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

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

In fact, the same process is going on there as in a nuclear explosion. Only now the explosion occurs very quickly, but in the reactor all this is stretched out for a long time. As a result, everything remains safe and sound, and we receive energy. Not so much that everything around was immediately blown up, but quite enough to provide the city with electricity.

Before you understand how a controlled nuclear reaction is going, you need to know what is nuclear reaction generally.

Nuclear reaction Is the process of transformation (fission) of atomic nuclei during their interaction with elementary particles and gamma quanta.

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

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

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

The history of the creation of a nuclear reactor

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

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

The principle of operation of a nuclear (atomic) reactor

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

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

As we have already said, during the decay of a heavy uranium nucleus, lighter elements and several neutrons are formed. The resulting neutrons collide with other nuclei, also causing their fission. In this case, the number of neutrons grows like an avalanche.

It should be mentioned here neutron multiplication factor ... So, if this coefficient exceeds a value equal to one, a nuclear explosion occurs. If the value is less than one, there are too few neutrons and the reaction is extinguished. But if you maintain the value of the coefficient equal to one, the reaction will proceed for a long time and stably.

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

How is a nuclear reactor started?

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

The critical mass is the mass of fissile matter required to start a nuclear chain reaction.

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

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

Device 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 the microparticles of a substance are in a state with a rest energy greater than in another possible one, the transition to which exists. A spontaneous transition is always hindered by an energy barrier, to overcome which a microparticle must receive from the outside a certain amount of energy - excitation energy. The exoenergetic reaction consists in the fact that in the transformation following the excitation, more energy is released than is required to excite the process. There are two ways to overcome the energy barrier: either due to the kinetic energy of colliding particles, or due to the binding energy of the joining particle.

If we bear in mind the macroscopic scales of energy release, then the kinetic energy necessary for the excitation of reactions must have all or, first, at least some fraction of the particles of the substance. This is achievable only when the temperature of the medium rises to a value at which the energy of the thermal motion approaches the value of the energy threshold, which limits the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of kelvin, 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 realized in practice only in the synthesis of the lightest nuclei, for which the Coulomb barriers are minimal (thermonuclear fusion).

Excitation by attaching 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 particles of the forces of attraction. But on the other hand, the particles themselves are needed to excite the reactions. And if we again have in mind not a separate act of reaction, but the production of energy on a macroscopic scale, then this is possible only when a chain reaction occurs. The latter arises when the particles that excite the reaction reappear as products of an exoenergetic reaction.

Design

Any nuclear reactor consists of the following parts:

• Core with nuclear fuel and moderator;
• A neutron reflector surrounding the core;
• Chain reaction control system, including emergency protection;
• Radiation protection;
• Remote control system.

Physical principles of work

See also the main articles:

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

These values ​​are characterized by the following values:

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

The condition for the criticality of a nuclear reactor:

, where

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

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

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

, where

• η is the neutron yield for two absorptions.

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

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

The least critical mass is possessed by reactors in which aqueous solutions of salts of pure fissile isotopes with a water reflector of neutrons serve as fuel. For 235 U this mass is 0.8 kg, for 239 Pu it is 0.5 kg. It is widely known, however, that the critical mass for the LOPO reactor (the world's first enriched uranium reactor) with a beryllium oxide reflector was 0.565 kg, despite the 235 isotope enrichment being only slightly over 14%. Theoretically, it has the smallest critical mass, for which this value is only 10 g.

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

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

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

Iodine pit

Main article: Iodine pit

Iodine well - the state of a nuclear reactor after its shutdown, characterized by the accumulation of a short-lived isotope of 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 appointment

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

• Power reactors, intended for the production of electrical and thermal energy used in the power industry, as well as for the desalination of sea water (reactors for desalination 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 is distinguished:
  • Transport reactors designed to supply energy to vehicle engines. The widest application groups are marine transport reactors used on submarines and various surface ships, as well as reactors used in space technology.
• Experimental reactors designed to study various physical quantities, the value of which is necessary for the design and operation of nuclear reactors; the power of such reactors does not exceed several kW.
• Research reactors, in which the fluxes of neutrons and gamma quanta generated in the core are used for research in the field of nuclear physics, solid state physics, radiation chemistry, biology, for testing materials intended for operation in intense neutron fluxes (including parts nuclear reactors), for the production of isotopes. The power of research reactors does not exceed 100 MW. The released energy is usually not used.
• Industrial (weapons, isotope) reactors used for the production of isotopes used in various fields. Most widely used for the production of nuclear weapons materials such as 239 Pu. Industrial reactors also include reactors used for desalination of seawater.

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

By neutron spectrum

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

By fuel placement

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

In a heterogeneous reactor, the fuel and the moderator can be 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 of homogeneity / heterogeneity is not the design, but the placement of fuel blocks at a distance exceeding the neutron moderation length in a given moderator. Thus, reactors with a so-called "tight grid" are calculated as homogeneous, although the fuel in them is usually separated from the moderator.

Blocks of nuclear fuel in a heterogeneous reactor are called fuel assemblies (FA), which are located in the core in the nodes of a regular grid, forming cell.

By type of fuel

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

By the degree of enrichment:

• natural uranium
• poorly 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 the nature of the moderator

• C (graphite, see Graphite-gas reactor, Graphite-water reactor)
• H 2 O (water, see Light Water Reactor, Water Moderated Reactor, VVER)
• D 2 O (heavy water, see Heavy water nuclear reactor, CANDU)
• Metal hydrides
• Without moderator (see Fast Reactor)

By design

By the way of generating steam

• Reactor with external steam generator (See Pressurized water reactor, VVER)

IAEA classification

• PWR (pressurized water reactors) - 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 widespread in the world are pressurized water (about 62%) and boiling (20%) reactors.

Reactor materials

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

The radiation instability of materials affects less 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, radiolysis of water is insignificant in non-boiling power reactors (for example, VVER), while in powerful research reactors a significant amount of an explosive mixture is released. The reactors have special systems for burning it.

Reactor materials are in contact with each other (fuel element cladding with coolant and nuclear fuel, fuel assemblies - with coolant and moderator, etc.). Naturally, the contacting materials must be chemically inert (compatible). An example of incompatibility is uranium and hot water, which react chemically.

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 materials of construction, especially for those parts of a power reactor that must withstand high pressures.

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 debris) and slagging(for stable isotopes).

The main reason for the poisoning of the reactor is the one with the largest neutron absorption cross section (2.6 · 10 6 barn). Half-life 135 Xe 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 ( 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:

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

Fission of nuclei produces a large number of stable fragments, which differ in absorption cross sections in comparison with the absorption cross section of a fissile isotope. The concentration of fragments with a large absorption cross section reaches saturation during the first few days of reactor operation. These are mainly fuel elements of different "ages".

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

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

The ratio of the number of fissile Pu isotopes formed in a nuclear reactor to the amount of 235 U burned out is called conversion rate K K. The K K value 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 rate of reproduction to the rate of burnup is called reproduction rate K V. In thermal reactors K B< 1, а для реакторов на быстрых нейтронах К В может достигать 1,4-1,5. Рост К В для реакторов на быстрых нейтронах объясняется главным образом тем, что, особенно в случае 239 Pu, для быстрых нейтронов g is growing and a falls.

Nuclear reactor control

Controlling a nuclear reactor is possible only due to the fact that part of the neutrons during fission are emitted from the fragments with a delay that can range from several milliseconds to several minutes.

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

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

Residual heat generation

Residual heat is an important issue directly related to nuclear safety. This is a specific feature of nuclear fuel, which consists in the fact that, after the termination of the fission chain reaction and the usual thermal inertia 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 release is a consequence of the β- and γ- decay of fission products that have accumulated in the fuel during the operation of the reactor. As a result of decay, the nuclei of fission products pass into a more stable or completely stable state with the release of significant energy.

Although the power of residual heat release rapidly decreases to values ​​that are small in comparison with stationary values, in powerful power reactors it is significant in absolute terms. For this reason, the residual heat release entails the need for a long time to provide heat removal from the reactor core after its shutdown. This task requires the presence in the design of the reactor plant of cooling systems with a reliable power supply, and also necessitates a long-term (for 3-4 years) storage of spent nuclear fuel in storage facilities with a special temperature regime - storage pools, which are usually located in the immediate vicinity of the reactor.

see also

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

Literature

• V.E. Levin 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 (edit)

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

Topic: Physical foundations of nuclear energy. Nuclear reactor.

Lesson objectives: updating existing knowledge; continue the formation of concepts: fission of uranium nuclei, nuclear chain reaction, conditions of its course, critical mass; introduce new concepts: a nuclear reactor, the main elements of a nuclear reactor, the design of a nuclear reactor and its principle of operation, control of a nuclear reaction, the classification of nuclear reactors and their use; continue the formation of skills to observe and draw conclusions, as well as develop the intellectual abilities and curiosity of students; continue fostering an attitude towards physics as an experimental science; to cultivate a conscientious attitude towards work, discipline, a positive attitude towards knowledge.

Lesson type: learning new material.

During the classes

1. Organizational moment.

Today in the lesson we will repeat the fission of uranium nuclei, the nuclear chain reaction, the conditions for its course, the critical mass, we will learn what a nuclear reactor is, the main elements of a nuclear reactor, the design of a nuclear reactor and its principle of operation, control of a nuclear reaction, the classification of nuclear reactors and their use.

2. Verification of the studied material.

Fission mechanism of uranium nuclei.

Tell us about the mechanism of the nuclear chain reaction.

Give an example of a nuclear fission reaction of a uranium nucleus.

What is called critical mass?

How is the chain reaction going in uranium if its mass is less than critical, more than critical?

What is the critical mass of uranium 295, is it possible to reduce the critical mass?

In what ways can the course of a nuclear chain reaction be changed?

What is the purpose of slowing down fast neutrons?

What substances are used as moderators?

3. Explanation of the new material.

: And what is the main part of any nuclear power plant? ( nuclear reactor)

Well done. So, guys, now let's dwell on this issue in more detail.

Historical reference.

Igor Vasilievich Kurchatov is an outstanding Soviet physicist, academician, founder and first director of the Institute of Atomic Energy from 1943 to 1960, the chief scientific leader of the atomic problem in the USSR, one of the founders of the use of nuclear energy for peaceful purposes. Academician of the USSR Academy of Sciences (1943). The tests of the first Soviet atomic bomb were carried out in 1949. Four years later, the world's first hydrogen bomb was successfully tested. And in 1949, Igor Vasilyevich Kurchatov began work on a project for a nuclear power plant. Nuclear power plant - a bulletin of the peaceful uses of atomic energy. The project was successfully completed: on July 27, 1954, our nuclear power plant became the first in the world! Kurchatov was jubilant and merry like a child!

Definition of a nuclear reactor.

A nuclear reactor is a device in which a controlled chain reaction of fission of some heavy nuclei is carried out and maintained.

The first nuclear reactor was built in 1942 in the USA under the leadership of E. Fermi. In our country, the first reactor was built in 1946 under the leadership of IV Kurchatov.

The main elements of a nuclear reactor are:

nuclear fuel (uranium 235, uranium 238, plutonium 239);

neutron moderator (heavy water, graphite, etc.);

coolant for the output of energy generated during the operation of the reactor (water, liquid sodium, etc.);

Control rods (boron, cadmium) - highly absorbing neutrons

Radiation-retarding protective shell (iron-filled concrete).

Operating principle nuclear reactor

Nuclear fuel is located in the core in the form of vertical rods called fuel elements (fuel rods). Fuel rods are designed to regulate the power of the reactor.

The mass of each fuel rod is much less than the critical one, therefore, a chain reaction cannot occur in one rod. It begins after all uranium rods are immersed in the core.

The core is surrounded by a layer of material that reflects neutrons (reflector) and a protective shell made of concrete, which traps neutrons and other particles.

Removing heat from fuel cells. Heat carrier - water washes the rod, heated to 300 ° C at high pressure, enters the heat exchangers.

The role of the heat exchanger is that water heated to 300 ° C gives off heat to ordinary water, turns into steam.

Nuclear reaction management

The reactor is controlled by rods containing cadmium or boron. With the rods extended from the reactor core K> 1, and with fully retracted - K< 1. Вдвигая стержни внутрь активной зоны, можно в любой момент времени приостановить развитие цепной реакции. Управление ядерными реакторами осуществляется дистанционно с помощью ЭВМ.

Slow neutron reactor.

The most efficient fission of uranium-235 nuclei occurs under the action of slow neutrons. Such reactors are called slow neutron reactors. The secondary neutrons produced by the fission reaction are fast. In order for their subsequent interaction with uranium-235 nuclei in a chain reaction to be most effective, they are slowed down by introducing a moderator into the core - a substance that reduces the kinetic energy of neutrons.

Fast neutron reactor.

Fast reactors cannot run on natural uranium. The reaction can be maintained only in an enriched mixture containing at least 15% of the uranium isotope. The advantage of fast reactors is that they generate a significant amount of plutonium, which can then be used as nuclear fuel.

Homogeneous and heterogeneous reactors.

Nuclear reactors, depending on the relative placement of the fuel and the moderator, are subdivided into homogeneous and heterogeneous. In a homogeneous reactor, the core is a homogeneous mass of fuel, moderator and coolant in the form of a solution, mixture, or melt. A reactor is called heterogeneous, in which fuel in the form of blocks or fuel assemblies is placed in a moderator, forming a regular geometric lattice in it.

Converting the internal energy of atomic nuclei into electrical energy.

A nuclear reactor is the main element of a nuclear power plant (NPP), which converts thermal nuclear energy into electrical energy. Energy conversion takes place according to the following scheme:

internal energy of uranium nuclei -

kinetic energy of neutrons and nuclear fragments -

internal energy of water -

internal energy of steam -

kinetic energy of steam -

kinetic energy of the turbine rotor and generator rotor -

Electric Energy.

Use of nuclear reactors.

Depending on the purpose, nuclear reactors are power, converters and breeders, research and multipurpose, transport and industrial.

Nuclear power reactors are used to generate electricity in nuclear power plants, ship power plants, nuclear power plants, and nuclear heating plants.

Reactors designed to produce secondary nuclear fuel from natural uranium and thorium are called converters or breeders. In the reactor-converter secondary nuclear fuel is formed less than the initially consumed.

In the breeder reactor, an expanded breeding of nuclear fuel is carried out, i.e. it turns out more than it was spent.

Research reactors are used to study the processes of interaction of neutrons with matter, to study the behavior of reactor materials in intense fields of neutron and gamma radiation, radiochemical in biological research, production of isotopes, experimental research of the physics of nuclear reactors.

The reactors have different capacities, stationary or pulsed operation. Multipurpose reactors are those that serve multiple purposes, such as power generation and nuclear fuel.

Environmental disasters at nuclear power plants

1957 - an accident in the UK

1966 - partial melting of the core after failure of the reactor cooling near Detroit.

1971 - A lot of contaminated water went into the US river

1979 - the largest accident in the United States

1982 - release of radioactive steam into the atmosphere

1983 - a terrible accident in Canada (radioactive water leaked out for 20 minutes - a ton per minute)

1986 - an accident in the UK

1986 - an accident in Germany

1986 - Chernobyl nuclear power plant

1988 - a fire at a nuclear power plant in Japan

Modern nuclear power plants are equipped with PCs, and earlier, even after an accident, the reactors continued to operate, since there was no automatic shutdown system.

4. Securing the material.

What is called a nuclear reactor?

What is nuclear fuel in a reactor?

What substance serves as a neutron moderator in a nuclear reactor?

What is the purpose of a neutron moderator?

What are control rods for? How are they used?

What is used as a coolant in nuclear reactors?

Why do you need the mass of each uranium rod to be less than the critical mass?

5. Execution of the test.

What particles are involved in the fission of uranium nuclei?
A. protons;
B. neutrons;
V. electrons;
G. helium nucleus.

What is the critical mass of uranium?
A. the highest, at which a chain reaction is possible;
B. any mass;
V. is the smallest, at which a chain reaction is possible;
G. the mass at which the reaction will stop.

What is approximately the critical mass of uranium 235?
A. 9 kg;
B. 20 kg;
H. 50 kg;
G. 90 kg.

Which substances from the following can be used in nuclear reactors as neutron moderators?
A. graphite;
B. cadmium;
B. heavy water;
G. bor.

For a nuclear chain reaction to occur at a nuclear power plant, the neutron multiplication factor must be:
A. is equal to 1;
B. is greater than 1;
V. less than 1.

Regulation of the rate of fission of heavy atoms in nuclear reactors is carried out:
A. due to the absorption of neutrons when lowering rods with an absorber;
B. due to an increase in heat removal with an increase in the speed of the coolant;
B. by increasing the supply of electricity to consumers;
G. by reducing the mass of nuclear fuel in the core when removing the fuel rods.

What energy transformations take place in a nuclear reactor?
A. the internal energy of atomic nuclei is converted into light energy;
B. the internal energy of atomic nuclei is converted into mechanical energy;
C. the internal energy of atomic nuclei is converted into electrical energy;
G. Among the answers there is no correct one.

In 1946, the first nuclear reactor was built in the Soviet Union. Who was the leader of this project?
A. S. Korolev;
B. I. Kurchatov;
V. D. Sakharov;
G. A. Prokhorov.

Which way do you consider the most acceptable for increasing the reliability of nuclear power plants and preventing contamination of the external environment?
A. development of reactors capable of automatically cooling the reactor core regardless of the operator's will;
B. increasing the literacy of NPP operation, the level of professional preparedness of NPP operators;
B. development of highly efficient technologies for dismantling nuclear power plants and processing radioactive waste;
D. location of reactors deep underground;
D. refusal to build and operate a nuclear power plant.

What sources of environmental pollution are associated with the operation of a nuclear power plant?
A. uranium industry;
B. nuclear reactors of various types;
B. radiochemical industry;
D. places of processing and disposal of radioactive waste;
D. use of radionuclides in the national economy; E. nuclear explosions.

Answers: 1 B; 2 B; 3V; 4 A, B; 5 A; 6 A; 7 B ;. 8 B; 9 B. V; 10 A, B, C, D, E.

6. Lesson summary.

What new have you learned in the lesson today?

What did you like in the lesson?

What questions do you have?

Nuclear reactors have one job: to split atoms in a controlled reaction and use the released energy to generate electrical power. For many years, reactors have been viewed as both a miracle and a threat.

When the first US commercial reactor went into operation at Shippingport, Pennsylvania, in 1956, the technology was hailed as a source of energy for the future, and some thought the reactors would make generating electricity too cheap. Currently, 442 nuclear reactors have been built around the world, about a quarter of these reactors are located in the United States. The world has become dependent on nuclear reactors for 14 percent of its electricity. Futurists even fantasized about atomic cars.

When the cooling system malfunctioned in 1979 at the Block 2 reactor at the Three Mile Island Power Plant in Pennsylvania and, as a result, its radioactive fuel partially melted, warm feelings about the reactors changed radically. Despite the blockage of the destroyed reactor and no significant radiation exposure, many people began to view the reactors as too complex and vulnerable, with potentially catastrophic consequences. People were also worried about the radioactive waste from the reactors. As a result, construction of new nuclear power plants in the United States has stalled. When a more serious accident occurred at the Chernobyl nuclear power plant in the Soviet Union in 1986, nuclear power seemed doomed.

But in the early 2000s, nuclear reactors began to make a comeback, thanks to rising energy needs and declining supplies of fossil fuels, as well as growing concerns about climate change from carbon dioxide emissions.

But in March 2011, another crisis hit - this time the earthquake hit Fukushima 1, a nuclear power plant in Japan.

Using a nuclear reaction

Simply put, in a nuclear reactor, atoms split and release the energy that holds their parts together.

If you've forgotten high school physics, we'll remind you how nuclear fission works. Atoms are like tiny solar systems, with a core like the sun and electrons like planets in orbit around it. The nucleus is made up of particles called protons and neutrons that are bound together. The force that binds the elements of the core is difficult even to imagine. It is many billions of times stronger than the force of gravity. Despite this enormous power, it is possible to split the nucleus by shooting neutrons at it. When this is done, a lot of energy will be released. When atoms disintegrate, their particles crash into nearby atoms, splitting them, and those, in turn, are next, next and next. There is a so-called chain reaction.

Uranium, an element with large atoms, is ideal for the fission process because the force that binds particles to its core is relatively weak compared to other elements. Nuclear reactors use a specific isotope called Haveearly235 ... Uranium-235 is rare in nature; ore from uranium mines contains only about 0.7% Uranium-235. This is why reactors use enrichedHavewounds which is created by separating and concentrating Uranium-235 through the process of gas diffusion.

A chain reaction process can be created in an atomic bomb, similar to those that were dropped on the Japanese cities of Hiroshima and Nagasaki during World War II. But in a nuclear reactor, the chain reaction is controlled by inserting control rods made of materials such as cadmium, hafnium or boron, which absorb some of the neutrons. This still allows the fission process to release enough energy to heat water to about 270 degrees Celsius and turn it into steam, which is used to turn the power plant's turbines and generate electricity. Basically, in this case, a controlled nuclear bomb works instead of coal, creating electricity, except that the energy for boiling water comes from splitting atoms instead of burning carbon.

Nuclear Reactor Components

There are several different types of nuclear reactors, but they all share some common characteristics. They all have a supply of radioactive fuel pellets - usually uranium oxide - that are located in pipes to form fuel rods in active zonesereactor.

The reactor also has the previously mentioned managingerodand- of neutron absorbing material such as cadmium, hafnium or boron, which is inserted to control or stop the reaction.

The reactor also has moderator, a substance that slows down neutrons and helps control the fission process. Most reactors in the United States use plain water, but reactors in other countries sometimes use graphite, or heavyyuwatersat, in which hydrogen is replaced by deuterium, an isotope of hydrogen with one proton and one neutron. Another important part of the system is coolingand Ifluidb usually ordinary water, which absorbs and transfers heat from the reactor to create steam to rotate the turbine and cools the reactor zone so that it does not reach the temperature at which the uranium will melt (about 3815 degrees Celsius).

Finally, the reactor is enclosed in shellat, a large, heavy structure, usually several meters thick, made of steel and concrete, which keeps radioactive gases and liquids inside where they cannot harm anyone.

There are a number of different reactor designs in use, but one of the most common is pressurized water power reactor (VVER)... In such a reactor, water is forced into contact with the core, and then remains there under such pressure that it cannot turn into steam. This water then comes into contact in the steam generator with water supplied without pressure, which turns into steam, which rotates the turbines. There is also a construction high power channel type reactor (RBMK) with one water circuit and fast reactor with two sodium and one water circuit.

How safe is a nuclear reactor?

It is rather difficult to answer this question and it depends on who you ask and how you understand “safe”. Are you worried about radiation or radioactive waste generated in reactors? Or are you more worried about the possibility of a catastrophic accident? What degree of risk do you consider to be an acceptable trade-off for the benefits of nuclear power? And to what extent do you trust the government and nuclear power?

"Radiation" is a compelling argument, mainly because we all know that high doses of radiation, such as from a nuclear bomb, can kill many thousands of people.

Nuclear proponents, however, point out that we are all regularly exposed to radiation from various sources, including cosmic rays and natural radiation emitted by the Earth. The average annual radiation dose is about 6.2 millisieverts (mSv), half from natural sources and half from artificial sources, ranging from chest x-rays, smoke detectors and luminous watch faces. How much radiation do we get from nuclear reactors? Only a fraction of a percent of our typical annual exposure is 0.0001 mSv.

While all nuclear power plants inevitably allow small amounts of radiation to escape, regulatory commissions keep the plant operators in strict compliance. They cannot expose people living around the station to more than 1 mSv per year, and workers at the plant have a threshold of 50 mSv per year. This may sound like a lot, but according to the Nuclear Regulatory Commission, there is no medical evidence that annual radiation doses below 100 mSv pose any risks to human health.

But it is important to note that not everyone agrees with such a complacent assessment of the radiation risks. For example, Physicians for Social Responsibility, a longtime critic of the nuclear industry, studied children living around German nuclear power plants. The study found that people living within 5 km of the plant had a double risk of contracting leukemia compared to those living further from the plant.

Nuclear Waste Reactor

Nuclear power is touted by its supporters as "clean" energy because the reactor does not emit large amounts of greenhouse gases into the atmosphere compared to coal-fired power plants. But critics point to another environmental problem - the disposal of nuclear waste. Some of the waste, spent fuel from reactors, still releases radioactivity. Another unnecessary material that needs to be retained is high level radioactive waste, the liquid residue from the reprocessing of spent fuel, in which partly uranium remains. Right now, most of this waste is stored locally at nuclear power plants in ponds of water, which absorb some of the remaining heat generated by the spent fuel and help shield workers from radiation exposure.

One problem with spent nuclear fuel is that it has been altered by fission; when large uranium atoms fission, they create byproducts - radioactive isotopes of several light elements such as Cesium-137 and Strontium-90, called fission products... They are hot and highly radioactive, but eventually, over a period of 30 years, they decay into less dangerous forms. This period is called for them NSperiodohmhalf-life... For other radioactive elements, the half-life will be different. In addition, some uranium atoms also capture neutrons, forming heavier elements such as plutonium. These transuranic elements do not generate as much heat or penetrating radiation as fission products, but they take much longer to decay. Plutonium-239, for example, has a half-life of 24,000 years.

These radioactiveeretreatNS high level from reactors are dangerous to humans and other life forms because they can emit a huge, lethal dose of radiation even from a short exposure. Ten years after removing the remaining fuel from the reactor, for example, they emit 200 times more radioactivity per hour than it takes to kill a person. And if waste ends up in groundwater or rivers, it can end up in the food chain and endanger large numbers of people.

Since waste is so dangerous, many people are in a difficult situation. 60,000 tons of waste is located at nuclear power plants close to large cities. But finding a safe place to store your waste is not easy.

What could go wrong with a nuclear reactor?

With government regulators looking back on their experiences, engineers have spent a lot of time over the years designing reactors for optimal safety. They just don't break down, work properly, and have back-up safety measures if something goes wrong. As a result, year after year, nuclear power plants seem to be quite safe compared to, say, air travel, which regularly kills 500 to 1,100 people a year around the world.

Nevertheless, nuclear reactors are overtaken by major breakdowns. On the international scale of nuclear events, which ranks reactor accidents from 1 to 7, there have been five accidents since 1957, which are rated 5 to 7.

The worst nightmare is a breakdown of the cooling system, which causes the fuel to overheat. The fuel turns into a liquid, and then burns through the containment, spewing out radioactive radiation. In 1979, Unit 2 at the Three Mile Island NPP (USA) was on the verge of this scenario. Fortunately, the well-designed containment system was strong enough to stop radiation from escaping.

The USSR was less fortunate. A severe nuclear accident happened in April 1986 at the 4th power unit at the Chernobyl nuclear power plant. This was caused by a combination of system failures, design flaws and poorly trained personnel. During a routine check, the reaction suddenly increased and the control rods jammed, preventing an emergency shutdown. The sudden build-up of steam caused two thermal explosions, propelling the reactor's graphite moderator into the air. In the absence of anything to cool the reactor fuel rods, their overheating began and their complete destruction, as a result of which the fuel took on a liquid form. Many workers of the station and liquidators of the accident were killed. A large amount of radiation spread over an area of ​​323,749 square kilometers. The number of deaths caused by radiation is still unclear, but the World Health Organization says it may have caused 9,000 cancer deaths.

The creators of nuclear reactors provide guarantees based on probabilistic assessmente in which they try to balance the potential harm from an incident with the likelihood that it actually occurs. But some critics say they should prepare, instead, for the rare, most unexpected, yet very dangerous events. A case in point is the accident in March 2011 at the Fukushima 1 nuclear power plant in Japan. The station was reportedly designed to withstand a massive earthquake, but not as catastrophic as the magnitude 9.0 quake that raised a 14-meter tsunami wave over dams designed to withstand the 5.4-meter wave. The tsunami onslaught destroyed the standby diesel generators that were meant to power the cooling system of the six reactors of the nuclear power plant in the event of a power outage. Thus, even after the control rods of the Fukushima reactors stopped the fission reaction, the still hot fuel allowed temperatures to rise dangerously inside the destroyed reactors.

Japanese officials resorted to at least - flooding the reactors with huge amounts of seawater with the addition of boric acid, which could prevent the disaster, but destroyed the reactor equipment. Eventually, with the help of fire engines and barges, the Japanese were able to pump fresh water into the reactors. But by then, monitoring had already revealed alarming levels of radiation in the surrounding land and water. In one village 40 km from this nuclear power plant, the radioactive element Cesium-137 was found at levels much higher than after the Chernobyl disaster, which raised doubts about the possibility of living in this zone.

What is a nuclear reactor?

A nuclear reactor, formerly known as a "nuclear boiler", is a device used to initiate and control a sustained nuclear chain reaction. Nuclear reactors are used in nuclear power plants to generate electricity and for ship engines. The heat from nuclear fission is transferred to a working fluid (water or gas) that passes through steam turbines. Water or gas drives the blades of a ship, or rotates electric generators. The steam generated by a nuclear reaction can in principle be used for the thermal industry or for district heating. Some reactors are used for the production of isotopes for medical and industrial purposes, or for the production of weapons-grade plutonium. Some of them are for research purposes only. Today there are about 450 nuclear power reactors that are used to generate electricity in about 30 countries around the world.

The principle of operation of a nuclear reactor

Just as conventional power plants generate electricity by using thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled fission into thermal energy for further conversion into mechanical or electrical forms.

The fission process of an atomic nucleus

When a significant number of decaying atomic nuclei (such as uranium-235 or plutonium-239) absorb a neutron, nuclear decay can occur. A heavy nucleus splits into two or more light nuclei (fission products), releasing kinetic energy, gamma radiation and free neutrons. Some of these neutrons can subsequently be absorbed by other fissioning atoms and cause further fission, which releases more neutrons, and so on. This process is known as a nuclear chain reaction.

To control such a nuclear chain reaction, neutron absorbers and moderators can alter the fraction of neutrons that go into fission of more nuclei. Nuclear reactors are controlled manually or automatically to be able to stop the decay reaction when dangerous situations are identified.

Commonly used neutron flux regulators are ordinary ("light") water (74.8% of reactors in the world), solid graphite (20% of reactors) and "heavy" water (5% of reactors). In some experimental types of reactors, it is proposed to use beryllium and hydrocarbons.

Heat release in a nuclear reactor

The working area of ​​the reactor generates heat in several ways:

• The kinetic energy of fission products is converted into thermal energy when nuclei collide with neighboring atoms.
• The reactor absorbs some of the gamma radiation generated during fission and converts its energy into heat.
• Heat is generated by the radioactive decay of fission products and those materials that have been exposed during the absorption of neutrons. This heat source will remain unchanged for some time, even after the reactor is shut down.

In nuclear reactions, a kilogram of uranium-235 (U-235) releases about three million times more energy than a conventional kilogram of coal burned (7.2 × 1013 joules per kilogram of uranium-235 versus 2.4 × 107 joules per kilogram coal),

Nuclear reactor cooling system

The coolant in a nuclear reactor - usually water, but sometimes gas, liquid metal (such as liquid sodium), or molten salt - circulates around the reactor core to absorb the generated heat. Heat is removed from the reactor and then used to generate steam. Most reactors use a cooling system that is physically isolated from the water that boils and generates steam used for turbines like a pressurized water reactor. However, in some reactors, steam turbine water boils directly in the reactor core; for example, in a pressurized water reactor.

Monitoring the neutron flux in the reactor

The power output of the reactor is controlled by controlling the number of neutrons capable of causing more fission.

Control rods that are made of "neutron poison" are used to absorb neutrons. The more neutrons are absorbed by the control rod, the fewer neutrons can cause further fission. Thus, immersing the absorption rods deep into the reactor decreases its output power and, conversely, removing the control rod will increase it.

At the first level of control in all nuclear reactors, the process of delayed neutron emission of a number of neutron-enriched fission isotopes is an important physical process. These delayed neutrons make up about 0.65% of the total number of neutrons produced during fission, and the rest (the so-called "fast neutrons") are produced immediately during fission. The fission products that form delayed neutrons have half-lives ranging from milliseconds to several minutes, and therefore it takes a significant amount of time to accurately determine when a reactor has reached a critical point. Maintaining the reactor in chain reactivity mode, where delayed neutrons are required to reach critical mass, is achieved by mechanical devices or human control to control the chain reaction in "real time"; otherwise, the time between reaching criticality and melting the core of a nuclear reactor as a result of an exponential surge in a normal nuclear chain reaction would be too short to intervene. This last step, where delayed neutrons are no longer required to maintain criticality, is known as prompt criticality. There is a scale for describing criticality in numerical form, in which the seed criticality is indicated by the term "zero dollars", the fast tipping point as "one dollar", other points in the process are interpolated in "cents".

In some reactors, the coolant also acts as a neutron moderator. The moderator increases the power of the reactor by causing the fast neutrons that are released during fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission. If the coolant is also a neutron moderator, then temperature changes can affect the density of the coolant / moderator and hence the change in reactor power output. The higher the coolant temperature, the less dense it will be, and therefore the less effective moderator.

In other types of reactors, the coolant acts as a "neutron poison" by absorbing neutrons in the same way as control rods. In these reactors, the power output can be increased by heating the coolant, making it less dense. Nuclear reactors generally have automatic and manual systems for shutting down the reactor for emergency shutdown. These systems put a large amount of "neutron poison" (often boron in the form of boric acid) into the reactor in order to stop the fission process if dangerous conditions are detected or suspected.

Most types of reactors are sensitive to a process known as a "xenon pit" or "iodine pit". The widespread fission product, xenon-135, plays the role of a neutron absorber that seeks to shut down the reactor. The accumulation of xenon-135 can be controlled by maintaining a power level high enough to destroy it by absorbing neutrons as quickly as it is produced. The fission also leads to the formation of iodine-135, which in turn decays (with a half-life of 6.57 hours) to form xenon-135. When the reactor is shut down, iodine-135 continues to decay to form xenon-135, which makes restarting the reactor more difficult within a day or two, as xenon-135 decays to form cesium-135, which is not a neutron absorber like xenon. 135, with a half-life of 9.2 hours. This temporary state is the "iodine pit". If the reactor has sufficient additional power, then it can be restarted. The more xenon-135 turns into xenon-136, which is less of a neutron absorber, and within a few hours the reactor experiences the so-called "xenon burnup stage". Additionally, control rods should be inserted into the reactor to compensate for the absorption of neutrons to replace the lost xenon-135. Failure to properly follow this procedure was the key reason for the accident at the Chernobyl nuclear power plant.

Reactors used in shipboard nuclear installations (especially nuclear submarines) often cannot be started in continuous power generation in the same way as land-based power reactors. In addition, such power plants must have a long period of operation without changing the fuel. For this reason, many designs use highly enriched uranium but contain a burnable neutron absorber in the fuel rods. This makes it possible to design a reactor with an excess of fissile material, which is relatively safe at the beginning of the burnup of the reactor fuel cycle due to the presence of neutron absorbing material, which is subsequently replaced by conventional long-lived neutron absorbers (more durable than xenon-135), which gradually accumulate over the life of the reactor. fuel.

How is electricity produced?

The energy generated in the fission process generates heat, some of which can be converted into usable energy. A common method of using this thermal energy is to use it to boil water and generate steam under pressure, which in turn rotates a steam turbine drive, which turns an alternator and generates electricity.

The history of the appearance of the first reactors

Neutrons were discovered in 1932. The scheme of a chain reaction provoked by nuclear reactions as a result of exposure to neutrons was first carried out by the Hungarian scientist Leo Sillard, in 1933. He applied for a patent for the idea of ​​his simple reactor during the next year at the Admiralty in London. However, Szilard's idea did not include the theory of nuclear fission as a source of neutrons, since this process had not yet been discovered. Szilard's ideas for nuclear reactors using a neutron-mediated nuclear chain reaction in light elements proved to be impracticable.

The impetus for creating a new type of reactor using uranium was the discovery of Lise Meitner, Fritz Strassmann and Otto Hahn in 1938, who "bombarded" uranium with neutrons (using the alpha decay reaction of beryllium, a "neutron gun") to form barium, which, as they believed originated from the decay of uranium nuclei. Subsequent studies in early 1939 (Szilard and Fermi) showed that some neutrons were also produced during the disintegration of the atom and this made possible the nuclear chain reaction that Szilard had foreseen six years ago.

On August 2, 1939, Albert Einstein signed a letter written by Szilard to President Franklin D. Roosevelt, which states that the discovery of uranium fission could lead to the creation of "extremely powerful bombs of a new type." This gave impetus to the study of reactors and radioactive decay. Szilard and Einstein knew each other well and worked together for many years, but Einstein never thought of such an opportunity for nuclear power until Szilard informed him, at the very beginning of his search, to write an Einstein-Szilard letter to warn the US government,

Shortly thereafter, in 1939, Nazi Germany invaded Poland, starting World War II in Europe. Officially, the US was not yet at war, but in October, when the Einstein-Szilard letter was delivered, Roosevelt noted that the purpose of the study is to make sure "the Nazis will not blow us up." The US nuclear project began, albeit with some delay, as skepticism remained (in particular from Fermi) and also due to the small number of government officials who initially oversaw the project.

The following year, the US government received the Frisch-Peierls memorandum from the UK, which stated that the amount of uranium required to carry out a chain reaction was significantly less than previously thought. The memorandum was created with the participation of Maud Commity, who worked on the atomic bomb project in the UK, later codenamed "Tube Alloys" and later accounted for as part of the Manhattan Project.

Ultimately, the first man-made nuclear reactor, named the Chicago Woodpile 1, was built at the University of Chicago by a team led by Enrico Fermi in late 1942. By this time, the US atomic program had already been accelerated by the country's entry into the war. The Chicago Woodpile reached its breaking point on December 2, 1942, at 15:25. The frame of the reactor was wooden, holding together a stack of graphite blocks (hence the name) with nested "briquettes" or "pseudospheres" of natural uranium oxide.

Beginning in 1943, shortly after the creation of the Chicago Woodpile, the US military developed a series of nuclear reactors for the Manhattan Project. The main goal of creating the largest reactors (located at the Hanford complex in Washington state) was the mass production of plutonium for nuclear weapons. Fermi and Szilard filed a patent application for the reactors on December 19, 1944. Its issuance was delayed for 10 years due to wartime secrecy.

"The first in the world" - This inscription was made on the site of the EBR-I reactor, which is now a museum near the city of Arco, Idaho. Originally named "Chicago Woodpile 4", this reactor was built under the direction of Walter Zinn for Aregonne National Laboratory. This experimental fast neutron breeder reactor was in the possession of the United States Atomic Energy Commission. The reactor produced 0.8 kW of power when tested on December 20, 1951, and 100 kW of power (electrical) the next day, with a design capacity of 200 kW (electrical).

Besides the military use of nuclear reactors, there were political reasons to continue researching atomic energy for peaceful purposes. US President Dwight D. Eisenhower delivered his famous Atoms for Peace speech to the UN General Assembly on December 8, 1953. This diplomatic move led to the proliferation of reactor technology both in the United States and around the world.

The first nuclear power plant built for civilian purposes was the "AM-1" nuclear power plant in Obninsk, launched on June 27, 1954 in the Soviet Union. It produced about 5 MW of electricity.

After World War II, the US military looked for other uses for nuclear reactor technology. Research carried out in the Army and Air Force has not been implemented; Nevertheless, the United States Navy achieved success by launching the nuclear submarine USS Nautilus (SSN-571) on January 17, 1955.

The first commercial nuclear power plant (Calder Hall in Sellafield, England) opened in 1956 with an initial capacity of 50 MW (later 200 MW).

The first portable nuclear reactor "Alco PM-2A" has been used to generate electricity (2 MW) for the American military base "Camp Century" since 1960.

Main components of a nuclear power plant

The main components of most types of nuclear power plants are:

Elements of a nuclear reactor

• Nuclear fuel (nuclear reactor core; neutron moderator)
• The original source of neutrons
• Neutron absorber
• Neutron gun (provides a constant source of neutrons to re-initiate the reaction after shutdown)
• Cooling system (often a neutron moderator and a cooler are the same, usually purified water)
• Control rods
• Nuclear Reactor Vessel (NRC)

Boiler water supply pump

• Steam generators (not in boiling-water reactors)
• Steam turbine
• Electricity generator
• Capacitor
• Cooling tower (not always required)
• Radioactive waste treatment system (part of the station for the disposal of radioactive waste)
• Nuclear fuel transfer site
• Spent fuel pool

Radiation safety system

• Rector protection system (SZR)
• Emergency diesel generators
• Emergency reactor core cooling system (ECCS)
• Emergency liquid control system (emergency boron injection, only in boiling-water reactors)
• Service water supply system for responsible consumers (SOTVOP)

Protective shell

• Remote Control
• Installation for emergency situations
• Nuclear training complex (as a rule, there is an imitation of the control panel)

Nuclear reactor classifications

Types of nuclear reactors

Nuclear reactors are classified in several ways; a summary of these classification methods is presented below.

Moderator classification of nuclear reactors

Used thermal reactors:

• Graphite reactors
  
• Pressurized water reactors
• Heavy water reactors(used in Canada, India, Argentina, China, Pakistan, Romania and South Korea).
• Light water reactors(LWR). Light water reactors (the most common type of thermal reactor) use ordinary water to control and cool the reactors. If the temperature of the water rises, then its density decreases, slowing down the neutron flux enough to cause further chain reactions. This negative feedback stabilizes the rate of the nuclear reaction. Graphite and heavy water reactors tend to heat up more intensely than light water reactors. Due to the additional heating, such reactors can use natural uranium / raw fuel.
• Reactors based on light element moderators.
• Molten salt moderated reactors(MSR) are controlled by the presence of light elements such as lithium or beryllium, which are found in the coolant / fuel matrix salts LiF and BEF2.
• Liquid Metal Cooled Reactors, where the coolant is a mixture of lead and bismuth, can use BeO oxide as a neutron absorber.
• Organic moderated reactors(OMR) uses diphenyl and terphenyl as moderator and cooling components.

Classification of nuclear reactors by type of coolant

• Water cooled reactor... There are 104 operating reactors in the United States. 69 of these are PWRs and 35 are boiling water reactors (BWRs). Pressurized water nuclear reactors (PWR) make up the overwhelming majority of all Western nuclear power plants. The main characteristic of the RVD type is the presence of a blower, a special high-pressure vessel. Most commercial high pressure and naval reactors use superchargers. During normal operation, the blower is partially filled with water and a steam bubble is maintained above it, which is created by heating the water with immersion heaters. In the normal mode, the supercharger is connected to the high-pressure reactor vessel (HPRR) and the pressure compensator ensures the presence of a cavity in the event of a change in the volume of water in the reactor. This scheme also provides control of the pressure in the reactor by increasing or decreasing the steam pressure in the compensator using heaters.

• Heavy water high pressure reactors They belong to a variety of pressurized water reactors (PWR), combining the principles of using pressure, an isolated thermal cycle, assuming the use of heavy water as a coolant and moderator, which is economically beneficial.
• Boiling water reactor(BWR). Boiling water reactor models are characterized by the presence of boiling water around the fuel rods at the bottom of the main reactor vessel. The boiling water reactor uses enriched 235U as fuel in the form of uranium dioxide. The fuel is assembled into rods housed in a steel vessel, which in turn is submerged in water. The nuclear fission process causes water to boil and steam to form. This steam passes through pipelines in turbines. The turbines are driven by steam, and this process generates electricity. During normal operation, the pressure is controlled by the amount of water vapor flowing from the reactor pressure vessel to the turbine.
• Pool type reactor
• Liquid metal cooled reactor... Since water is a neutron moderator, it cannot be used as a coolant in a fast neutron reactor. Liquid metal coolants include sodium, NaK, lead, lead-bismuth eutectic and, for early reactors, mercury.
• Sodium-cooled fast reactor.
• Lead-cooled fast neutron reactor.
• Gas cooled reactors cooled by circulating inert gas, conceived by helium in high-temperature structures. At the same time, carbon dioxide was used earlier at British and French nuclear power plants. Nitrogen was also used. The use of heat depends on the type of reactor. Some reactors are so hot that the gas can directly drive a gas turbine. Older reactor designs typically involved passing gas through a heat exchanger to generate steam for a steam turbine.
• Molten salt reactors(MSR) are cooled by circulating molten salt (usually eutectic mixtures of fluoride salts such as FLiBe). In a typical MSR, the heat transfer fluid is also used as the matrix in which the fissile material is dissolved.

Generations of nuclear reactors

• First generation reactor(early prototypes, research reactors, non-commercial power reactors)
• Second generation reactor(most modern nuclear power plants 1965-1996)
• Third generation reactor(evolutionary improvements to existing designs 1996 - present)
• Fourth Generation Reactor(technologies are still under development, unknown date of commencement of operation, possibly 2030)

In 2003, the French Atomic Energy Commission (CEA) introduced the designation "Gen II" for the first time during the Nucleonics Week.

The first mention of "Gen III" was made in 2000 in connection with the start of the Generation IV International Forum (GIF).

"Gen IV" was named in 2000 by the United States Department of Energy (DOE) for the development of new types of power plants.

Classification of nuclear reactors by type of fuel

• Solid fuel reactor
• Liquid fueled reactor
• Water Cooled Homogeneous Reactor
• Molten Salt Reactor
• Gas fired reactors (theoretical)

Classification of nuclear reactors by purpose

• Electricity generation
• Nuclear power plants, including small cluster reactors
• Self-propelled devices (see nuclear power plants)
• Nuclear offshore installations
• Various types of rocket engines offered
• Other uses of heat
• Desalination
• Heat generation for domestic and industrial heating
• Hydrogen production for use in hydrogen energy
• Production reactors for transformation of elements
• Breeder reactors capable of producing more fissile material than they consume in a chain reaction (by converting the parent isotopes U-238 to Pu-239, or Th-232 to U-233). Thus, after completing one cycle, the uranium breeder reactor can be refueled with natural or even depleted uranium. In turn, the thorium breeder reactor can be refueled with thorium. However, an initial supply of fissile material is required.
• Creation of various radioactive isotopes, such as americium for use in smoke detectors and cobalt-60, molybdenum-99 and others, used as indicators and for treatment.
• Production of materials for nuclear weapons such as weapons-grade plutonium
• Creation of a source of neutron radiation (for example, a pulsed reactor "Lady Godiva") and positron radiation (for example, neutron activation analysis and dating by the potassium-argon method)
• Research Reactor: Typically, reactors are used for research and teaching, material testing, or the production of radioisotopes for medicine and industry. They are much smaller than power reactors or ship reactors. Many of these reactors are on campus. There are about 280 such reactors operating in 56 countries. Some work with highly enriched uranium fuel. International efforts are underway to replace low enrichment fuels.

Modern nuclear reactors

Pressurized water reactors (PWR)

These reactors use a pressure vessel to contain nuclear fuel, control rods, moderator and coolant. Cooling of reactors and moderation of neutrons occurs with liquid water under high pressure. The hot radioactive water that leaves the pressure vessel passes through the steam generator circuit, which in turn heats the secondary (non-radioactive) circuit. These reactors make up the majority of modern reactors. It is a device for the heating structure of a neutron reactor, the newest of which are the VVER-1200, the Advanced Pressurized Water Reactor and the European Pressurized Water Reactor. The US Navy reactors are of this type.

Boiling water reactors (BWR)

Boiling water reactors are like pressurized water reactors without a steam generator. Boiling water reactors also use water as a coolant and neutron moderator as pressurized water reactors, but at a lower pressure, allowing water to boil inside a boiler, creating steam that drives turbines. Unlike a pressurized water reactor, there is no primary or secondary circuit. The heating capacity of these reactors can be higher, and they can be structurally simpler, and even more stable and safer. It is a thermal reactor device, the newest of which are the advanced boiling water reactor and the economical simplified boiling water nuclear reactor.

Pressurized Heavy Water Moderated Reactor (PHWR)

Canadian development (known as CANDU), these are heavy water moderated and pressurized coolant reactors. Instead of using a single pressure vessel, as in pressurized water reactors, the fuel is stored in hundreds of high pressure passages. These reactors run on natural uranium and are thermal neutron reactors. Heavy water reactors can be refueled while operating at full power, making them very efficient when using uranium (this allows for precise control of core flow). Heavy water CANDU reactors have been built in Canada, Argentina, China, India, Pakistan, Romania and South Korea. India also operates a number of heavy water reactors, often referred to as "CANDU-derivatives", built after the Canadian government broke off nuclear relations with India following the 1974 Smiling Buddha nuclear weapon test.

High Power Channel Reactor (RBMK)

Soviet development, designed for the production of plutonium, as well as electricity. RBMKs use water as a coolant and graphite as a neutron moderator. RBMKs are similar in some respects to CANDUs in that they can be recharged during operation and use pressure tubes instead of a pressure vessel (as in pressurized water reactors). However, unlike CANDU, they are very unstable and bulky, making the reactor cap expensive. A number of critical safety flaws have also been identified in RBMK designs, although some of these flaws have been corrected after the Chernobyl disaster. Their main feature is the use of light water and unenriched uranium. As of 2010, 11 reactors remain open, largely due to safety improvements and support from international safety organizations such as the US Department of Energy. Despite these improvements, RBMK reactors are still considered one of the most dangerous reactor designs to use. RBMK reactors were only used in the former Soviet Union.

Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor (AGR)

They typically use a graphite neutron moderator and a CO2 coolant. Because of their high operating temperatures, they can be more efficient for generating heat than pressurized water reactors. There are a number of operating reactors of this design, mainly in the United Kingdom, where the concept was developed. Older developments (i.e. Magnox stations) are either closed or will be closed in the near future. However, the improved gas-cooled reactors have an estimated operating life of another 10 to 20 years. Reactors of this type are thermal reactors. The cost of decommissioning such reactors can be high due to the large core volume.

Fast Breeder Reactor (LMFBR)

The design of this reactor is liquid metal cooled, without moderator and produces more fuel than it consumes. They are said to "multiply" the fuel because they produce fissile fuel by capturing neutrons. Such reactors can function in the same way as pressurized water reactors in terms of efficiency, they need to compensate for the increased pressure, since they use a liquid metal that does not create an excess of pressure even at very high temperatures. BN-350 and BN-600 in the USSR and Superphenix in France were reactors of this type, as was Fermi-I in the United States. The Monju reactor in Japan, damaged by a sodium leak in 1995, resumed operations in May 2010. All of these reactors use / have used liquid sodium. These reactors are fast reactors and do not belong to thermal reactors. These reactors are of two types:

Lead cooled

The use of lead as a liquid metal provides excellent protection against radioactive radiation and allows operation at very high temperatures. In addition, lead is (mostly) transparent to neutrons, so less neutrons are lost in the coolant and the coolant does not become radioactive. Unlike sodium, lead is generally inert, so there is less risk of explosion or accident, but such large quantities of lead can cause toxicity and waste disposal problems. Lead-bismuth eutectic mixtures can often be used in reactors of this type. In this case, bismuth will present little interference to radiation, since it is not completely transparent to neutrons, and can be transformed into another isotope more easily than lead. The Russian Alpha-class submarine uses a lead-bismuth cooled fast breeder reactor as its primary power generation system.

Sodium cooled

Most liquid metal breeder reactors (LMFBRs) are of this type. Sodium is relatively easy to obtain and easy to work with, and it also helps prevent corrosion of the various parts of the reactor immersed in it. However, sodium reacts violently when in contact with water, so care must be taken, although such explosions will not be much more powerful than, for example, superheated liquid leaks from SCWR or RWD reactors. EBR-I is the first reactor of its type where the core consists of a melt.

Ball Reactor (PBR)

They use fuel pressed into ceramic balls in which gas is circulated through the balls. The result is efficient, unpretentious, very safe reactors with inexpensive, unified fuel. The prototype was the AVR reactor.

Molten salt reactors

In them, the fuel is dissolved in fluoride salts, or fluorides are used as a heat carrier. Their diverse safety systems, high efficiency and high energy density are suitable for vehicles. It is noteworthy that they have no parts subject to high pressures or combustible components in the core. The prototype was the MSRE reactor, which also used a thorium fuel cycle. As a breeder reactor, it reprocesses spent fuel, extracting both uranium and transuranium elements, leaving only 0.1% of the transuranium waste compared to conventional straight-through uranium light water reactors currently in operation. A separate issue is radioactive fission products, which do not undergo reprocessing and must be disposed of in conventional reactors.

Water homogeneous reactor (AHR)

These reactors use fuel in the form of soluble salts that are dissolved in water and mixed with a coolant and neutron moderator.

Innovative nuclear systems and projects

Advanced reactors

More than a dozen advanced reactor designs are at various stages of development. Some of them have evolved from the designs of RWD, BWR and PHWR reactors, some differ more significantly. The former include the Advanced Boiling Water Reactor (ABWR) (two of which are currently operational and the others under construction), as well as the planned Economical Lightweight Boiling Water Reactor with Passive Safety System (ESBWR) and AP1000 installations (Ref. Nuclear Power Program 2010).

Integral Fast Breeder Nuclear Reactor(IFR) was built, tested and tested during the 1980s and then decommissioned following the resignation of the Clinton administration in the 1990s due to nuclear non-proliferation policies. Reprocessing of spent nuclear fuel is at the core of its design and, therefore, it produces only a fraction of the waste from operating reactors.

Modular High Temperature Gas Cooled Reactor reactor (HTGCR), is designed in such a way that high temperatures reduce the output power due to the Doppler broadening of the neutron beam cross section. The reactor uses a ceramic type of fuel, so its safe operating temperatures exceed the power derating temperature range. Most structures are cooled with inert helium. Helium cannot lead to an explosion due to vapor expansion, is not an absorber of neutrons, which would lead to radioactivity, and does not dissolve pollutants that may be radioactive. Typical designs consist of more layers of passive protection (up to 7) than in light water reactors (usually 3). A unique feature that can provide safety is that the fuel balls actually form a core and are replaced one by one over time. The design features of fuel cells make them expensive to recycle.

Small, closed, mobile, autonomous reactor (SSTAR) was originally tested and developed in the USA. The reactor was conceived as a fast neutron reactor, with a passive protection system, which can be shut down remotely in case a malfunction is suspected.

Clean and environmentally friendly advanced reactor (CAESAR) is the concept of a nuclear reactor that uses steam as a neutron moderator - a design still in development.

The scaled-down water-moderated reactor is based on the Advanced Boiling Water Reactor (ABWR), which is currently in operation. This is not a full fast reactor, but mainly uses epithermal neutrons, which have intermediate speeds between thermal and fast.

Self-regulating nuclear power module with hydrogen neutron moderator (HPM) is a structural type of reactor manufactured by Los Alamos National Laboratory that uses uranium hydride as fuel.

Subcritical nuclear reactors are designed to be safer and more stable-working, but are difficult in engineering and economic terms. One example is the "Energy Booster".

Thorium-based reactors... Thorium-232 can be converted to U-233 in reactors designed specifically for this purpose. In this way, thorium, which is four times more abundant than uranium, can be used to produce nuclear fuel based on U-233. It is believed that U-233 has favorable nuclear properties over the traditionally used U-235, in particular a better neutron efficiency and a reduction in the amount of long lived transuranic waste produced.

Improved Heavy Water Reactor (AHWR)- the proposed heavy water reactor, which will represent the next generation PHWR type development. Under development at Bhabha Nuclear Research Center (BARC), India.

KAMINI- a unique reactor using the uranium-233 isotope as fuel. Built in India at the BARC Research Center and the Indira Gandhi Nuclear Research Center (IGCAR).

India also plans to build fast reactors using the thorium-uranium-233 fuel cycle. FBTR (Fast Breeder Reactor) (Kalpakkam, India) uses plutonium as fuel and liquid sodium as coolant during operation.

What are fourth generation reactors?

The fourth generation of reactors is a collection of different theoretical designs that are currently being considered. These projects are unlikely to be implemented by 2030. Modern reactors in operation are generally considered second or third generation systems. First generation systems have not been used for some time. The development of this fourth generation of reactors was formally launched at the Generation IV International Forum (GIF) with eight technology goals. The main objectives were to improve nuclear safety, increase proliferation security, minimize waste and use natural resources, and to reduce the cost of building and launching such plants.

• Gas-cooled fast neutron reactor
• Lead Cooled Fast Reactor
• Liquid salt reactor
• Sodium cooled fast reactor
• Water cooled supercritical nuclear reactor
• Ultra high temperature nuclear reactor

What are Fifth Generation Reactors?

The fifth generation of reactors are projects, the implementation of which is possible from a theoretical point of view, but which are not the object of active consideration and research at the present time. While such reactors can be built in the current or short term, they generate little interest for reasons of economic viability, practicality or safety.

• Liquid phase reactor... A closed loop with liquid in the core of a nuclear reactor, where the fissile material is in the form of molten uranium or uranium solution cooled with a working gas, injected into through holes in the base of the holding vessel.
• Gas phase reactor in the core... A variant of a closed cycle for a rocket with a nuclear engine, where the fissile material is gaseous uranium hexafluoride, located in a quartz vessel. A working gas (such as hydrogen) will flow around this vessel and absorb ultraviolet radiation from the nuclear reaction. This design could be used as a rocket engine, as mentioned in Harry Harrison's 1976 science fiction novel Skyfall. In theory, using uranium hexafluoride as a nuclear fuel (rather than as an intermediate, as is currently done) would result in lower energy generation costs and would also significantly reduce the size of the reactors. In practice, a reactor operating at such high power densities would produce an uncontrollable flux of neutrons, weakening the strength properties of most of the reactor materials. Thus, the flow would be similar to the flow of particles released in thermonuclear installations. In turn, this would require the use of such materials that are similar to those used in the framework of the International Project for the Implementation of a Facility for Irradiating Materials in a Fusion Reaction.
• Gas-phase electromagnetic reactor... Same as a gas-phase reactor, but with photovoltaic cells converting ultraviolet light directly into electricity.
• Fission Reactor
• Hybrid nuclear fusion... The neutrons are used, emitted during the fusion and decay of the original or "substance in the breeding zone". For example, transmutation of U-238, Th-232 or spent fuel / radioactive waste from another reactor into relatively benign isotopes.

Gas phase reactor in the core. A variant of a closed cycle for a rocket with a nuclear engine, where the fissile material is gaseous uranium hexafluoride, located in a quartz vessel. A working gas (such as hydrogen) will flow around this vessel and absorb ultraviolet radiation from the nuclear reaction. This design could be used as a rocket engine, as mentioned in Harry Harrison's 1976 science fiction novel Skyfall. In theory, using uranium hexafluoride as a nuclear fuel (rather than as an intermediate, as is currently done) would result in lower energy generation costs and would also significantly reduce the size of the reactors. In practice, a reactor operating at such high power densities would produce an uncontrolled flux of neutrons, weakening the strength properties of most of the reactor materials. Thus, the flow would be similar to the flow of particles released in thermonuclear installations. In turn, this would require the use of such materials that are similar to those used in the framework of the International Project for the Implementation of a Facility for Irradiating Materials in a Fusion Reaction.

Gas-phase electromagnetic reactor. Same as a gas-phase reactor, but with photovoltaic cells converting ultraviolet light directly into electricity.

Fission Reactor

Hybrid nuclear fusion. The neutrons are used, emitted during the fusion and decay of the original or "substance in the breeding zone". For example, transmutation of U-238, Th-232, or spent fuel / radioactive waste from another reactor into relatively benign isotopes.

Fusion reactors

Controlled fusion can be used in fusion power plants to generate electricity without the complications associated with handling actinides. However, serious scientific and technological obstacles remain. Several thermonuclear reactors have been built, but only recently has it been possible to ensure that the reactors release more energy than they consume. Despite the fact that research began in the 1950s, it is assumed that a commercial fusion reactor will not function until 2050. Efforts are currently under way within the ITER project to use thermonuclear energy.

Nuclear fuel cycle

Thermal reactors generally depend on the degree of purification and enrichment of uranium. Some nuclear reactors can operate on a mixture of plutonium and uranium (see MOX fuel). The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed and disposed of is known as the nuclear fuel cycle.

Up to 1% of uranium in nature is the readily fissionable isotope U-235. Thus, the design of most reactors involves the use of enriched fuel. Enrichment involves an increase in the proportion of U-235 and, as a rule, is carried out using gas diffusion or in a gas centrifuge. The enriched product is further converted into uranium dioxide powder, which is compressed and fired into granules. These granules are placed in tubes, which are then sealed. These tubes are called fuel rods. Each nuclear reactor uses many of these fuel rods.

Most commercial BWR and PWR reactors use uranium enriched to 4% U-235 approximately. In addition, some industrial high neutron economy reactors do not require enriched fuel at all (that is, they can use natural uranium). According to the International Atomic Energy Agency, there are at least 100 research reactors in the world that use highly enriched fuel (weapons grade / 90% enriched uranium). The risk of theft of this type of fuel (possibly for use in the production of nuclear weapons) has led to a campaign calling for a switch to reactors with low enriched uranium (which poses less proliferation threat).

Fissile U-235 and non-fissionable U-238 capable of nuclear fission are used in the nuclear transformation process. U-235 is fissioned by thermal (i.e. slowly moving) neutrons. A thermal neutron is a neutron that moves at approximately the same speed as the atoms around it. Since the vibrational frequency of atoms is proportional to their absolute temperature, the thermal neutron has a greater ability to split U-235 when it moves at the same vibrational speed. On the other hand, U-238 is more likely to capture a neutron if the neutron is moving very quickly. The U-239 atom decays as quickly as possible with the formation of plutonium-239, which itself is a fuel. Pu-239 is a full-fledged fuel and should be taken into account even when using highly enriched uranium fuel. Plutonium decay processes will prevail over U-235 fission processes in some reactors. Especially after the original loaded U-235 is depleted. Plutonium fissions in both fast and thermal reactors, making it ideal for both nuclear reactors and nuclear bombs.

Most of the existing reactors are thermal reactors, which typically use water as a neutron moderator (moderator means it slows down a neutron to a thermal velocity) and also as a coolant. However, in a fast neutron reactor, a slightly different type of coolant is used, which will not slow down the neutron flux too much. This allows fast neutrons to prevail, which can be effectively used to continuously replenish the fuel supply. Just by placing cheap, unenriched uranium in the core, the spontaneously non-fissionable U-238 will turn into Pu-239, "breeding" the fuel.

In a thorium-based fuel cycle, thorium-232 absorbs a neutron in both fast and thermal reactors. The beta decay of thorium leads to the formation of protactinium-233 and then uranium-233, which in turn is used as fuel. Therefore, like uranium-238, thorium-232 is fertile material.

Maintenance of nuclear reactors

The amount of energy in a nuclear fuel tank is often expressed in the term “full day”, which is the number of 24 hour periods (days) of operating a reactor at full power to generate heat. The days of operation at full power in the reactor operating cycle (between the intervals required for refueling) are related to the amount of decaying uranium-235 (U-235) contained in the fuel assemblies at the beginning of the cycle. The higher the percentage of U-235 in the core at the beginning of the cycle, the more days of operation at full power will allow the reactor to operate.

At the end of the working cycle, the fuel in some assemblies is "processed", unloaded and replaced in the form of new (fresh) fuel assemblies. Also, such a reaction of the accumulation of fission products in nuclear fuel determines the service life of nuclear fuel in the reactor. Even long before the final process of fuel fission occurs, the reactor will have time to accumulate long-lived neutron-absorbing decay by-products, preventing the chain reaction from proceeding. The fraction of the reactor core that is replaced during refueling is typically one quarter for a boiling water reactor and one third for a pressurized water reactor. Utilization and storage of this spent fuel is one of the most difficult tasks in organizing the operation of an industrial nuclear power plant. Such nuclear waste is highly radioactive and toxic for thousands of years.

Not all reactors need to be taken out of service for refueling; for example, nuclear reactors packed with spherical fuel elements, RBMK reactors (high power channel reactor), molten salt reactors, Magnox, AGR and CANDU reactors allow fuel cells to be moved while the plant is running. In a CANDU reactor, it is possible to place individual fuel cells in the core in such a way as to adjust the U-235 content in the fuel cell.

The amount of energy recovered from a nuclear fuel is called its burnup, which is expressed in terms of the thermal energy generated by the original unit of fuel weight. Burn-up is usually expressed in the form of thermal megawatt days per tonne of starting heavy metal.

Nuclear Power Safety

Nuclear safety is actions aimed at preventing nuclear and radiation accidents or localizing their consequences. Nuclear power has improved the safety and performance of reactors, and has also proposed new, safer reactor designs (which have not generally been tested). However, there is no guarantee that such reactors will be designed, built and will be able to operate reliably. Mistakes occur when reactor designers at the Fukushima nuclear power plant in Japan did not expect the earthquake tsunami to shut down the backup system that was supposed to stabilize the reactor after the earthquake, despite numerous warnings from the NRG (National Research Group) and the Japanese administration. on nuclear safety. According to UBS AG, the Fukushima I nuclear accidents call into question whether even advanced economies like Japan can ensure nuclear safety. Catastrophic scenarios are also possible, including terrorist attacks. An interdisciplinary team from MIT (Massachusetts Institute of Technology) has calculated that given the expected growth in nuclear power, there are at least four serious nuclear accidents to be expected between 2005-2055.

Nuclear and radiation accidents

Some serious nuclear and radiation accidents that have occurred. Nuclear power plant accidents include Incident SL-1 (1961), Three Mile Island Accident (1979), Chernobyl Accident (1986), and Fukushima Daichi Nuclear Accident (2011). Nuclear-powered accidents include reactor accidents at K-19 (1961), K-27 (1968), and K-431 (1985).

Nuclear reactors have been launched into orbit around the Earth at least 34 times. A series of incidents involving the Soviet unmanned RORSAT satellite, powered by a nuclear installation, led to the penetration of spent nuclear fuel into the Earth's atmosphere from orbit.

Natural nuclear reactors

While fission reactors are often believed to be the product of modern technology, the first nuclear reactors exist in the wild. A natural nuclear reactor can be formed under certain conditions that simulate the conditions in a designed reactor. To date, up to fifteen natural nuclear reactors have been discovered within three separate ore deposits at the Oklo uranium mine in Gabon, West Africa. The well-known "dead" reactors of Okllo were first discovered in 1972 by the French physicist Francis Perrin. The self-sustaining fission reaction took place in these reactors about 1.5 billion years ago, and has been sustained for several hundred thousand years, generating an average of 100 kW of power output during this period. The concept of a natural nuclear reactor was explained in terms of theory back in 1956 by Paul Kuroda at the University of Arkansas.

Such reactors can no longer form on Earth: radioactive decay during this huge period of time has reduced the proportion of U-235 in natural uranium below the level required to maintain a chain reaction.

Natural nuclear reactors formed when a mineral rich uranium deposit began to fill with groundwater, which acted as a neutron moderator and set off a significant chain reaction. The neutron moderator in the form of water vaporized, accelerating the reaction, and then condensed back, leading to slowing down the nuclear reaction and preventing melting. The fission reaction has persisted for hundreds of thousands of years.

Such natural reactors have been studied in detail by scientists interested in the disposal of radioactive waste in a geological setting. They propose a case study of how radioactive isotopes will migrate through the earth's crust. This is a key point for critics of geological landfilling, who fear that isotopes in waste may end up in water supplies or migrate into the environment.

Environmental problems of nuclear power

A nuclear reactor releases small amounts of tritium, Sr-90 into the air and groundwater. Water contaminated with tritium is colorless and odorless. Large doses of Sr-90 increase the risk of bone cancer and leukemia in animals, and presumably in humans.