Sinusitis

Nuclear reactor. Nuclear reactor

First Nuclear reactor built in December 1942 in the USA under the direction of E. Fermi . In Europe, the first nuclear reactor was launched in December 1946 in Moscow under the leadership of I.V. Kurchatova . By 1978, there were already about a thousand nuclear reactors operating in the world. various types. Components any Nuclear reactor are: core With nuclear fuel, usually surrounded by a neutron reflector, coolant, regulation system chain reaction, radiation protection, system remote control (rice. 1). The main characteristic of a nuclear reactor is its power. Power at 1 Mv corresponds to a chain reaction in which 3 10 16 acts of fission into 1 occur sec.
Design of power nuclear reactors.

A nuclear power reactor is a device in which a controlled chain reaction of fission of nuclei of heavy elements is carried out, and the released thermal energy is removed by the coolant. The main element of a nuclear reactor is the core. It houses nuclear fuel and carries out a fission chain reaction. The core is a collection of fuel elements containing nuclear fuel placed in a certain way. Thermal neutron reactors use a moderator. Coolant is pumped through the core to cool the fuel elements. In some types of reactors, the role of moderator and coolant is performed by the same substance, for example ordinary or heavy water.

Homogeneous reactor diagram: 1-reactor body, 2-core, 3-volume compensator, 4-heat exchanger, 5-steam outlet, 6-feedwater inlet, 7-circulation pump

To control the operation of the reactor, control rods made of materials with a large neutron absorption cross section are introduced into the core. The core of power reactors is surrounded by a neutron reflector - a layer of moderator material to reduce the leakage of neutrons from the core. In addition, thanks to the reflector, the neutron density and energy release are equalized throughout the volume of the core, which makes it possible to obtain greater power for a given zone size, achieve more uniform fuel burnout, increase the operating time of the reactor without overloading the fuel, and simplify the heat removal system. The reflector is heated by the energy of slowing down and absorbed neutrons and gamma quanta, so its cooling is provided. The core, reflector and other elements are housed in a sealed housing or casing, usually surrounded by biological shielding.

In the core of a nuclear reactor there is nuclear fuel, a chain reaction occurs nuclear fission and energy is released. The state of the nuclear reactor is characterized by effective coefficient Kef neutron multiplication or reactivity r:

R = (K ¥ - 1)/K eff. (1)

If K ef > 1, then the chain reaction increases over time, the nuclear reactor is in a supercritical state and its reactivity r > 0; If K eff< 1 , then the reaction dies out, the reactor is subcritical, r< 0; при TO ¥ = 1, r = 0 the reactor is in critical condition, there is a stationary process and the number of divisions is constant over time. To initiate a chain reaction when starting a nuclear reactor, a neutron source (a mixture of Ra and Be, 252 Cf, etc.) is usually introduced into the core, although this is not necessary, since the spontaneous fission of uranium nuclei and cosmic rays provide a sufficient number of initial neutrons for the development of a chain reaction at K ef > 1.

235 U is used as a fissile substance in most nuclear reactors. If the core, in addition to nuclear fuel (natural or enriched uranium), contains a neutron moderator (graphite, water and other substances containing light nuclei, see Neutron moderation), then the main part of divisions occurs under the influence thermal neutrons (thermal reactor). A nuclear reactor using thermal neutrons can use natural uranium that is not enriched with 235 U (such were the first nuclear reactors). If there is no moderator in the core, then the main part of the fissions is caused fast neutrons with energy x n > 10 kev (fast reactor). Intermediate neutron reactors with energies of 1-1000 are also possible ev.

The criticality condition for a nuclear reactor has the form:

Keff = K ¥ × P = 1 , (1)

Where 1 - P is the probability of neutrons escaping (leakage) from the core of the Nuclear Reactor, TO ¥ - the neutron multiplication factor in the core is infinite large sizes, determined for thermal nuclear reactors by the so-called “four factor formula”:

TO¥ = neju. (2)

Here n is the average number of secondary (fast) neutrons produced during the fission of a 235 U nucleus by thermal neutrons, e is the multiplication factor by fast neutrons (an increase in the number of neutrons due to the fission of nuclei, mainly 238 U nuclei, by fast neutrons); j is the probability that a neutron will not be captured by the 238 U nucleus during the slowdown process, u is the probability that a thermal neutron will cause fission. The value h = n/(l + a) is often used, where a is the ratio of the radiation capture cross section s p to the fission cross section s d.

Condition (1) determines the dimensions of the Nuclear Reactor For example, for a Nuclear Reactor made of natural uranium and graphite n = 2.4. e » 1.03, eju » 0.44, from where TO¥ =1.08. This means that for TO ¥ > 1 necessary P<0,93, что соответствует (как показывает теория Ядерный реактор) размерам активной зоны Ядерный реактор ~ 5-10 m. The volume of a modern energy nuclear reactor reaches hundreds m 3 and is determined mainly by heat removal capabilities, and not by criticality conditions. The volume of the active zone of a nuclear reactor in a critical state is called the critical volume of the nuclear reactor, and the mass of the fissile material is called the critical mass. A nuclear reactor with fuel in the form of solutions of salts of pure fissile isotopes in water and with a water neutron reflector have the lowest critical mass. For 235 U this mass is 0.8 kg, For 239 Pu - 0,5 kg . 251 Cf has the smallest critical mass (theoretically 10 g). Critical parameters of graphite nuclear reactor with natural uranium: uranium mass 45 T, graphite volume 450 m 3 . To reduce neutron leakage, the core is given a spherical or nearly spherical shape, for example, a cylinder with a height on the order of the diameter or a cube (the smallest surface-to-volume ratio).

The value of n is known for thermal neutrons with an accuracy of 0.3% (Table 1). As the energy x n of the neutron that caused fission increases, n increases according to the law: n = n t + 0.15x n (x n in Mev), where n t corresponds to fission by thermal neutrons.

Table 1. - Values n and h) for thermal neutrons (according to data for 1977)

233U

235 U

239 Pu

241 Pu

The value (e-1) is usually only a few%; nevertheless, the role of fast neutron multiplication is significant, since for large nuclear reactors ( TO ¥ - 1) << 1 (графитовые Ядерный реактор с естественным ураном, в которых впервые была осуществлена цепная реакция, невозможно было бы создать, если бы не существовало деления на быстрых нейтронах).

The maximum possible value of J is achieved in a nuclear reactor, which contains only fissile nuclei. Energy nuclear reactors use weakly enriched uranium (concentration of 235 U ~ 3-5%), and 238 U nuclei absorb a significant portion of neutrons. Thus, for a natural mixture of uranium isotopes, the maximum value of nJ = 1.32. The absorption of neutrons in the moderator and structural materials usually does not exceed 5-20% of the absorption of all isotopes of nuclear fuel. Of the moderators, heavy water has the lowest absorption of neutrons, and of structural materials - Al and Zr.

The probability of resonant capture of neutrons by 238 U nuclei during the moderation process (1-j) is significantly reduced in a heterogeneous nuclear reactor. The decrease (1 - j) is due to the fact that the number of neutrons with energy close to resonance sharply decreases inside the fuel block and in resonant absorption Only the outer layer of the block is involved. The heterogeneous structure of the nuclear reactor makes it possible to carry out a chain process using natural uranium. It reduces the value of O, but this loss in reactivity is significantly less than the gain due to a decrease in resonant absorption.

To calculate the thermal properties of a nuclear reactor, it is necessary to determine the spectrum of thermal neutrons. If neutron absorption is very weak and the neutron manages to collide with moderator nuclei many times before absorption, then thermodynamic equilibrium (neutron thermalization) is established between the moderating medium and the neutron gas, and the spectrum of thermal neutrons is described Maxwell distribution . In reality, the absorption of neutrons in the core of a nuclear reactor is quite high. This leads to a deviation from the Maxwell distribution - the average energy of neutrons is greater than the average energy of the molecules of the medium. The thermalization process is influenced by the movements of nuclei, chemical bonds of atoms, etc.

Burnout and reproduction of nuclear fuel. During the operation of a nuclear reactor, a change in the composition of the fuel occurs due to the accumulation of fission fragments in it (see. Nuclear fission) and with education transuranic elements, mainly Pu isotopes. The influence of fission fragments on reactivity A nuclear reactor is called poisoning (for radioactive fragments) and slagging (for stable ones). Poisoning is caused mainly by 135 Xe which has the largest neutron absorption cross section (2.6 10 6 barn). Its half-life T 1/2 = 9.2 hours, the fission yield is 6-7%. The bulk of 135 Xe is formed as a result of the decay of 135 ]( Shopping center = 6,8 h). When poisoned, Cef changes by 1-3%. The large absorption cross section of 135 Xe and the presence of the intermediate isotope 135 I lead to two important phenomena: 1) to an increase in the concentration of 135 Xe and, consequently, to a decrease in the reactivity of a nuclear reactor after it is stopped or the power is reduced (“iodine pit”). This forces an additional reserve of reactivity in the regulatory bodies or makes short-term stops and power fluctuations impossible. The depth and duration of the iodine well depend on the neutron flux Ф: at Ф = 5·10 13 neutron/cm 2 × sec Duration of iodine well ~ 30 h, and the depth is 2 times greater than the stationary change K eff, caused by 135 Xe poisoning. 2) Due to poisoning, spatiotemporal oscillations of the neutron flux F, and therefore power, can occur. Nuclear reactor These oscillations occur at F> 10 13 neutrons/cm 2 × sec and large sizes Nuclear reactor Oscillation periods ~ 10 h.

The number of different stable fragments resulting from nuclear fission is large. There are fragments with large and small absorption cross sections compared to the absorption cross section of the fissile isotope. The concentration of the former reaches saturation during the first few days of operation of the nuclear reactor (mainly 149 Sm, changing Keff by 1%). The concentration of the latter and the negative reactivity they introduce increase linearly with time.

The formation of transuranium elements in a nuclear reactor occurs according to the following schemes:

Here 3 means neutron capture, the number under the arrow is the half-life.

The accumulation of 239 Pu (nuclear fuel) at the beginning of operation of a nuclear reactor occurs linearly in time, and the faster (with a fixed burnup of 235 U) the lower the uranium enrichment. Then the concentration of 239 Pu tends to a constant value, which does not depend on the degree of enrichment, but is determined by the ratio of the neutron capture cross sections of 238 U and 239 Pu . Characteristic time to establish equilibrium concentration 239 Pu ~ 3/ F years (F in units 10 13 neutrons/ cm 2×sec). The isotopes 240 Pu and 241 Pu reach equilibrium concentrations only when fuel is re-burned in a nuclear reactor after regeneration of nuclear fuel.

Nuclear fuel burnout is characterized by the total energy released into the nuclear reactor per 1 T fuel. For a nuclear reactor operating on natural uranium, the maximum burnup is ~10 GW × day/t(heavy water nuclear reactor). B Nuclear reactor with weakly enriched uranium (2-3% 235 U) burnout ~ 20-30 is achieved GW-day/t. In fast neutron nuclear reactor - up to 100 GW-day/t. Burnout 1 GW-day/t corresponds to the combustion of 0.1% nuclear fuel.

When nuclear fuel burns out, the reactivity of a nuclear reactor decreases (in a nuclear reactor using natural uranium, at small burnups, a slight increase in reactivity occurs). Replacement of burnt fuel can be carried out immediately from the entire core or gradually along the fuel rods so that the core contains fuel rods of all ages - a continuous overload mode (intermediate options are possible). In the first case, a nuclear reactor with fresh fuel has excess reactivity that must be compensated. In the second case, such compensation is needed only during initial startup, before entering continuous overload mode. Continuous reloading makes it possible to increase the burnup depth, since the reactivity of a nuclear reactor is determined by the average concentrations of fissile nuclides (fuel elements with a minimum concentration of fissile nuclides are unloaded). Table 2 shows the composition of the recovered nuclear fuel (in kg) V pressurized water reactor power 3 Gvt. The entire core is unloaded simultaneously after operating the nuclear reactor for 3 years and "excerpts" 3 years(Ф = 3×10 13 neutron/cm 2 ×sec). Initial composition: 238 U - 77350, 235 U - 2630, 234 U - 20.

Table 2. - Composition of the unloaded fuel, kg

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 it is at least 10 7 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).

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 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 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 designed to produce electrical and thermal energy used in the energy sector, as well as for 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 days of nuclear power, were designed primarily for experimentation. 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 at high temperatures. The mobility of atoms becomes so great that the probability of the return of atoms knocked out of the crystal lattice to their place or the recombination of hydrogen and oxygen into a water molecule increases markedly. Thus, the radiolysis of water is insignificant in 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 reactor sizes. Oscillation periods ˜ 10 hours.

Nuclear fission produces a large number of stable fragments, which differ in absorption cross sections compared to the absorption cross section of the fissile isotope. The concentration of fragments with a large absorption cross section reaches saturation within the first few days of reactor operation. These are mainly fuel rods of different “ages”.

In the case of a complete fuel change, the reactor has excess reactivity that needs to be compensated, while in the second case compensation is required only when the reactor is first started. 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 by 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 is a specific feature of 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 heat release 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 high-power power reactors it is significant in absolute terms. For this reason, residual heat generation entails the need for a long period of time to ensure heat removal from the reactor core after it is shut down. 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 a special temperature regime - 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. -

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

How does a nuclear reactor work?

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

When a neutron is absorbed, nuclear fuel (most often this uranium-235 or plutonium-239) fission of the atomic nucleus occurs;
Kinetic energy, gamma radiation and free neutrons are released;
Kinetic energy is converted into thermal energy (when nuclei collide with surrounding atoms), gamma radiation is absorbed by the reactor itself and also turns into heat;
Some of the neutrons produced are absorbed by fuel atoms, which causes a chain reaction. To control it, neutron absorbers and moderators are used;
With the help of a coolant (water, gas or liquid sodium), heat is removed from the reaction site;
Pressurized steam from heated water is used to drive steam turbines;
With the help of a generator, the mechanical energy of turbine rotation 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 energy, widespread only in some research institutes);
By coolant. In the vast majority of cases, water (boiling or heavy) is used for this purpose. Alternative solutions are sometimes used: liquid metal (sodium, lead-bismuth, mercury), gas (helium, carbon dioxide or nitrogen), molten salt (fluoride salts);
By generation. The first was early prototypes that made no commercial sense. Second, most of the nuclear power plants currently in use 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 fuel currently exists only on paper);
By purpose of use(for electricity production, engine starting, hydrogen production, desalination, elemental transmutation, obtaining neural radiation, theoretical and investigative purposes).

Nuclear reactor structure

The main components of reactors in most power plants are:

Nuclear fuel is a substance needed to produce heat for power turbines (usually low-enriched uranium);
The nuclear reactor core is where the nuclear reaction takes place;
Neutron moderator - reduces the speed of fast neutrons, turning them into thermal neutrons;
Starting neutron source - used for reliable and stable starting of a nuclear reaction;
Neutron absorber - available in some power plants to reduce the high reactivity of fresh fuel;
Neutron howitzer - used to re-initiate a reaction after shutdown;
Coolant (purified water);
Control rods - to regulate the rate of fission of uranium or plutonium nuclei;
Water pump - pumps water into the steam boiler;
Steam turbine - converts the thermal energy of steam into rotational mechanical energy;
Cooling tower - a device for removing excess heat into the atmosphere;
Radioactive waste reception and storage system;
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 structure of their operation are at the development stage. According to modern data, these modifications will differ from existing models in such advantages:

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 operate at such high temperatures, specific raw materials will be required: composite ceramic materials and actinide compounds;
It is possible to use lead or a lead-bismuth alloy as the primary coolant. These materials have a low neutron absorption rate and a relatively low melting point;
Also, a mixture of molten salts can be used as the main coolant. This will make it possible to operate at higher temperatures than modern water-cooled counterparts.

Natural analogues in nature

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

The reactor was formed due to the flooding of uranium rocks by 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 away and stopped 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, approximately 0.1 million watts of power output was provided;
Such a wonder of the world on Earth is the only one known. The emergence 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 there in South Korea?

Poor in natural resources, but industrialized and overpopulated, the Republic of Korea has an extraordinary need for energy. Against the backdrop of Germany's refusal to use peaceful atoms, 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 will be more than 40 gigawatts;
The country does not have atomic weapons, but research on nuclear physics is ongoing. Korean scientists have developed designs for modern reactors: modular, hydrogen, with liquid metal, etc.;
The successes of local researchers make it possible to sell technologies abroad. The country is expected to export 80 such units in the next 15-20 years;
But as of today, most nuclear power plants were built with the assistance of American or French scientists;
The number of operating plants is relatively small (only four), but each of them has a significant number of reactors - a total of 40, and this figure will grow.

When bombarded by neutrons, nuclear fuel goes into a chain reaction, resulting in the production of a huge amount of heat. The water in the system takes this heat and turns into steam, which turns turbines that produce electricity. Here is a simple diagram of the 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 and their detailed structure:

The immense energy of a tiny atom

“Good science - physics! Only life is short." These words belong to a scientist who has done a surprising amount in physics. They were once said by an academician Igor Vasilievich Kurchatov, creator of the world's first nuclear power plant.

On June 27, 1954, this unique power plant came into operation. Humanity now has another powerful source of electricity.

The path to mastering the energy of the atom was long and difficult. It began in the first decades of the 20th century with the discovery of natural radioactivity by the Curies, with Bohr's postulates, Rutherford's planetary model of the atom and the proof of what now seems to be an obvious fact - the nucleus of any atom consists of positively charged protons and neutral neutrons.

In 1934, the couple Frédéric and Irène Joliot-Curie (daughter of Marie Skłodowska-Curie and Pierre Curie) discovered that bombarding them with alpha particles (the nuclei of helium atoms) could transform ordinary chemical elements into radioactive ones. The new phenomenon is called artificial radioactivity.

I.V. Kurchatov (right) and A.I. Alikhanov (center) with their teacher A.F. Ioffe. (Early 30s.)

If such bombardment is carried out with very fast and heavy particles, then a cascade of chemical transformations begins. Elements with artificial radioactivity will gradually give way to stable elements that will no longer decay.

With the help of irradiation or bombardment, it is easy to make the alchemists' dream come true - to make gold from other chemical elements. Only the cost of such a transformation will significantly exceed the price of the resulting gold...

Uranium nuclear fission

What was discovered in 1938-1939 by a group of German physicists and chemists brought more benefit (and, unfortunately, anxiety) to humanity. fission of uranium nuclei. When irradiated with neutrons, heavy uranium nuclei decay into lighter chemical elements belonging to the middle part of the periodic table of Mendeleev, and release several neutrons. For the nuclei of light elements, these neutrons turn out to be superfluous... When uranium nuclei “split,” a chain reaction can begin: each of the two or three resulting neutrons is capable, in turn, of producing several neutrons, falling into the nucleus of a neighboring atom.

The total mass of the products of such a nuclear reaction turned out to be, as scientists calculated, less than the mass of the nuclei of the original substance - uranium.

According to Einstein's equation, which relates mass to energy, one can easily determine that enormous energy must be released in this case! And this will happen in a negligibly short time. If, of course, the chain reaction becomes uncontrollable and goes to the end...

On a walk after the conference, E. Fermi (right) with his student B. Pontecorvo. (Basel, 1949)

He was one of the first to appreciate the enormous physical and technical capabilities hidden in the process of uranium fission. Enrico Fermi, in those distant thirties of our century, still a very young, but already recognized head of the Italian school of physicists. Long before the Second World War, he and a group of talented collaborators studied the behavior of various substances under neutron irradiation and determined that the efficiency of the uranium fission process could be significantly increased ... by slowing down the movement of neutrons. Strange as it may seem at first glance, as the speed of neutrons decreases, the probability of their capture by uranium nuclei increases. Effective “moderators” of neutrons are quite accessible substances: paraffin, carbon, water...

After moving to the United States, Fermi continued to be the brain and heart of nuclear research conducted there. Two talents, usually mutually exclusive, were combined in Fermi: an outstanding theorist and a brilliant experimenter. “It will still be a long time before we can see his equal,” wrote the eminent scientist W. Zinn after Fermi’s untimely death from a malignant tumor in 1954 at the age of 53.

A team of scientists who rallied around Fermi during the Second World War decided to create a weapon of unprecedented destructive power based on the chain reaction of uranium fission - atomic bomb. Scientists were in a hurry: what if Nazi Germany manages to manufacture new weapons before anyone else and uses them in its inhumane quest to enslave other peoples?

Construction of a nuclear reactor in our country

Already in 1942, scientists managed to assemble and launch it on the territory of the University of Chicago stadium first nuclear reactor. The uranium rods in the reactor were interspersed with carbon “bricks” - moderators, and if the chain reaction still became too violent, it could be quickly stopped by introducing cadmium plates into the reactor, which separated the uranium rods and completely absorbed the neutrons.

The researchers were very proud of the simple reactor adaptations they came up with, which now make us smile. One of Fermi’s collaborators in Chicago, the famous physicist G. Anderson, recalls that cadmium tin was nailed to a wooden block, which, if necessary, instantly fell into the boiler under the influence of its own gravity, which was the reason for giving it the name “instant”. G. Anderson writes: “Before starting the boiler, this rod should have been pulled up and secured with a rope. In the event of an accident, the rope could be cut and the “moment” would take its place inside the boiler.”

A controlled chain reaction was achieved at a nuclear reactor, and theoretical calculations and predictions were tested. A chain of chemical transformations took place in the reactor, as a result of which a new chemical element, plutonium, accumulated. It, like uranium, can be used to create an atomic bomb.

Scientists have determined that there is a "critical mass" of uranium or plutonium. If there is a sufficiently large amount of atomic substance, the chain reaction leads to an explosion; if it is small, less than the “critical mass,” then simply heat is released.

Construction of a nuclear power plant

In an atomic bomb of the simplest design, two pieces of uranium or plutonium are placed side by side, and the mass of each is slightly less than critical. At the right moment, a fuse from a conventional explosive connects the pieces, the mass of atomic fuel exceeds a critical value - and the release of destructive energy of monstrous force occurs instantly...

Dazzling light radiation, a shock wave that swept away everything in its path, and penetrating radioactive radiation hit the residents of two Japanese cities - Hiroshima and Nagasaki - after the explosion of American atomic bombs in 1945, raising anxiety in the hearts of people about the terrible consequences of the use of atomic bombs. weapons.

Under the unifying scientific leadership of I.V. Kurchatov, Soviet physicists developed atomic weapons.

But the leader of these works did not stop thinking about the peaceful use of atomic energy. After all, a nuclear reactor has to be intensively cooled, so why not “give” this heat to a steam or gas turbine or use it to heat houses?

Tubes containing liquid low-melting metal were passed through a nuclear reactor. The heated metal entered the heat exchanger, where it transferred its heat to water. The water turned into superheated steam, and the turbine began to operate. The reactor was surrounded by a protective shell made of concrete with a metal filler: radioactive radiation should not escape outside.

The nuclear reactor has turned into a nuclear power plant, bringing calm light, cozy warmth, and desired peace to people...

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The first nuclear reactor built in the Soviet Union (uranium-graphite) operated on natural uranium without special cooling.

The first nuclear reactor, created under the leadership of Fermi, was launched in 1942. U-235, Pu-239, U-238, and Th-232 are used as raw materials and fissile substances in the reactors. In the natural mixture of uranium isotopes, the isotope U-238 is found in. To understand the processes occurring in a reactor with a natural mixture of isotopes, it is necessary to take into account the differences noted in § 18.8 in the conditions under which fission of the nuclei of both isotopes of uranium occurs. These neutrons are capable of causing the fission of only U-235 nuclei. Those few prompt neutrons whose energy exceeds the fission activation energy of the U-238 nucleus are more likely to undergo inelastic scattering, and their energy is, as a rule, below the fission threshold of the U-238 nucleus. As a result of a series of collisions with uranium nuclei, neutrons lose energy in small portions, slow down and experience radiative capture by U-238 nuclei or are absorbed by U-235 nuclei. The absorption of neutrons by U-235 nuclei promotes the development of a chain reaction, while their absorption by U-238 nuclei removes neutrons from the chain reaction and leads to the termination of the reaction chains. Calculations show that in a natural mixture of uranium isotopes the probability of chain termination exceeds the probability of reaction branching and a fission chain reaction cannot develop with either fast or slow neutrons.

The first nuclear reactor, created under the leadership of Fermi, was launched in 1942. U-235, Pu-239, U-238, and Th-232 are used as raw materials and fissile substances in the reactors. In the natural mixture of uranium isotopes, the isotope U-238 contains 140 times more than the isotope U-235. To understand the processes occurring in a reactor with a natural mixture of isotopes, it is necessary to take into account the differences noted in § 18.8 in the conditions under which fission of the nuclei of both isotopes of uranium occurs. These neutrons are capable of causing the fission of only U-235 nuclei. Those few prompt neutrons whose energy exceeds the fission activation energy of the U-238 nucleus are more likely to undergo inelastic scattering, and their energy is, as a rule, below the fission threshold of the U-238 nucleus. As a result of a series of collisions with uranium nuclei, neutrons lose energy in small portions, slow down and experience radiative capture by U-238 nuclei or are absorbed by U-235 nuclei. The absorption of neutrons by U-235 nuclei promotes the development of a chain reaction, while their absorption by U-238 nuclei removes neutrons from the chain reaction and leads to the termination of the reaction chains. Calculations show that in a natural mixture of uranium isotopes the probability of chain termination exceeds the probability of reaction branching and a fission chain reaction cannot develop with either fast or slow neutrons.

The first nuclear reactors were built to meet the pressing requirements of the atomic weapons program; These requirements have been dominant in reactor design for 10 years. Reactors for military purposes were used essentially only for the production of plutonium, and the main effort was aimed at separating plutonium from natural or low-enriched uranium. The fuel elements in such reactors were usually enclosed in shells made of aluminum or magnesium alloys.

The first nuclear reactor was built at the end of 1942 in the USA by the Italian physicist Fermi.

The first nuclear reactor was built from uranium and graphite by Fermi and his colleagues at the end of 1942 in the USA.

The first fast neutron nuclear reactors were built in our country - this is the Beloyarsk nuclear power plant, as well as the nuclear power plant in the city of Shevchenko. For the reactor to reach its design capacity, it is necessary that almost all Np (T / z 2 35 days) be converted into Pu. In addition, the resulting Pu must be separated from the remaining original uranium and fragmentation elements. Thus, the chemistry of nuclear reactors is very complex.

Chain reaction using dominoes as an example.

The first nuclear reactors were developed during the Second World War.

The first nuclear reactor was not intended to produce energy, it was needed to accumulate materials and knowledge.

The first critical-size uranium nuclear reactor was installed at the University of Chicago. By that time, about 6 tons of pure uranium had already been produced; uranium and graphite were laid in successive layers - 57 layers in total - in which holes were left for cadmium adjustment rods.

Although the first nuclear reactor was launched only 12 years ago, entire volumes could already be written about these extraordinary installations. Today, all over the globe - in the Soviet Union and the United States of America, in France and Canada, in Norway and in England - various types of reactors operate. Some of them serve research purposes, others generate energy, and others are real factories for the production of huge quantities of various radioactive isotopes. Let us dwell at least briefly on the design and operation of nuclear reactors.

In the first nuclear reactors, special graphite was used as a moderator. In graphite (density 1 67), a neutron travels an average of 2 53 cm between collisions with carbon nuclei and loses 0 158 of its energy. Consequently, the moderating ability will be equal to 0.0625 and during I cm of travel through graphite, the fast neutron will lose 6-25% of its energy.