Creation of the first nuclear reactor. Nuclear power plant: how it works

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. 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 that have 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 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 coal cars, 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.

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 special machines delivered on board cargo aircraft.

Nuclear fuel for nuclear power plants weighs prohibitively much, tk. uranium is one of the heaviest metals on the planet. His specific gravity 2.5 times more than 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'S 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, it starts to happen. chain reaction. 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.

Organization IAEA (international agency for nuclear power) 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 runs on an electric current of 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.

There are 4 physical barriers in the NPP scheme that prevent the exit 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 of 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, (that same large tower - a containment area) which "holds" all radioactive processes in itself. 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 the last hope of the safety system comes into play - 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.

: ... quite banal, but nevertheless I never found the information in a digestible form - how a nuclear reactor BEGINS to work. Everything about the principle and operation of the device has already been chewed and understood 300 times, but here's how the fuel is obtained and from what, and why it is not so dangerous until it is in the reactor and why it does not react before being immersed in the reactor! - after all, it warms up only inside, nevertheless, before loading the fuel rods are cold and everything is fine, so what causes the elements to heat up is not entirely clear how they are affected, and so on, preferably not scientifically).

Of course, it is difficult to arrange such a topic not “according to science”, but I will try. Let's first understand what these very TVELs are.

Nuclear fuel is black tablets with a diameter of about 1 cm and a height of about 1.5 cm. They contain 2% uranium dioxide 235, and 98% uranium 238, 236, 239. In all cases, with any amount of nuclear fuel nuclear explosion cannot develop, because for an avalanche-like rapid fission reaction, characteristic of a nuclear explosion, a concentration of uranium 235 of more than 60% is required.

Two hundred nuclear fuel pellets are loaded into a tube made of zirconium metal. The length of this tube is 3.5m. diameter 1.35 cm. This tube is called TVEL - fuel element. 36 TVELs are assembled into a cassette (another name is "assembly").

The device of the fuel element of the RBMK reactor: 1 - plug; 2 - tablets of uranium dioxide; 3 - zirconium shell; 4 - spring; 5 - bushing; 6 - tip.

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 hindered 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 thermal motion approaches the energy threshold limiting the course of the process. In the case of molecular transformations, that is chemical reactions, such an increase is usually hundreds of degrees Kelvin, while in the case of nuclear reactions it is at least 107 K due to the very high height of the Coulomb barriers of 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.

To control and protect a nuclear reactor, control rods are used that 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 the deep introduction of the rods, the chain reaction becomes impossible, since the 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 decay. In this way, the power of the reactor is regulated.

Leningrad NPP, RBMK reactor

Reactor start:

At the initial moment of time after the first loading with fuel, there is no fission chain reaction in the reactor, the reactor is in a subcritical state. The coolant temperature is much lower than the operating temperature.

As we already mentioned here, in order to start a chain reaction, the fissile material must form a critical mass - a sufficient amount of spontaneously fissile material in a sufficiently small space, the condition under which the number of neutrons released during nuclear fission must be more number absorbed neutrons. This can be done by increasing the content of uranium-235 (the number of loaded fuel elements), or by slowing down the speed of neutrons so that they do not fly past the uranium-235 nuclei.

The reactor is brought to power in several stages. With the help of the reactivity regulators, the reactor is transferred to the supercritical state Kef>1 and the reactor power increases to a level of 1-2% of the nominal. At this stage, the reactor is heated up to the operating parameters of the coolant, and the heating rate is limited. During the warm-up process, the controls keep the power at a constant level. Then the circulation pumps are started and the heat removal system is put into operation. After that, the reactor power can be increased to any level in the range from 2 to 100% of the rated power.

When the reactor is heated, the reactivity changes due to changes in the temperature and density of the core materials. Sometimes, during heating, the mutual position of the core and the control elements that enter the core or leave it changes, causing a reactivity effect in the absence of active movement of the control elements.

Control by solid, moving absorber elements

In the vast majority of cases, solid mobile absorbers are used to quickly change the reactivity. In the RBMK reactor, the control rods contain boron carbide bushings enclosed in an aluminum alloy tube with a diameter of 50 or 70 mm. Each control rod is placed in a separate channel and cooled with water from the CPS circuit (control and protection system) at average temperature 50 ° C. According to their purpose, the rods are divided into rods AZ (emergency protection), in RBMK there are 24 such rods. Automatic control rods - 12 pieces, Local automatic control rods - 12 pieces, manual control rods -131, and 32 shortened absorber rods (USP). There are 211 rods in total. Moreover, shortened rods are introduced into the AZ from the bottom, the rest from the top.

VVER 1000 reactor. 1 - CPS drive; 2 - reactor cover; 3 - reactor vessel; 4 - block of protective pipes (BZT); 5 - mine; 6 - core baffle; 7 - fuel assemblies (FA) and control rods;

Burn-out absorbing elements.

Burnable poisons are often used to compensate for excess reactivity after fresh fuel has been loaded. The principle of operation of which is that they, like fuel, after the capture of a neutron, subsequently cease to absorb neutrons (burn out). Moreover, the rate of decline as a result of the absorption of neutrons, absorber nuclei, is less than or equal to the rate of loss, as a result of fission, of fuel nuclei. If we load into the reactor core fuel designed for operation during the year, then it is obvious that the number of fissile fuel nuclei at the beginning of work will be greater than at the end, and we must compensate for the excess reactivity by placing absorbers in the core. If control rods are used for this purpose, then we must constantly move them as the number of fuel nuclei decreases. The use of burnable poisons makes it possible to reduce the use of moving rods. At present, burnable poisons are often incorporated directly into fuel pellets during their manufacture.

Liquid regulation of reactivity.

Such regulation is used, in particular, during the operation of a VVER-type reactor, boric acid H3BO3 containing 10B nuclei absorbing neutrons is introduced into the coolant. By changing the concentration of boric acid in the coolant path, we thereby change the reactivity in the core. In the initial period of the reactor operation, when there are many fuel nuclei, the acid concentration is maximum. As the fuel burns out, the acid concentration decreases.

chain reaction mechanism

A nuclear reactor can operate at a given power for a long time only if it has a reactivity margin at the beginning of operation. The exception is subcritical reactors with an external source of thermal neutrons. The release of bound reactivity as it decreases due to natural causes ensures that the critical state of the reactor is maintained at every moment of its operation. The initial reactivity margin is created by building a core with dimensions that are much larger than the critical ones. To prevent the reactor from becoming supercritical, k0 of the breeding medium is artificially reduced at the same time. This is achieved by introducing neutron absorbers into the core, which can be subsequently removed from the core. As in the elements of chain reaction control, absorbent substances are included in the material of rods of one or another cross-section, moving along the corresponding channels in the core. But if one, two or several rods are sufficient for regulation, then the number of rods can reach hundreds to compensate for the initial excess of reactivity. These rods are called compensating. Control and compensating rods are not necessarily various elements by constructive design. A number of compensating rods can be control rods, but the functions of both are different. The control rods are designed to maintain a critical state at any time, to stop, start the reactor, switch from one power level to another. All these operations require small changes in reactivity. Compensating rods are gradually withdrawn from the reactor core, ensuring a critical state during the entire time of its operation.

Sometimes control rods are made not from absorbent materials, but from fissile or scatter material. In thermal reactors, these are mainly neutron absorbers, while there are no effective fast neutron absorbers. Such absorbers as cadmium, hafnium and others strongly absorb only thermal neutrons due to the proximity of the first resonance to the thermal region, and outside the latter they do not differ from other substances in their absorbing properties. An exception is boron, whose neutron absorption cross section decreases with energy much more slowly than that of the indicated substances, according to the l / v law. Therefore, boron absorbs fast neutrons, although weakly, but somewhat better than other substances. Only boron, if possible enriched in the 10B isotope, can serve as an absorbent material in a fast neutron reactor. In addition to boron, fissile materials are also used for control rods in fast neutron reactors. A compensating rod made of fissile material performs the same function as a neutron absorber rod: it increases the reactivity of the reactor with its natural decrease. However, unlike an absorber, such a rod is located outside the core at the beginning of the reactor operation, and then it is introduced into the core.

Of the scatterer materials in fast reactors, nickel is used, which has a scattering cross section for fast neutrons somewhat larger than the cross sections for other substances. Scatterer rods are located along the periphery of the core and their immersion in the corresponding channel causes a decrease in neutron leakage from the core and, consequently, an increase in reactivity. In some special cases, the purpose of controlling a chain reaction is the moving parts of the neutron reflectors, which, when moving, change the leakage of neutrons from the core. The control, compensating and emergency rods, together with all the equipment that ensures their normal functioning, form the reactor control and protection system (CPS).

Emergency protection:

Nuclear reactor emergency protection - 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.

The executive elements of emergency protection are, in most cases, rods with a substance that absorbs neutrons well (boron or cadmium). Sometimes a liquid scavenger is injected into the coolant loop to shut down the reactor.

In addition to active protection, many modern projects also include elements passive protection. For example, modern options 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 circuit of the reactor), the contents of these tanks are by gravity inside the reactor core and the nuclear chain reaction is quenched by a large amount of a boron-containing substance that absorbs neutrons well.

According to the rules nuclear safety reactor installations of nuclear power plants”, at least one of the provided reactor shutdown systems must perform the function of emergency protection (EP). Emergency protection must have at least two independent groups of working bodies. At the signal of the AZ, the working bodies of the AZ must be actuated from any working or intermediate positions.

The AZ equipment must consist of at least two independent sets.

Each set of AZ equipment must be designed in such a way that, in the range of neutron flux density changes from 7% to 120% of the nominal value, protection is provided for:

1. According to the density of the neutron flux - at least three independent channels;
2. According to the rate of increase in the neutron flux density - by at least three independent channels.

Each set of AZ equipment must be designed in such a way that, in the entire range of process parameter changes established in the reactor plant (RP) design, emergency protection is provided by at least three independent channels for each process parameter for which protection is necessary.

The control commands of each set for AZ actuators must be transmitted over at least two channels. When one channel is taken out of operation in one of the AZ equipment sets without this set being taken out of operation, an alarm signal should be automatically generated for this channel.

Tripping of emergency protection should occur at least in the following cases:

1. Upon reaching the AZ setpoint in terms of neutron flux density.
2. Upon reaching the AZ setpoint in terms of the rate of increase in the neutron flux density.
3. In the event of a power failure in any set of AZ equipment and CPS power supply buses that have not been taken out of operation.
4. In case of failure of any two of the three protection channels in terms of the neutron flux density or in terms of the rate of neutron flux increase in any set of AZ equipment that has not been decommissioned.
5. When the AZ settings are reached by the technological parameters, according to which it is necessary to carry out protection.
6. When initiating the operation of the AZ from the key from the block control point (BCR) or the backup control point (RCP).

Maybe someone will be able to explain briefly even less scientifically how the power unit of a nuclear power plant starts working? :-)

Recall a topic like The original article is on the website InfoGlaz.rf Link to the article from which this copy is made -

Nuclear reactor, principle of operation, operation of a nuclear reactor.

Every day we use electricity and do not think about how it is produced and how it came to us. However, it is one of the most important parts modern civilization. Without electricity, there would be nothing - no light, no heat, no movement.

Everyone knows that electricity is generated at power plants, including nuclear ones. The heart of every nuclear power plant is nuclear reactor. That is what we will be discussing in this article.

Nuclear reactor, a device in which a controlled nuclear chain reaction occurs with the release of heat. These devices are mainly used to generate electricity and as a drive. big ships. In order to imagine the power and efficiency of nuclear reactors, one can give an example. Where an average nuclear reactor would need 30 kilograms of uranium, an average thermal power plant would need 60 wagons of coal or 40 tanks of fuel oil.

prototype nuclear reactor was built in December 1942 in the USA under the direction of E. Fermi. It was the so-called "Chicago stack". Chicago Pile (subsequently the word"Pile" along with other meanings began to denote a nuclear reactor). This name was given to him due to the fact that he resembled a large stack of graphite blocks laid one on top of the other.

Between the blocks was placed spherical "working bodies" of natural uranium and its dioxide.

In the USSR, the first reactor was built under the leadership of Academician IV Kurchatov. The F-1 reactor was put into operation on December 25, 1946. The reactor was in the form of a ball and had a diameter of about 7.5 meters. It did not have a cooling system, so it worked at very low power levels.

Research continued and on June 27, 1954, the world's first nuclear power plant with a capacity of 5 MW in Obninsk.

The principle of operation of a nuclear reactor.

During the decay of uranium U 235, heat is released, accompanied by the release of two or three neutrons. According to statistics - 2.5. These neutrons collide with other uranium atoms U 235 . In a collision, uranium U 235 turns into an unstable isotope U 236, which almost immediately decays into Kr 92 and Ba 141 + these same 2-3 neutrons. The decay is accompanied by the release of energy in the form of gamma radiation and heat.

This is called a chain reaction. Atoms divide, the number of decays increases exponentially, which ultimately leads to a lightning-fast, by our standards, release of a huge amount of energy - an atomic explosion occurs, as a consequence of an uncontrolled chain reaction.

However, in nuclear reactor we are dealing with controlled nuclear reaction. How this becomes possible is described further.

The device of a nuclear reactor.

At present, there are two types of nuclear reactors VVER (pressure water power reactor) and RBMK (high power channel reactor). The difference is that RBMK is a boiling water reactor, while VVER uses water under pressure of 120 atmospheres.

VVER 1000 reactor. 1 - CPS drive; 2 - reactor cover; 3 - reactor vessel; 4 - block of protective pipes (BZT); 5 - mine; 6 - core baffle; 7 - fuel assemblies (FA) and control rods;

Each industrial-type nuclear reactor is a boiler through which a coolant flows. As a rule, this is ordinary water (approx. 75% in the world), liquid graphite (20%) and heavy water (5%). For experimental purposes, beryllium was used and a hydrocarbon was assumed.

TVEL- (fuel element). These are rods in a zirconium shell with niobium alloying, inside of which there are tablets of uranium dioxide.

TVEL raktor RBMK. The device of the fuel element of the RBMK reactor: 1 - plug; 2 - tablets of uranium dioxide; 3 - zirconium shell; 4 - spring; 5 - bushing; 6 - tip.

TVEL also includes a spring system for holding fuel pellets at the same level, which makes it possible to more accurately control the depth of immersion/removal of fuel into the core. They are assembled into hexagonal cassettes, each of which includes several dozen fuel rods. The coolant flows through the channels in each cassette.

The fuel elements in the cassette are highlighted in green.

Fuel cassette assembly.

The reactor core consists of hundreds of cassettes, placed vertically and united together by a metal shell - the body, which also plays the role of a neutron reflector. Among the cassettes, control rods and emergency protection rods of the reactor are inserted at regular intervals, which, in case of overheating, are designed to shut down the reactor.

Let us give as an example the data on the VVER-440 reactor:

The controllers can move up and down by sinking, or vice versa, leaving the core, where the reaction is most intense. This is provided by powerful electric motors, together with the control system. Emergency protection rods are designed to shut down the reactor in case of an emergency, falling into the core and absorbing more free neutrons.

Each reactor has a lid through which the used and new cassettes are loaded and unloaded.

Thermal insulation is usually installed on top of the reactor vessel. The next barrier is biological protection. This is usually a reinforced concrete bunker, the entrance to which is closed by an airlock with sealed doors. Biological protection is designed not to release radioactive steam and pieces of the reactor into the atmosphere, if an explosion does occur.

A nuclear explosion in modern reactors is extremely unlikely. Because the fuel is not sufficiently enriched, and is divided into TVELs. Even if the core melts, the fuel will not be able to react so actively. The maximum that can happen is a thermal explosion, like at Chernobyl, when the pressure in the reactor reached such values ​​that the metal case was simply torn apart, and the reactor lid, weighing 5000 tons, made a flip jump, breaking through the roof of the reactor compartment and releasing steam out. If Chernobyl nuclear power plant was equipped with the correct biological protection, like today's sarcophagus, the catastrophe cost humanity much less.

The work of a nuclear power plant.

In a nutshell, the raboboa looks like this.

Nuclear power plant. (clickable)

After entering the reactor core with the help of pumps, the water is heated from 250 to 300 degrees and exits from the “other side” of the reactor. This is called the first circuit. Then it goes to the heat exchanger, where it meets with the second circuit. After that, the steam under pressure enters the turbine blades. Turbines generate electricity.

For ordinary person modern high-tech devices are so mysterious and mysterious that it is just right to worship them, as the ancients worshiped lightning. School lessons physicists, replete with mathematical calculations, do not solve the problem. But it’s interesting to tell even about a nuclear reactor, the principle of operation of which is clear even to a teenager.

How does a nuclear reactor work?

The principle of operation of this high-tech device is as follows:

1. When a neutron is absorbed, nuclear fuel (most often this uranium-235 or plutonium-239) the division of the atomic nucleus occurs;
2. released kinetic energy, gamma radiation and free neutrons;
3. Kinetic energy is converted into thermal energy (when nuclei collide with surrounding atoms), gamma radiation is absorbed by the reactor itself and is also converted into heat;
4. Some of the generated neutrons are absorbed by the fuel atoms, which causes a chain reaction. To control it, neutron absorbers and moderators are used;
5. With the help of a coolant (water, gas or liquid sodium), heat is removed from the reaction site;
6. Pressurized steam from heated water is used to drive steam turbines;
7. With the help of a generator, the mechanical energy of the rotation of the turbines is converted into alternating electric current.

Approaches to classification

There can be many reasons for the typology of reactors:

• By type of nuclear reaction. Fission (all commercial installations) or fusion (thermonuclear power, is widespread only in some research institutes);
• By coolant. In the vast majority of cases, water (boiling or heavy) is used for this purpose. Sometimes used alternative solutions: liquid metal (sodium, lead-bismuth alloy, mercury), gas (helium, carbon dioxide or nitrogen), molten salt (fluoride salts);
• By generation. The first is the early prototypes, which didn't make any commercial sense. The second is the majority of currently used nuclear power plants that were built before 1996. The third generation differs from the previous one only in minor improvements. Work on the fourth generation is still underway;
• By state of aggregation fuel (gas still exists only on paper);
• By purpose of use(for the production of electricity, engine start, hydrogen production, desalination, transmutation of elements, obtaining neural radiation, theoretical and investigative purposes).

Nuclear reactor device

The main components of reactors in most power plants are:

1. Nuclear fuel - a substance that is necessary for the production of heat for power turbines (usually low enriched uranium);
2. The active zone of the nuclear reactor - this is where the nuclear reaction takes place;
3. Neutron moderator - reduces the speed of fast neutrons, turning them into thermal neutrons;
4. Starting neutron source - used for reliable and stable launch of a nuclear reaction;
5. Neutron absorber - available in some power plants to reduce the high reactivity of fresh fuel;
6. Neutron howitzer - used to re-initiate a reaction after being turned off;
7. Coolant (purified water);
8. Control rods - to control the rate of fission of uranium or plutonium nuclei;
9. Water pump - pumps water to the steam boiler;
10. Steam turbine - converts the thermal energy of steam into rotational mechanical energy;
11. Cooling tower - a device for removing excess heat into the atmosphere;
12. System for receiving and storing radioactive waste;
13. Safety systems (emergency diesel generators, devices for emergency core cooling).

How the latest models work

The latest 4th generation of reactors will be available for commercial operation no earlier than 2030. Currently, the principle and arrangement of their work are at the development stage. According to current data, these modifications will differ from existing models such benefits:

• Rapid gas cooling system. It is assumed that helium will be used as a coolant. According to the design documentation, reactors with a temperature of 850 °C can be cooled in this way. To work with such high temperatures specific raw materials will also be required: composite ceramic materials and actinide compounds;
• It is possible to use lead or a lead-bismuth alloy as a primary coolant. These materials have a low neutron absorption and are relatively low temperature melting;
• Also, a mixture of molten salts can be used as the main coolant. Thus, it will be possible to work at higher temperatures than modern analogues with water cooling.

Natural analogues in nature

The nuclear reactor is perceived in the public mind solely as a product high technology. However, in fact the first the device is of natural origin. It was discovered in the Oklo region, in the Central African state of Gabon:

• The reactor was formed due to the flooding of uranium rocks groundwater. They acted as neutron moderators;
• The thermal energy released during the decay of uranium turns water into steam, and the chain reaction stops;
• After the coolant temperature drops, everything repeats again;
• If the liquid had not boiled off and stopped the course of the reaction, humanity would have faced a new natural disaster;
• Self-sustaining nuclear fission began in this reactor about one and a half billion years ago. During this time, about 0.1 million watts of output power was allocated;
• Such a wonder of the world on Earth is the only one known. The appearance of new ones is impossible: the proportion of uranium-235 in natural raw materials is much lower than the level necessary to maintain a chain reaction.

How many nuclear reactors are in South Korea?

Poor on Natural resources, but the industrialized and overpopulated Republic of Korea is in dire need of energy. Against the backdrop of Germany's rejection of the peaceful atom, this country has high hopes for curbing nuclear technology:

• It is planned that by 2035 the share of electricity generated by nuclear power plants will reach 60%, and the total production - more than 40 gigawatts;
• The country does not have atomic weapons, but research in nuclear physics is ongoing. Korean scientists have developed designs for modern reactors: modular, hydrogen, liquid metal and etc.;
• The success of local researchers allows you to sell technology abroad. It is expected that in the next 15-20 years the country will export 80 such units;
• But as of today most of The nuclear power plant was built with the assistance of American or French scientists;
• The number of operating stations is relatively small (only four), but each of them has a significant number of reactors - 40 in total, and this figure will grow.

When bombarded with neutrons, nuclear fuel enters a chain reaction, as a result of which great amount heat. The water in the system takes this heat and turns it into steam, which turns turbines that produce electricity. Here simple circuit operation of a nuclear reactor, the most powerful source of energy on Earth.

Video: how nuclear reactors work

In this video, nuclear physicist Vladimir Chaikin will tell you how electricity is generated in nuclear reactors, their detailed structure:

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 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.

Before you can understand how a controlled nuclear reaction works, you need to know what nuclear reaction generally.

nuclear reaction - this is the process of transformation (fission) of atomic nuclei when they interact 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.

Nuclear reactor - This 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 . Isotopes are the most commonly used fuel in reactors. uranium (235, 238, 233), plutonium (239) and thorium (232). The active zone is a boiler through which ordinary water (coolant) flows. 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. The electricity itself is generated by the same method as in other types of power plants - steam rotates the turbine, and the energy of movement is converted into electrical energy.

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

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.

It needs to be mentioned here 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 (TVELah). These are rods in which, in the form of small tablets, nuclear fuel . 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, among them are control rods and emergency protection rods . 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 a concept critical mass .

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.

In this article, we have tried to give you a general idea of ​​the structure and principle of operation of a nuclear (atomic) reactor. If you have any questions on the topic or the university asked a problem in nuclear physics, please contact 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!