities of heat removal.

Critical volume nuclear reactor - the volume of the reactor core in a critical state. Critical mass is the mass of the fissile material of the reactor, which is in a critical state.

Reactors with the lowest critical mass have aqueous solutions salts of pure fissile isotopes with a water neutron reflector. For 235 U this mass is 0.8 kg, for 239 Pu it is 0.5 kg. It is widely known, however, that the critical mass for the LOPO reactor (the world's first enriched uranium reactor), which had a beryllium oxide reflector, was 0.565 kg, despite the fact that the degree of enrichment in the 235 isotope was only slightly more than 14%. Theoretically, the smallest critical mass has, for which this value is only 10 g.

In order to reduce neutron leakage, the core is given a spherical or close to spherical shape, such as a short cylinder or cube, since these figures have the smallest ratio of surface area to volume.

Despite the fact that the value (e - 1) is usually small, the role of fast neutron multiplication is quite large, since for large nuclear reactors (K ∞ - 1)<< 1. Без этого процесса было бы невозможным создание первых графитовых реакторов на естественном уране.

To start a chain reaction, usually enough neutrons are produced during the spontaneous fission of uranium nuclei. It is also possible to use an external source of neutrons to start the reactor, for example, a mixture of and, or other substances.

iodine pit

Iodine pit - the state of a nuclear reactor after it has been shut down, characterized by the accumulation of the short-lived xenon isotope. This process leads to the temporary appearance of significant negative reactivity, which, in turn, makes it impossible to bring the reactor to its design capacity for a certain period (about 1-2 days).

Classification

By appointment

According to the nature of the use of nuclear reactors are divided into:

• Power reactors designed to produce electrical and thermal energy used in the energy sector, as well as for seawater desalination (desalination reactors are also classified as industrial). Such reactors were mainly used in nuclear power plants. The thermal power of modern power reactors reaches 5 GW. In a separate group allocate:
  • Transport reactors designed to supply energy to vehicle engines. The widest application groups are marine transport reactors used on submarines and various surface vessels, as well as reactors used in space technology.
• Experimental reactors, designed to study various physical quantities, the value of which is necessary for the design and operation of nuclear reactors; the power of such reactors does not exceed a few kW.
• Research reactors, in which neutron and gamma-ray fluxes created in the core are used for research in the field of nuclear physics, solid state physics, radiation chemistry, biology, for testing materials intended for operation in intense neutron fluxes (including parts nuclear reactors), for the production of isotopes. The power of research reactors does not exceed 100 MW. The released energy is usually not used.
• Industrial (weapons, isotope) reactors used to produce isotopes used in various fields. Most widely used for the production of nuclear weapons-grade materials, such as 239 Pu. Also industrial include reactors used for sea water desalination.

Often reactors are used to solve two or more different tasks, in which case they are called multipurpose. For example, some power reactors, especially at the dawn of nuclear energy, were intended mainly for experiments. Fast neutron reactors can be both power-generating and producing isotopes at the same time. Industrial reactors, in addition to their main task, often generate electrical and thermal energy.

According to the neutron spectrum

• Thermal (slow) neutron reactor ("thermal reactor")
• Fast neutron reactor ("fast reactor")

By fuel placement

• Heterogeneous reactors, where the fuel is placed in the core discretely in the form of blocks, between which there is a moderator;
• Homogeneous reactors, where the fuel and moderator are a homogeneous mixture (homogeneous system).

In a heterogeneous reactor, the fuel and the moderator can be spaced apart, in particular, in a cavity reactor, the moderator-reflector surrounds the cavity with fuel that does not contain a moderator. From a nuclear-physical point of view, the criterion of homogeneity/heterogeneity is not the design, but the placement of fuel blocks at a distance exceeding the neutron moderation length in a given moderator. For example, so-called “close-lattice” reactors are designed to be homogeneous, although the fuel is usually separated from the moderator in them.

Blocks of nuclear fuel in a heterogeneous reactor are called fuel assemblies (FA), which are placed in the core at the nodes of a regular lattice, forming cells.

By type of fuel

• uranium isotopes 235, 238, 233 ( 235 U , 238 U , 233 U)
• plutonium isotope 239 ( 239 Pu), also isotopes 239-242 Pu as a mixture with 238 U (MOX fuel)
• thorium isotope 232 (232 Th) (via conversion to 233 U)

According to the degree of enrichment:

• natural uranium
• low enriched uranium
• highly enriched uranium

By chemical composition:

• metal U
• UC (uranium carbide), etc.

By type of coolant

• Gas, (see Graphite-gas reactor)
• D 2 O (heavy water, see Heavy water nuclear reactor, CANDU)

By type of moderator

• C (graphite, see Graphite-gas reactor, Graphite-water reactor)
• H 2 O (water, see Light water reactor, Pressurized water reactor, VVER)
• D 2 O (heavy water, see Heavy water nuclear reactor, CANDU)
• Metal hydrides
• Without moderator (see fast neutron reactor)

By design

steam generation method

• Reactor with an external steam generator (See PWR, VVER)

IAEA classification

• PWR (pressurized water reactors) - pressurized water reactor (pressurized water reactor);
• BWR (boiling water reactor) - boiling water reactor;
• FBR (fast breeder reactor) - fast breeder reactor;
• GCR (gas-cooled reactor) - gas-cooled reactor;
• LWGR (light water graphite reactor) - graphite-water reactor
• PHWR (pressurised heavy water reactor) - heavy water reactor

The most common in the world are pressurized water (about 62%) and boiling water (20%) reactors.

Reactor materials

The materials from which the reactors are built operate at high temperature in the field of neutrons, γ-quanta and fission fragments. Therefore, not all materials used in other branches of technology are suitable for reactor construction. When choosing reactor materials, their radiation resistance, chemical inertness, absorption cross section, and other properties are taken into account.

Radiation instability of materials is less affected at high temperatures. The mobility of atoms becomes so great that the probability of the return of atoms knocked out of the crystal lattice to their place or the recombination of hydrogen and oxygen into a water molecule increases markedly. Thus, the radiolysis of water is insignificant in power non-boiling reactors (for example, VVER), while in powerful research reactors a significant amount of explosive mixture is released. The reactors have special systems for burning it.

Reactor materials come into contact with each other (a fuel element cladding with coolant and nuclear fuel, fuel cassettes with coolant and moderator, etc.). Naturally, the contacting materials must be chemically inert (compatible). An example of incompatibility is uranium and hot water entering into a chemical reaction.

For most materials, strength properties deteriorate sharply with increasing temperature. In power reactors, structural materials operate at high temperatures. This limits the choice of structural materials, especially for those parts of a power reactor that must withstand high pressure.

Burnup and reproduction of nuclear fuel

During the operation of a nuclear reactor, due to the accumulation of fission fragments in the fuel, its isotopic and chemical composition changes, and transuranium elements, mainly isotopes, are formed. The influence of fission fragments on the reactivity of a nuclear reactor is called poisoning(for radioactive fragments) and slagging(for stable isotopes).

The main reason for the poisoning of the reactor is, which has the largest neutron absorption cross section (2.6 10 6 barn). Half-life of 135 Xe T 1/2 = 9.2 h; the division yield is 6-7%. The main part of 135 Xe is formed as a result of decay ( T 1/2 = 6.8 hours). In case of poisoning, Kef changes by 1-3%. The large absorption cross section of 135 Xe and the presence of the intermediate isotope 135 I lead to two important phenomena:

1. To an increase in the concentration of 135 Xe and, consequently, to a decrease in the reactivity of the reactor after its shutdown or power reduction (“iodine pit”), which makes it impossible for short-term shutdowns and fluctuations in output power. This effect is overcome by introducing a reactivity margin in the regulatory bodies. The depth and duration of the iodine well depend on the neutron flux Ф: at Ф = 5 10 18 neutron/(cm² sec), the duration of the iodine well is ˜ 30 h, and the depth is 2 times greater than the stationary change in Keff caused by 135 Xe poisoning.
2. Due to poisoning, spatio-temporal fluctuations of the neutron flux Ф, and, consequently, of the reactor power, can occur. These fluctuations occur at Ф > 10 18 neutrons/(cm² sec) and large reactor sizes. Oscillation periods ˜ 10 h.

During nuclear fission, big number stable fragments that differ in their absorption cross sections compared to the absorption cross section of a fissile isotope. Fragment concentration with great value the absorption cross section reaches saturation during the first few days of reactor operation. These are mainly TVELs of different "ages".

In the case of complete fuel replacement, the reactor has excess reactivity, which must be compensated, while in the second case, compensation is required only at the first start of the reactor. Continuous refueling makes it possible to increase the burnup depth, since the reactivity of the reactor is determined by the average concentrations of fissile isotopes.

The mass of the loaded fuel exceeds the mass of the unloaded due to the "weight" of the released energy. After the shutdown of the reactor, first mainly due to fission by delayed neutrons, and then, after 1-2 minutes, due to β- and γ-radiation of fission fragments and transuranium elements, energy continues to be released in the fuel. If the reactor worked long enough before shutdown, then 2 minutes after shutdown, the energy release is about 3%, after 1 hour - 1%, after a day - 0.4%, after a year - 0.05% of the initial power.

The ratio of the number of fissile Pu isotopes formed in a nuclear reactor to the amount of 235 U burned out is called conversion rate K K . The value of K K increases with decreasing enrichment and burnup. For a heavy water reactor running on natural uranium, with a burnup of 10 GW day/t K K = 0.55, and for small burnups (in this case, K K is called initial plutonium coefficient) K K = 0.8. If a nuclear reactor burns and produces the same isotopes (breeder reactor), then the ratio of the reproduction rate to the burn-up rate is called reproduction rate K V. In thermal reactors K V< 1, а для реакторов на быстрых нейтронах К В может достигать 1,4-1,5. Рост К В для реакторов на быстрых нейтронах объясняется главным образом тем, что, особенно в случае 239 Pu, для быстрых нейтронов g is growing and a falls.

Nuclear reactor control

The control of a nuclear reactor is only possible due to the fact that during fission some of the neutrons fly out of the fragments with a delay, which can range from several milliseconds to several minutes.

To control the reactor, absorbing rods are used, introduced into the core, made of materials that strongly absorb neutrons (mainly, and some others) and / or a solution of boric acid, added to the coolant in a certain concentration (boron regulation). The movement of the rods is controlled by special mechanisms, drives, operating on signals from the operator or equipment for automatic control of the neutron flux.

In case of various emergencies in each reactor, an emergency termination of the chain reaction is provided, carried out by dropping all absorbing rods into the core - an emergency protection system.

Residual heat

An important issue directly related to nuclear safety is decay heat. This is a specific feature of nuclear fuel, which consists in the fact that, after the termination of the fission chain reaction and thermal inertia, which is common for any energy source, heat release in the reactor continues for a long time, which creates a number of technically complex problems.

Decay heat is a consequence of the β- and γ-decay of fission products, which have accumulated in the fuel during the operation of the reactor. The nuclei of fission products, as a result of decay, pass into a more stable or completely stable state with the release of significant energy.

Although the residual heat release rate rapidly drops to values ​​that are small compared to stationary values, in high-power power reactors it is significant in absolute terms. For this reason, decay heat release requires a long time to provide heat removal from the reactor core after it has been shut down. This task requires the presence of cooling systems with reliable power supply in the design of the reactor facility, and also necessitates long-term (within 3-4 years) storage of spent nuclear fuel in storage facilities with a special temperature regime - spent fuel pools, which are usually located in the immediate vicinity of the reactor.

see also

• List of nuclear reactors designed and built in the Soviet Union

Literature

• Levin V. E. Nuclear physics and nuclear reactors. 4th ed. - M.: Atomizdat, 1979.
• Shukolyukov A. Yu. “Uranium. natural nuclear reactor. "Chemistry and Life" No. 6, 1980, p. 20-24

Notes

1. "ZEEP - Canada's First Nuclear Reactor", Canada Science and Technology Museum.
2. Greshilov A. A., Egupov N. D., Matushchenko A. M. Nuclear shield. - M .: Logos, 2008. - 438 p. -