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Why was a nuclear reactor created? Great encyclopedia of oil and gas

The importance of nuclear energy in the modern world

Nuclear energy has made huge strides over the past few decades, becoming one of the most important sources of electricity for many countries. At the same time, it should be remembered that the development of this industry National economy It is worth the enormous efforts of tens of thousands of scientists, engineers and ordinary workers who are doing everything to ensure that the “peaceful atom” does not turn into a real threat to millions of people. The real core of any nuclear power plant is the nuclear reactor.

History of the creation of a nuclear reactor

The first such device was built at the height of the Second World War in the USA by the famous scientist and engineer E. Fermi. Because of his unusual looking, resembling a stack of graphite blocks stacked on top of each other, this nuclear reactor was called the Chicago Stack. It is worth noting that this device operated on uranium, which was placed just between the blocks.

Creation of a nuclear reactor in the Soviet Union

In our country, nuclear issues were also given increased attention. Despite the fact that the main efforts of scientists were concentrated on the military use of the atom, they actively used the results obtained for peaceful purposes. The first nuclear reactor, codenamed F-1, was built by a group of scientists led by the famous physicist I. Kurchatov at the end of December 1946. Its significant drawback was the absence of any cooling system, so the power of energy it released was extremely insignificant. At the same time, Soviet researchers completed the work they had begun, which resulted in the opening just eight years later of the world's first nuclear power plant in the city of Obninsk.

Operating principle of the reactor

A nuclear reactor is an extremely complex and dangerous technical device. Its principle of operation is based on the fact that during the decay of uranium, several neutrons are released, which, in turn, knock out elementary particles from neighboring uranium atoms. This chain reaction releases a significant amount of energy in the form of heat and gamma rays. At the same time, one should take into account the fact that if this reaction is not controlled in any way, then the fission of uranium atoms will short time can lead to powerful explosion with undesirable consequences.

In order for the reaction to proceed within strictly defined limits, the design of a nuclear reactor is of great importance. Currently, each such structure is a kind of boiler through which coolant flows. Water is usually used in this capacity, but there are nuclear power plants that use liquid graphite or heavy water. It is impossible to imagine a modern nuclear reactor without hundreds of special hexagonal cassettes. They contain fuel-generating elements, through the channels of which coolants flow. This cassette is coated with a special layer that is capable of reflecting neutrons and thereby slowing down the chain reaction

Nuclear reactor and its protection

It has several levels of protection. In addition to the body itself, it is covered on top with special thermal insulation and biological protection. From an engineering point of view this building It is a powerful reinforced concrete bunker, the doors to which are closed as tightly as possible.

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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 production program. atomic weapons; 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. So the chemistry works nuclear reactors 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 on everything globe- in the Soviet Union and the United States of America, in France and Canada, in Norway and in England - valid different kinds reactors. 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.

Nuclear reactors.

A nuclear (atomic) reactor is a device designed to organize a controlled self-sustaining chain reaction of atomic fission, which is accompanied by the release large quantity energy.

Nuclear reactors are the main element of modern nuclear power plants.

The first nuclear reactors.

The first nuclear reactor was built and launched in December 1942 in the USA under the leadership of E. Fermi.

The first reactor built outside the United States was ZEEP, launched in Canada on September 5, 1945.

In Europe, the first nuclear reactor was the F-1 installation, which started operating on December 25, 1946 in Moscow under the leadership of I.V. Kurchatov.

By 1978, there were already about a hundred nuclear reactors operating in the world. various types.

History of the creation of nuclear reactors.

Scientific work in Germany.

The theoretical group “Uranium Project” of Nazi Germany, working in the Kaiser Wilhelm Society, was headed by Weizsäcker, but only formally. The actual leader was Heisenberg, who developed theoretical basis chain reaction, Weizsäcker and a group of participants focused on creating a “uranium machine” - the first reactor.

In the late spring of 1940, one of the group's scientists, Harteck, conducted the first experiment attempting to create a chain reaction using uranium oxide and a solid graphite moderator. However, the available fissile material was not sufficient to achieve this goal.

In 1941, at the University of Leipzig, a member of Heisenberg's group, Doepel, built a stand with a heavy water moderator, in experiments on which, by May 1942, it was possible to achieve the production of neutrons in quantities exceeding their absorption.

German scientists managed to achieve a full-fledged chain reaction in February 1945 in an experiment conducted in a mine working near Haigerloch. However, after a few weeks nuclear program Germany ceased to exist.

Scientific work in the USA.

The nuclear fission chain reaction (chain reaction for short) was first carried out by American scientists in December 1942. A group of physicists at the University of Chicago, led by E. Fermi, created the world's first nuclear reactor, called the Chicago Pile-1 (CP-1). It consisted of graphite blocks, between which were located balls of natural uranium and its dioxide. Fast neutrons appearing after the fission of 235U nuclei were slowed down by graphite to thermal energies, and then caused new nuclear fissions. Reactors like SR-1, in which the majority of fissions occur under the influence of thermal neutrons, are called thermal neutron reactors. They contain a lot of moderator compared to nuclear fuel.

Scientific work in the USSR.

In the USSR, theoretical and experimental studies features of the start-up, operation and control of reactors were carried out by a group of physicists and engineers under the leadership of Academician I.V. Kurchatov.

The first Soviet reactor F-1 was built in Laboratory No. 2 of the USSR Academy of Sciences (Moscow). This reactor was brought into critical condition on December 25, 1946. The F-1 reactor was assembled from graphite blocks and had the shape of a ball with a diameter of approximately 7.5 m. In the central part of the ball with a diameter of 6 m, uranium rods were placed through holes in the graphite blocks. The F-1 reactor, like the CP-1 reactor, did not have a cooling system, so it operated at very low power levels (Average power did not exceed 20 W. For comparison, the first American CP-1 reactor rarely exceeded 1 W of power). The results of research at the F-1 reactor became the basis for projects of more complex industrial reactors. In 1948, the I-1 reactor (according to other sources it was called A-1) for the production of plutonium was put into operation.

June 27, 1954 started to work world's first nuclear power plant with an electrical capacity of 5 MW in the city of Obninsk.

Physical principles of operation of a nuclear reactor.

Diagram of a thermal neutron nuclear reactor:

1 - Control rod.

2 - Radiation protection.

3 - Thermal insulation.

4 - Retarder.

5 - Nuclear fuel.

6 - Coolant.

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

Thus, the following options for the development of a chain reaction of atomic fission are possible:

1. ρ<0, Кэф

2. ρ>0, Kef>1 - the reactor is supercritical, the reaction intensity and reactor power increase.

3. ρ=0, Kef=1 - the reactor is critical, the reaction intensity and reactor power are constant.

Classification of nuclear reactors.

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

Energy reactors designed to produce electrical and thermal energy used in the energy sector, as well as for desalination 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.

Transport reactors designed to supply energy to engines Vehicle. 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 designed to study various physical quantities, the significance of which is necessary for the design and operation of nuclear reactors. The power of such reactors usually does not exceed several kW.

Research reactors, in which fluxes of neutrons and gamma rays created in the core are used for research in the field of nuclear physics, physics solid, radiation chemistry, biology, for testing materials intended to operate in intense neutron fluxes (including parts of nuclear reactors), for the production of isotopes. The power of research reactors is usually no more than 100 MW. The released energy is usually not used.

Industrial (weapons, isotope) reactors used to produce isotopes used in various areas. Most widely used for the production of nuclear weapons materials, such as 239Pu. Industrial nuclear reactors also include reactors used for desalination of sea water.

Nuclear reactors are often 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.

Nuclear reactor. Atomic reactor.

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 initially 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 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 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;
System remote control.

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.

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

By 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 different emergency situations In each reactor, an emergency termination of the chain reaction is provided, carried out by dropping all absorbing rods into the core - an emergency protection system.

Residual Heat

An important issue directly related to nuclear safety is decay heat. This is a specific feature of nuclear fuel, which consists in the fact that, after the 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 high-power power reactors it is significant in absolute terms. 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 include 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. -

The first nuclear reactor was built in December 1942 in the USA under the leadership 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 of various types operating in the world. Components any Nuclear reactor are: core With nuclear fuel, usually surrounded by a neutron reflector, coolant, chain reaction control system, radiation protection, remote control system ( 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 of nuclear fission occurs and energy is released. State Nuclear reactor is characterized by an 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 a critical state, a stationary process is in progress and the number of fissions 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 thermal neutron nuclear reactor can use natural uranium that is not enriched with 235 U (this was the first nuclear reactor). If there is no moderator in the core, then the bulk of fissions is caused by 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 resulting from 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)

233 U

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 noticeable 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 the absorption of neutrons is very weak and the neutron manages to collide with the 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 main part 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

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