Operating principle of the reactor. Everyone has heard but no one knows. How does a nuclear (nuclear) reactor work?

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 got to us. However, this 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 . This is what we will be looking at 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 to drive large ships. In order to imagine the power and efficiency of nuclear reactors, we can give an example. Where an average nuclear reactor will require 30 kilograms of uranium, an average thermal power plant will require 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 (later the word“Pile”, along with other meanings, has come to mean a nuclear reactor). It was given this name because it resembled a large stack of graphite blocks placed one on top of the other.

Between the blocks were placed spherical “working fluids” made of natural uranium and its dioxide.

In the USSR, the first reactor was built under the leadership of Academician I.V. Kurchatov. The F-1 reactor was operational on December 25, 1946. The reactor was spherical in shape and had a diameter of about 7.5 meters. It had no cooling system, so it operated 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 operating principle 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 U235. During 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 fission, the number of decays increases in geometric progression, which ultimately leads to a lightning-fast, by our standards, release of a huge amount of energy - happens atomic explosion, as a consequence of an uncontrollable chain reaction.

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

The structure of a nuclear reactor.

Currently, there are two types of nuclear reactors: VVER (water-cooled 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 - control system drive; 2 - reactor cover; 3 - reactor body; 4 - block of protective pipes (BZT); 5 - shaft; 6 - core enclosure; 7 - fuel assemblies (FA) and control rods;

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

TVEL– (fuel element). These are rods in a zirconium shell with niobium alloy, inside of which uranium dioxide tablets are located.

TVEL raktor RBMK. RBMK reactor fuel element design: 1 - plug; 2 - uranium dioxide tablets; 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 regulate the depth of immersion/removal of fuel into the core. They are assembled into hexagonal-shaped cassettes, each of which includes several dozen fuel rods. The coolant flows through the channels in each cassette.

The fuel rods 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 - a body, which also plays the role of a neutron reflector. Among the cassettes, control rods and reactor emergency protection rods are inserted at regular intervals, which are designed to shut down the reactor in case of overheating.

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

The controllers can move up and down, plunging, or vice versa, leaving the active zone, where the reaction is most intense. This is ensured by powerful electric motors, in conjunction with a control system. The emergency protection rods are designed to shut down the reactor in the event of an emergency, falling into the core and absorbing more free neutrons.

Each reactor has a lid through which 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 to prevent the release of 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 quite slightly enriched and divided into fuel elements. Even if the core melts, the fuel will not be able to react as actively. The worst that can happen is a thermal explosion like at Chernobyl, when the pressure in the reactor reached such values that the metal casing simply burst, and the reactor cover, weighing 5,000 tons, made an inverted jump, breaking through the roof of the reactor compartment and releasing steam outside. If the Chernobyl nuclear power plant had been equipped with the correct biological protection, like today’s sarcophagus, the disaster cost humanity much less.

Operation of a nuclear power plant.

In a nutshell, this is what raboboa looks like.

Nuclear power plant. (Clickable)

After entering the reactor core using 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. After which it is sent to the heat exchanger, where it meets the second circuit. After which the steam under pressure flows onto the turbine blades. Turbines generate electricity.

Design and principle of operation

Energy release mechanism

The transformation of a substance is accompanied by the release of free energy only if the substance has a reserve of energy. The latter means that microparticles of a substance are in a state with a rest energy greater than in another possible state to which a transition exists. A spontaneous transition is always prevented by an energy barrier, to overcome which the microparticle must receive a certain amount of energy from the outside - excitation energy. The exoenergetic reaction consists in the fact that in the transformation following 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 colliding particles, or due to the binding energy of the joining particle.

If we keep in mind the macroscopic scale of energy release, then all or at first at least some fraction of particles of the substance must have the kinetic energy necessary to excite reactions. This is achievable only by increasing the temperature of the medium to a value at which the energy of 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 kelvins, but in the case of nuclear reactions- this is a minimum of 10 7 due to the very high altitude Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions is carried out in practice only during the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

Excitation by joining particles does not require large kinetic energy, and, therefore, does not depend on the temperature of the medium, since it occurs due to unused bonds inherent in the attractive forces of particles. But to excite reactions, the particles themselves are necessary. And if we again mean not an individual act of reaction, but the production of energy on a macroscopic scale, then this is possible only when a chain reaction occurs. The latter occurs 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 surrounding the core;
Chain reaction control system, including emergency protection;
Radiation protection;
Remote control system.

Physical principles of operation

Iodine pit

Main article: Iodine pit

Iodine pit - a state of a nuclear reactor after it is turned off, characterized by the accumulation of the short-lived isotope xenon. 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 within a certain period (about 1-2 days).

Classification

By purpose

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

Power reactors, intended for the production of electrical and thermal energy used in the energy sector, as well as for the desalination of sea water (desalination reactors are also classified as industrial). Such reactors are mainly used in nuclear power plants. The thermal power of modern power reactors reaches 5 GW. A separate group includes:
- Transport reactors, designed to supply energy to vehicle engines. The widest groups of applications are marine transport reactors used on submarines and various surface vessels, as well as reactors used in space technology.
Experimental reactors, intended for the study of 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 several kW.
Research reactors, in which fluxes of neutrons and gamma quanta created in the core are used for research in the field of nuclear physics, solid state physics, radiation chemistry, biology, for testing materials intended to operate 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 to produce nuclear weapons materials, such as 239 Pu. Also classified as industrial are reactors used for desalination of sea water.

Often reactors are used to solve two or more different problems, in which case they are called multi-purpose. For example, some power reactors, especially in the early nuclear energy, were intended mainly for experiments. Fast neutron reactors can simultaneously produce energy and produce isotopes. 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 fuel is placed discretely in the core 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 moderator can be spatially separated, in particular, in a cavity reactor, the moderator-reflector surrounds a cavity with fuel that does not contain a moderator. From a nuclear physical point of view, the criterion for homogeneity/heterogeneity is not the design, but the placement of fuel blocks at a distance exceeding the neutron moderation length in a given moderator. Thus, reactors with the so-called “close lattice” are designed as homogeneous, although in them the fuel is usually separated from the moderator.

Nuclear fuel blocks in a heterogeneous reactor are called fuel assemblies (FA), which are located in the core at the nodes of a regular lattice, forming cells.

By fuel type

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

By degree of enrichment:

natural uranium
weakly 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)
H2O (water, see Light water reactor, Water-cooled reactor, VVER)
D 2 O (heavy water, see Heavy water nuclear reactor, CANDU)
Metal hydrides
Without moderator (see Fast reactor)

By design

By steam generation method

Reactor with external steam generator (See Water-water reactor, VVER)

IAEA classification

PWR (pressurized water reactors) - water-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 (pressurized 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 reactors are built operate at high temperatures in a 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.

The radiation instability of materials has less effect when high temperatures Oh. 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 energy non-boiling reactors (for example, VVER), while in powerful research reactors a significant amount of explosive mixture is released. Reactors have special systems for burning it.

Reactor materials are in contact with each other (fuel shell 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, the strength properties deteriorate sharply with increasing temperature. In power reactors, structural materials operate at high temperatures. This limits the choice of construction materials, especially for those parts of the power reactor that must withstand high pressure.

Burnout 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 transuranic elements, mainly isotopes, are formed. The effect 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 reactor poisoning is , which has the largest neutron absorption cross section (2.6·10 6 barn). Half-life of 135 Xe T 1/2 = 9.2 hours; The yield during division is 6-7%. The bulk of 135 Xe is formed as a result of the decay ( T 1/2 = 6.8 hours). In case of poisoning, Keff 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:

To an increase in the concentration of 135 Xe and, consequently, to a decrease in the reactivity of the reactor after it is stopped or the power is reduced (“iodine pit”), which makes short-term stops and fluctuations in output power impossible. This effect is overcome by introducing a reactivity reserve in 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 hours, and the depth is 2 times greater than the stationary change in Keff caused by 135 Xe poisoning.
Due to poisoning, spatiotemporal fluctuations in the neutron flux F, and, consequently, in the reactor power, can occur. These oscillations occur at Ф > 10 18 neutrons/(cm² sec) and large sizes reactor. Oscillation periods ˜ 10 hours.

When nuclear fission occurs large number stable fragments that differ in absorption cross sections compared to the absorption cross section of the fissile isotope. Concentration of fragments with great value The absorption cross section reaches saturation within the first few days of reactor operation. These are mainly fuel rods of different “ages”.

In case complete replacement fuel, the reactor has excess reactivity that needs to be compensated, whereas in the second case compensation is required only during the first start-up of the reactor. Continuous overloading 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 loaded fuel exceeds the mass of unloaded fuel due to the “weight” of the released energy. After the reactor is shut down, 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, the release of energy in the fuel continues. If the reactor worked long enough before stopping, then 2 minutes after stopping 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 burnt 235 U is called conversion rate K K . The value of K K increases with decreasing enrichment and burnup. For a heavy water reactor using natural uranium, with a burnup of 10 GW day/t K K = 0.55, and with 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 burnup rate is called reproduction rate K V. In nuclear reactors using thermal neutrons 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 grows and A falls.

Nuclear reactor control

Control of a nuclear reactor is possible only 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, absorber 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 control). The movement of the rods is controlled special mechanisms, drives operating according to signals from the operator or equipment for automatic control of the neutron flux.

In case of various emergency situations, each reactor is provided with an emergency termination of the chain reaction, 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 specific feature nuclear fuel, which consists in the fact that, after the cessation of the fission chain reaction and the thermal inertia usual for any energy source, the release of heat in the reactor continues for a long time, which creates a number of technically complex problems.

Residual heat is a consequence of the β- and γ-decay of fission products that accumulated in the fuel during the operation of the reactor. Fission product nuclei, due to decay, transform into a more stable or completely stable state with the release of significant energy.

Although the decay heat release rate quickly decreases to values small compared to steady-state values, in powerful power reactors it is significant in absolute values. For this reason, residual heat generation necessitates long time ensure heat removal from the reactor core after shutdown. This task requires the design of the reactor installation to have cooling systems with a reliable power supply, and also necessitates long-term (3-4 years) storage of spent nuclear fuel in storage facilities with special temperature conditions- cooling pools, which are usually located in close proximity to the reactor.

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

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

: ... quite banal, but nevertheless I still haven’t found the information in a digestible form - how a nuclear reactor STARTS to work. Everything about the principle and structure of work has already been chewed over 300 times and is clear, 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 only heats up inside, nevertheless, before loading the fuel is cold and everything is fine, so what causes the heating of the elements is not entirely clear, how they are affected, and so on, preferably not scientifically).

It’s difficult, of course, to frame such a topic in a non-scientific way, but I’ll try. Let's first figure out what these fuel rods are.

Nuclear fuel is a black tablet 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 fuel element - fuel element. 36 fuel rods are assembled into a cassette (another name is “assembly”).

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

If we keep in mind the macroscopic scale of energy release, then all or at first at least some fraction of particles of the substance must have the kinetic energy necessary to excite reactions. This is achievable only by increasing the temperature of the medium to a value at which the energy of 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, but 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 is carried out in practice only during the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

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 of substances that strongly absorb neutrons - for example, boron or cadmium. When the rods are inserted deeply, a chain reaction becomes impossible, since neutrons are strongly absorbed and removed from the reaction zone.

The rods are moved remotely from the control panel. With a slight movement of the rods, the chain process will either develop or fade. In this way the power of the reactor is regulated.

Leningrad NPP, RBMK reactor

Start of reactor operation:

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

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

The reactor is brought up to power in several stages. With the help of 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 one. At this stage, the reactor is heated to the operating parameters of the coolant, and the heating rate is limited. During the heating process, the controls maintain the power at a constant level. Then the circulation pumps are started and the heat removal system is put into operation. After this, the reactor power can be increased to any level in the range from 2 to 100% of the rated power.

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

Regulation by solid, moving absorbent elements

To quickly change reactivity, in the vast majority of cases, solid movable absorbers are used. 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 is cooled by water from the control and protection system (control and protection system) circuit at average temperature 50 ° C. According to their purpose, the rods are divided into AZ (emergency protection) rods; there are 24 such rods in the RBMK. 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, the shortened rods are inserted into the core from the bottom, the rest from the top.

VVER 1000 reactor. 1 - control system drive; 2 - reactor cover; 3 - reactor body; 4 - block of protective pipes (BZT); 5 - shaft; 6 - core enclosure; 7 - fuel assemblies (FA) and control rods;

Burnable absorbing elements.

To compensate for excess reactivity after loading fresh fuel, burnable absorbers are often used. The operating principle of which is that they, like fuel, after capturing a neutron, subsequently cease to absorb neutrons (burn out). Moreover, the rate of decrease as a result of the absorption of neutrons by absorber nuclei is less than or equal to the rate of decrease as a result of fission of fuel nuclei. If we load a reactor core with fuel designed to operate for a year, then it is obvious that the number of fissile fuel nuclei at the beginning of operation 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, we must continually move them as the number of fuel nuclei decreases. The use of burnable absorbers reduces the use of moving rods. Nowadays, burnable absorbents are often added directly to fuel pellets during their manufacture.

Fluid reactivity control.

Such regulation is used, in particular, during the operation of a VVER-type reactor, boric acid H3BO3 containing 10B neutron-absorbing nuclei 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 initial period During 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 reserve 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 reasons ensures the maintenance of the critical state of the reactor at every moment of its operation. The initial reactivity reserve is created by constructing a core with dimensions significantly exceeding the critical ones. To prevent the reactor from becoming supercritical, k0 of the breeding medium is simultaneously artificially reduced. This is achieved by introducing neutron absorber substances into the core, which can be subsequently removed from the core. As in the chain reaction control elements, absorbent substances are included in the material of rods of one or another cross-section moving through the corresponding channels in the core. But if one or two or several rods are enough for regulation, then to compensate for the initial excess reactivity the number of rods can reach hundreds. These rods are called compensating rods. Control and compensating rods do not necessarily represent different design elements. A number of compensating rods can be control rods, but the functions of both are different. Control rods are designed to maintain a critical state at any time, to stop and start the reactor, and to transition from one power level to another. All these operations require small changes in reactivity. The compensating rods are gradually removed from the reactor core, ensuring a critical state throughout its operation.

Sometimes control rods are made not from absorbent materials, but from fissile material or scattering material. In thermal reactors, these are mainly neutron absorbers; there are no effective fast neutron absorbers. Absorbers such 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 are no different from other substances in their absorbing properties. The 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. The absorber material in a fast neutron reactor can only be boron, if possible enriched with the 10B isotope. 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 while it naturally decreases. However, unlike an absorber, such a rod is located outside the core at the beginning of the reactor operation and is then introduced into the core.

The scatterer materials used in fast reactors are nickel, which has a scattering cross section for fast neutrons that is slightly larger than the cross sections of other substances. The 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 purposes of controlling the chain reaction are the moving parts of the neutron reflectors, which, when moved, change the leakage of neutrons from the core. 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:

Emergency protection of a nuclear reactor is 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 could lead to an accident. Such parameters can be: temperature, pressure and coolant flow, level and speed 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, to shut down the reactor, a liquid absorber is injected into the coolant loop.

In addition to active protection, many modern projects also include elements of passive protection. For example, modern versions of VVER reactors include an “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 first reactor cooling circuit), the contents of these tanks end up inside the reactor core by gravity and the nuclear chain reaction is extinguished by a large amount of boron-containing substance, which absorbs neutrons well.

According to the “Nuclear Safety Rules for Reactor Facilities 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 elements. At the AZ signal, the AZ working parts must be activated 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 protection is provided in the range of changes in neutron flux density from 7% to 120% of the nominal:

1. By neutron flux density - no less than three independent channels;
2. According to the rate of increase in neutron flux density - no less than three independent channels.

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

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

Emergency protection must be triggered at least in the following cases:

1. Upon reaching the AZ setting for neutron flux density.
2. Upon reaching the AZ setting for the rate of increase in neutron flux density.
3. If the voltage disappears in any set of emergency protection equipment and the 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 for the neutron flux density or for the rate of increase of the neutron flux in any set of AZ equipment that has not been taken out of service.
5. When the AZ settings are reached by the technological parameters for which protection must be carried out.
6. When triggering the AZ from a key from a block control point (BCP) or a reserve control point (RCP).

Maybe someone can explain briefly in an even less scientific way how a nuclear power plant unit starts operating? :-)

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

Today we will commit short trip into the world of nuclear physics. The theme of our excursion will be a nuclear reactor. You will learn how it works, what physical principles are the basis of its operation and where this device is used.

The Birth of Nuclear Energy

The world's first nuclear reactor was created in 1942 in the USA experimental group of physicists led by the laureate Nobel Prize Enrico Fermi. At the same time, they carried out a self-sustaining reaction of uranium fission. The atomic genie has been released.

The first Soviet nuclear reactor was launched in 1946, and 8 years later, the world’s first nuclear power plant in the city of Obninsk generated current. The chief scientific director of work in the nuclear energy industry of the USSR was an outstanding physicist Igor Vasilievich Kurchatov.

Since then, several generations of nuclear reactors have changed, but the main elements of its design have remained unchanged.

Anatomy of a nuclear reactor

This nuclear installation is a thick-walled steel tank with a cylindrical capacity ranging from several cubic centimeters to many cubic meters.

Inside this cylinder is the holy of holies - reactor core. This is where the nuclear fission chain reaction occurs.

Let's look at how this process occurs.

Cores heavy elements, in particular Uranium-235 (U-235), under the influence of a small energy shock they are capable of falling apart into 2 fragments of approximately equal mass. The causative agent of this process is the neutron.

The fragments are most often barium and krypton nuclei. Each of them carries a positive charge, so Coulomb repulsion forces force them to fly apart in different directions at a speed of about 1/30 of the speed of light. These fragments are carriers of colossal kinetic energy.

For practical use energy, it is necessary that its release be self-sustaining. Chain reaction, The fission in question is especially interesting because each fission event is accompanied by the emission of new neutrons. On average, 2-3 new neutrons are produced per initial neutron. The number of fissile uranium nuclei is increasing like an avalanche, causing the release of enormous energy. If this process is not controlled, a nuclear explosion will occur. It takes place in .

To regulate the number of neutrons materials that absorb neutrons are introduced into the system, ensuring a smooth release of energy. Cadmium or boron are used as neutron absorbers.

How to curb and use the enormous kinetic energy of fragments? The coolant is used for these purposes, i.e. a special environment, moving in which the fragments are slowed down and heat it to extremely high temperatures. Such a medium can be ordinary or heavy water, liquid metals (sodium), as well as some gases. In order not to cause the transition of the coolant into a vapor state, high pressure is maintained in the core (up to 160 atm). For this reason, the reactor walls are made of ten-centimeter steel of special grades.

If neutrons escape beyond the nuclear fuel, the chain reaction may be interrupted. Therefore, there is a critical mass of fissile material, i.e. its minimum mass at which a chain reaction will be maintained. It depends on various parameters, including the presence of a reflector surrounding the reactor core. It serves to prevent neutron leakage into environment. The most common material for this structural element is graphite.

The processes occurring in the reactor are accompanied by the release of the dangerous looking radiation – gamma radiation. To minimize this danger, it is equipped with anti-radiation protection.

How does a nuclear reactor work?

Nuclear fuel, called fuel rods, is placed in the reactor core. They are tablets formed from crushable material and placed in thin tubes about 3.5 m long and 10 mm in diameter.

Hundreds of similar fuel assemblies are placed in the core, and they become sources of thermal energy released during the chain reaction. The coolant flowing around the fuel rods forms the first circuit of the reactor.

Heated to high parameters, it is pumped into a steam generator, where it transfers its energy to the secondary circuit water, turning it into steam. The resulting steam rotates the turbogenerator. The electricity generated by this unit is transmitted to the consumer. And the exhaust steam, cooled by water from the cooling pond, in the form of condensate, returns to the steam generator. The cycle is completed.

This double-circuit operation of a nuclear installation eliminates the penetration of radiation accompanying the processes occurring in the core beyond its boundaries.

So, a chain of energy transformations occurs in the reactor: nuclear energy of the fissionable material → into kinetic energy of fragments → thermal energy of the coolant → kinetic energy of the turbine → and into electrical energy in the generator.

Inevitable energy losses lead to The efficiency of nuclear power plants is relatively low, 33-34%.

In addition to generating electrical energy at nuclear power plants, nuclear reactors are used to produce various radioactive isotopes, for research in many areas of industry, and to study the permissible parameters of industrial reactors. Transport reactors, which provide energy for vehicle engines, are becoming increasingly widespread.

Types of nuclear reactors

Typically, nuclear reactors run on U-235 uranium. However, its content is natural material extremely small, only 0.7%. The bulk of natural uranium is the isotope U-238. Only slow neutrons can cause a chain reaction in U-235, and the U-238 isotope is split only by fast neutrons. As a result of the splitting of the nucleus, both slow and fast neutrons are born. Fast neutrons, experiencing inhibition in the coolant (water), become slow. But the amount of the U-235 isotope in natural uranium is so small that it is necessary to resort to its enrichment, bringing its concentration to 3-5%. This process is very expensive and economically unprofitable. In addition, the time for depletion of the natural resources of this isotope is estimated at only 100-120 years.

Therefore, in the nuclear industry There is a gradual transition to reactors operating on fast neutrons.

Their main difference is that they use liquid metals as a coolant, which do not slow down neutrons, and U-238 is used as nuclear fuel. The nuclei of this isotope pass through a chain of nuclear transformations into Plutonium-239, which is subject to a chain reaction in the same way as U-235. That is, nuclear fuel is reproduced, and in quantities exceeding its consumption.

According to experts reserves of the isotope Uranium-238 should be enough for 3000 years. This time is enough for humanity to have enough time to develop other technologies.

Problems of using nuclear energy

Along with the obvious advantages of nuclear energy, the scale of the problems associated with the operation of nuclear facilities cannot be underestimated.

The first one is disposal of radioactive waste and dismantled equipment nuclear energy. These elements have an active background radiation that persists for a long period. To dispose of this waste, special lead containers are used. They are supposed to be buried in permafrost areas at a depth of up to 600 meters. Therefore, work is constantly underway to find a way to recycle radioactive waste, which should solve the problem of disposal and help preserve the ecology of our planet.

The second no less serious problem is ensuring safety during NPP operation. Major accidents, similar to Chernobyl, are capable of carrying away many human lives and put vast areas out of use.

The accident at the Japanese nuclear power plant Fukushima-1 only confirmed potential danger, which manifests itself when an emergency situation occurs at nuclear facilities.

However, the possibilities of nuclear energy are so great that environmental problems fade into the background.

Today, humanity has no other way to satisfy its ever-increasing energy hunger. The basis of the nuclear energy of the future will probably be “fast” reactors with the function of reproducing nuclear fuel.

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In the mid-twentieth century, humanity's attention was focused around the atom and scientists' explanation of the nuclear reaction, which they initially decided to use for military purposes, inventing the first nuclear bombs. But in the 50s of the 20th century, the nuclear reactor in the USSR was used for peaceful purposes. It is well known that on June 27, 1954, the world's first nuclear power plant with a capacity of 5000 kW entered the service of humanity. Today, a nuclear reactor makes it possible to generate electricity of 4000 MW or more, that is, 800 times more than half a century ago.

What is a nuclear reactor: basic definition and main components of the unit

A nuclear reactor is a special unit that produces energy as a result of properly maintaining a controlled nuclear reaction. It is permissible to use the word “atomic” in combination with the word “reactor”. Many generally consider the concepts “nuclear” and “atomic” to be synonymous, since they do not find a fundamental difference between them. But representatives of science are inclined to a more correct combination - “nuclear reactor”.

Interesting fact! Nuclear reactions can occur with the release or absorption of energy.

The main components in the design of a nuclear reactor are the following elements:

Moderator;
Control rods;
Rods containing an enriched mixture of uranium isotopes;
Special protective elements against radiation;
Coolant;
Steam generator;
Turbine;
Generator;
Capacitor;
Nuclear fuel.

What fundamental principles of the operation of a nuclear reactor are determined by physicists and why they are unshakable

The fundamental operating principle of a nuclear reactor is based on the peculiarities of the manifestation of a nuclear reaction. At the moment of a standard physical chain nuclear process, the particle interacts with atomic nucleus As a result, the nucleus turns into a new one with the release of secondary particles, which scientists call gamma rays. During a nuclear chain reaction, enormous amounts of thermal energy are released. The space in which the chain reaction occurs is called the reactor core.

Interesting fact! The active zone externally resembles a boiler through which ordinary water flows, acting as a coolant.

To prevent the loss of neutrons, the reactor asset zone is surrounded by a special neutron reflector. Its primary task is to discard most of neutrons emitted into the core. The same substance that serves as a moderator is usually used as a reflector.

The main control of a nuclear reactor occurs using special control rods. It is known that these rods are introduced into the reactor core and create all the conditions for the operation of the unit. Typically control rods are made from chemical compounds boron and cadmium. Why are these particular elements used? Yes, all because boron or cadmium are able to effectively absorb thermal neutrons. And as soon as the launch is planned, according to the operating principle of a nuclear reactor, control rods are inserted into the core. Their primary task is to absorb a significant portion of neutrons, thereby provoking the development of a chain reaction. The result should reach the desired level. When the power increases above the set level, automatic machines are switched on, necessarily immersing the control rods deep into the reactor core.

Thus, it becomes clear that the control or control rods play important role in the operation of a thermal nuclear reactor.

And to reduce neutron leakage, the reactor core is surrounded by a neutron reflector, which throws a significant mass of freely escaping neutrons into the core. The reflector usually uses the same substance as the moderator.

According to the standard, the nucleus of the atoms of the moderator substance has a relatively small mass, so that when colliding with a light nucleus, the neutron present in the chain loses more energy than when colliding with a heavy one. The most common moderators are ordinary water or graphite.

Interesting fact! Neutrons in the process of a nuclear reaction are characterized extremely high speed movement, which is why a moderator is required, pushing neutrons to lose part of their energy.

Not a single reactor in the world can function normally without the help of a coolant, since its purpose is to remove the energy that is generated in the heart of the reactor. Liquid or gases must be used as a coolant, since they are not capable of absorbing neutrons. Let's give an example of a coolant for a compact nuclear reactor - water, carbon dioxide, and sometimes even liquid sodium metal.

Thus, the principles of operation of a nuclear reactor are entirely based on the laws of the chain reaction and its course. All components of the reactor - moderator, rods, coolant, nuclear fuel - perform their assigned tasks, ensuring the normal operation of the reactor.

What fuel is used for nuclear reactors and why these chemical elements are chosen

The main fuel in reactors can be isotopes of uranium, plutonium or thorium.

Back in 1934, F. Joliot-Curie, having observed the process of fission of the uranium nucleus, noticed that as a result chemical reaction the uranium nucleus is divided into fragments-nuclei and two or three free neutrons. This means that there is a possibility that free neutrons will join other uranium nuclei and trigger another fission. And so, as the chain reaction predicts: six to nine neutrons will be released from three uranium nuclei, and they will again join the newly formed nuclei. And so on ad infinitum.

Important to remember! Neutrons appearing during nuclear fission are capable of provoking the fission of nuclei of a uranium isotope with a mass number of 235, and to destroy the nuclei of a uranium isotope with a mass number of 238 there may be little energy generated during the decay process.

Uranium number 235 is rarely found in nature. It accounts for only 0.7%, but natural uranium-238 occupies a larger niche and makes up 99.3%.

Despite such a small proportion of uranium-235 in nature, physicists and chemists still cannot refuse it, because it is most effective for the operation of a nuclear reactor, reducing the cost of energy production for humanity.

When did the first nuclear reactors appear and where are they commonly used today?

Back in 1919, physicists had already triumphed when Rutherford discovered and described the process of formation of moving protons as a result of the collision of alpha particles with the nuclei of nitrogen atoms. This discovery meant that a nitrogen isotope nucleus, as a result of a collision with an alpha particle, was transformed into an oxygen isotope nucleus.

Before the first nuclear reactors appeared, the world learned several new laws of physics that deal with all the important aspects of nuclear reactions. Thus, in 1934, F. Joliot-Curie, H. Halban, L. Kowarski first proposed to society and the circle of world scientists a theoretical assumption and evidence base about the possibility of carrying out nuclear reactions. All experiments were related to the observation of the fission of a uranium nucleus.

In 1939, E. Fermi, I. Joliot-Curie, O. Gan, O. Frisch tracked the fission reaction of uranium nuclei when bombarded with neutrons. During the research, scientists found that when one accelerated neutron hits a uranium nucleus, the existing nucleus is divided into two or three parts.

The chain reaction was practically proven in the middle of the 20th century. Scientists managed to prove in 1939 that the fission of one uranium nucleus releases about 200 MeV of energy. But approximately 165 MeV is allocated to the kinetic energy of fragment nuclei, and the remainder is carried away by gamma quanta. This discovery made a breakthrough in quantum physics.

E. Fermi continued his work and research for several more years and launched the first nuclear reactor in 1942 in the USA. The implemented project was named “Chicago Woodpile” and was put on the rails. On September 5, 1945, Canada launched its ZEEP nuclear reactor. The European continent was not far behind, and at the same time the F-1 installation was being built. And for Russians there is another memorable date– On December 25, 1946, a reactor was launched in Moscow under the leadership of I. Kurchatov. These were not the most powerful nuclear reactors, but it was the beginning of man's mastery of the atom.

For peaceful purposes, a scientific nuclear reactor was created in 1954 in the USSR. The world's first peaceful nuclear-powered ship power plant- nuclear icebreaker "Lenin" - was built in the Soviet Union in 1959. And another achievement of our state is the nuclear icebreaker “Arktika”. For the first time in the world, this surface ship reached North Pole. This happened in 1975.

The first portable nuclear reactors used slow neutrons.

Where are nuclear reactors used and what types does humanity use?

Industrial reactors. They are used to generate energy at nuclear power plants.
Nuclear reactors acting as propulsion units for nuclear submarines.
Experimental (portable, small) reactors. Not a single modern day can pass without them. scientific experience or research.

Today scientific light learned to desalinate using special reactors sea water, provide the population with quality drinking water. There are a lot of operating nuclear reactors in Russia. Thus, according to statistics, as of 2018, about 37 units operate in the state.

And according to classification they can be as follows:

Research (historical). These include the F-1 station, which was created as an experimental site for the production of plutonium. I.V. Kurchatov worked at F-1 and led the first physical reactor.
Research (active).
Armory. As an example of a reactor - A-1, which went down in history as the first reactor with cooling. The past power of the nuclear reactor is small, but functional.
Energy.
Ship's. It is known that on ships and submarines, out of necessity and technical feasibility, water-cooled or liquid metal reactors are used.
Space. As an example, let’s call the installation “Yenisei” on spaceships, which comes into effect if it is necessary to obtain additional energy, and it will have to be obtained using solar panels and isotope sources.

Thus, the topic of nuclear reactors is quite extensive, and therefore requires in-depth study and understanding of the laws of quantum physics. But the importance of nuclear reactors for the energy and economy of the state is already, undoubtedly, surrounded by an aura of usefulness and benefit.