Diagram of processes in a nuclear reactor. How does a nuclear reactor work?

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

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

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

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

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

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

If we keep in mind the macroscopic scale of energy release, then all or initially at least some fraction of particles of the substance must have the kinetic energy necessary to excite reactions. This is achievable only by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the energy threshold limiting the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of degrees Kelvin, but in the case of nuclear reactions it is at least 107 K due to the very high height of the Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions is carried out in practice only during the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).

Excitation by joining particles does not require large kinetic energy, and, therefore, does not depend on the temperature of the medium, since it occurs due to unused bonds inherent in the attractive forces of particles. But to excite reactions, the particles themselves are necessary. And if we again mean not a separate act of reaction, but the production of energy on a macroscopic scale, then this is possible only when a chain reaction occurs. The latter occurs when the particles that excite the reaction reappear as products of an exoenergetic reaction.

To control and protect a nuclear reactor, control rods are used that can be moved along the entire height of the core. The rods are made of substances that strongly absorb neutrons - for example, boron or cadmium. When the rods are inserted deeply, a chain reaction becomes impossible, since neutrons are strongly absorbed and removed from the reaction zone.

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

Leningrad NPP, RBMK reactor

Start of reactor operation:

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

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

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

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

Regulation by solid, moving absorbent elements

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

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

Burnable absorbing elements.

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

Fluid reactivity control.

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

Chain reaction mechanism

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

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

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

Emergency protection:

Emergency protection of a nuclear reactor is a set of devices designed to quickly stop a nuclear chain reaction in the reactor core.

Active emergency protection is automatically triggered when one of the parameters of a nuclear reactor reaches a value that could lead to an accident. Such parameters may include: temperature, pressure and coolant flow, level and speed of power increase.

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

In addition to active protection, many modern designs also include elements of passive protection. For example, modern versions of VVER reactors include an “Emergency Core Cooling System” (ECCS) - special tanks with boric acid located above the reactor. In the event of a maximum design basis accident (rupture of the first cooling circuit of the reactor), the contents of these tanks end up inside the reactor core by gravity and the nuclear chain reaction is extinguished by a large amount of boron-containing substance, which absorbs neutrons well.

According to the “Nuclear Safety Rules for Reactor Facilities of Nuclear Power Plants”, at least one of the provided reactor shutdown systems must perform the function of emergency protection (EP). Emergency protection must have at least two independent groups of working elements. At the AZ signal, the AZ working parts must be activated from any working or intermediate positions.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The first portable nuclear reactors used slow neutrons.

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

• Industrial reactors. They are used to generate energy at nuclear power plants.
• Nuclear reactors acting as propulsion units for nuclear submarines.
• Experimental (portable, small) reactors. Without them, not a single modern scientific experiment or research takes place.

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

And according to classification they can be as follows:

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

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

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 for power generation 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 sets the ship's blades in motion or rotates electric generators. Steam generated as a result of a nuclear reaction can, in principle, be used for the thermal industry or for district heating. Some reactors are used to produce isotopes used for medical and industrial purposes or to produce weapons-grade plutonium. Some of them are for research purposes only. Today there are about 450 nuclear power reactors used to generate electricity in about 30 countries around the world.

Operating principle 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 nuclear fission into thermal energy for further conversion into mechanical or electrical forms.

The process of nuclear fission

When a significant number of decaying atomic nuclei (such as uranium-235 or plutonium-239) absorb a neutron, nuclear fission can occur. A heavy nucleus breaks down 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 fissile atoms and cause further fission, which releases even 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 change the proportion of neutrons that go into fissioning more nuclei. Nuclear reactors are controlled manually or automatically to be able to stop the decay reaction when dangerous situations are detected.

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 reactor work area generates heat in several ways:

• The kinetic energy of fission products is converted into thermal energy when the 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 exposed during the absorption of neutrons. This heat source will remain unchanged for some time, even after the reactor is shut down.

During nuclear reactions, a kilogram of uranium-235 (U-235) releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2 × 1013 joules per kilogram of uranium-235 compared to 2.4 × 107 joules per kilogram coal),

Nuclear reactor cooling system

A nuclear reactor's coolant—usually water, but sometimes gas, liquid metal (such as liquid sodium), or molten salt—circulates around the reactor core to absorb the heat generated. The 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 the steam used for turbines, like a pressurized water reactor. However, in some reactors, the water for the steam turbines boils directly in the reactor core; for example, in a pressurized water type reactor.

Monitoring the neutron flux in the reactor

The reactor's power output is regulated by controlling the number of neutrons capable of causing more fissions.

Control rods, which are made of "neutron poison" are used to absorb neutrons. The more neutrons that are absorbed by the control rod, the fewer neutrons that can cause further fission. Thus, immersing the absorption rods deep into the reactor reduces 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 from 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 by fission, while the rest (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 considerable time to accurately determine when the reactor reaches the critical point. Maintaining the reactor in chain reactivity mode, where delayed neutrons are needed to reach critical mass, is achieved using mechanical devices or human control to control the chain reaction in "real time"; otherwise, the time between reaching criticality and melting the nuclear reactor core as a result of the exponential voltage surge during a normal nuclear chain reaction will be too short to intervene. This final stage, where delayed neutrons are no longer required to maintain criticality, is known as prompt neutron criticality. There is a scale for describing criticality in numerical form, in which initial criticality is designated as "zero dollars", fast criticality 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 changes in temperature can affect the density of the coolant/moderator and therefore the change in reactor power output. The higher the temperature of the coolant, the less dense it will be, and therefore the less effective the retarder.

In other types of reactors, the coolant acts as a "neutron poison", 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 typically have automatic and manual systems for shutting down the reactor for emergency shutdown. These systems place large amounts 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 the "xenon pit" or "iodine pit". The widespread decay product xenon-135, resulting from the fission reaction, plays the role of a neutron absorber that tends 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. Fission also results in 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. 135, with a half-life of 9.2 hours. This temporary state is an “iodine hole.” If the reactor has sufficient additional power, 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 what is called a "xenon burnup stage." Additionally, control rods must be inserted into the reactor to compensate for the absorption of neutrons to replace the lost xenon-135. The failure to correctly follow such a procedure was a key cause of the Chernobyl accident.

Reactors used in shipboard nuclear power plants (especially nuclear submarines) often cannot be run continuously to produce power in the same way as land-based power reactors. In addition, such power plants must have a long period of operation without changing 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 burn-up of the reactor fuel cycle due to the presence of neutron absorbing material, which is subsequently replaced by conventional long-life neutron absorbers (more durable than xenon-135), which gradually accumulate over the operating life fuel.

How is electricity produced?

The energy generated during fission generates heat, some of which can be converted into useful energy. A common method of using this thermal energy is to use it to boil water and produce steam under pressure, which in turn drives a steam turbine, which turns an alternator and produces electricity.

The history of the first reactors

Neutrons were discovered in 1932. The chain reaction scheme triggered by nuclear reactions as a result of exposure to neutrons was first implemented by the Hungarian scientist Leo Sillard in 1933. He applied for a patent for his simple reactor idea during the next year of work 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 neutron-mediated nuclear chain reactions in light elements proved unfeasible.

The impetus for creating a new type of reactor using uranium was the discovery by 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 produce barium, which they believed it arose from the decay of uranium nuclei. Subsequent research in early 1939 (Szilard and Fermi) showed that some neutrons were also produced by atomic fission, making possible the nuclear chain reaction that Szilard had envisioned six years earlier.

On August 2, 1939, Albert Einstein signed a letter written by Szilard to President Franklin D. Roosevelt, which stated 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 had worked together for many years, but Einstein had never thought about this possibility for nuclear power until Szilard informed him early in his quest to write a letter to Einstein-Szilard to warn US Government,

Shortly thereafter, in 1939, Hitler's Germany attacked Poland, beginning the Second World War. world war in Europe. The US was not yet officially at war, but in October, when the Einstein-Szilard letter was delivered, Roosevelt noted that the purpose of the study was to make sure "the Nazis don't blow us up." The US nuclear project began, although with some delay, because skepticism remained (particularly from Fermi) and because of the small number of government officials who initially oversaw the project.

The following year, the US government received the Frisch-Peierls Memorandum from Great Britain, which stated that the amount of uranium required to carry out the chain reaction was much less than previously thought. The memorandum was created with the participation of Maud Committee, which worked on the atomic bomb project in Great Britain, later known under the code name "Tube Alloys" and later included in the Manhattan Project.

Ultimately, the first man-made nuclear reactor, called 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 due to the country's entry into the war. The Chicago Woodpile reached its critical point on December 2, 1942, at 3:25 p.m. The reactor frame was made of wood, holding together a stack of graphite blocks (hence the name) with nested "briquettes" or "pseudo-spheres" 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 purpose of the largest reactors (located at the Hanford complex in Washington State) was to mass produce plutonium for nuclear weapons. Fermi and Szilard filed a patent application for the reactors on December 19, 1944. Its grant was delayed for 10 years due to wartime secrecy.

"World's First" is the inscription on the site of the EBR-I reactor, which is now a museum near Arco, Idaho. Originally called Chicago Woodpile 4, this reactor was created under the direction of Walter Sinn for the Aregon National Laboratory. This experimental fast breeder reactor was operated by the US 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, having a design capacity of 200 kW (electrical power).

In addition to the military use of nuclear reactors, there were political reasons continue research into atomic energy for peaceful purposes. US President Dwight Eisenhower made his famous "Atoms for Peace" speech at the UN General Assembly on December 8, 1953. This diplomatic move led to the spread of reactor technology both in the US 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 electrical energy.

After World War II, the US military sought other applications for nuclear reactor technology. Research conducted by the Army and Air Force was not implemented; Nevertheless, the US Navy achieved success by launching a nuclear submarine USS boat Nautilus (SSN-571) January 17, 1955.

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

The first portable nuclear reactor, the Alco PM-2A, was used to generate electricity (2 MW) for the US military base Camp Century in 1960.

Main components of a nuclear power plant

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

Nuclear reactor elements

• Nuclear fuel (nuclear reactor core; neutron moderator)
• Original neutron source
• Neutron absorber
• Neutron gun (provides a constant source of neutrons to re-initiate the reaction after shutdown)
• Cooling system (often the neutron moderator and coolant are the same thing, usually purified water)
• Control rods
• Nuclear reactor vessel (NRP)

Boiler water supply pump

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

Radiation safety system

• Rector protection system (RPS)
• Emergency diesel generators
• Emergency reactor core cooling system (ECCS)
• Emergency liquid control system (emergency boron injection, only in boiling-water nuclear reactors)
• System for supplying process water to responsible consumers (SOTVOP)

Protective shell

• Remote Control
• Emergency installation
• Nuclear training complex (as a rule, there is an imitation control panel)

Classifications of nuclear reactors

Types of nuclear reactors

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

Classification of nuclear reactors by moderator type

Thermal reactors used:

• Graphite reactors
  
• Pressurized water reactors
• Heavy water reactors(used in Canada, India, Argentina, China, Pakistan, Romania and South Korea).
• Light water reactors(LVR). Light water reactors (the most common type of thermal reactor) use ordinary water to control and cool the reactors. If the temperature of water increases, its density decreases, slowing down the flow of neutrons enough to cause further chain reactions. This is negative Feedback stabilizes the rate of 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/unenriched fuel.
• Reactors based on light element moderators.
• Molten salt moderated reactors(MSR) are driven by the presence of light elements such as lithium or beryllium, which are found in the LiF and BEF2 coolant/fuel matrix salts.
• Reactors with liquid metal coolers, where the coolant is a mixture of lead and bismuth, can use BeO oxide as a neutron absorber.
• Reactors based on organic moderator(OMR) use biphenyl 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 pressurized water reactors (PWRs) and 35 are boiling water reactors (BWRs). Nuclear pressurized water reactors (PWRs) make up the vast majority of all Western nuclear power plants. The main characteristic of the RVD type is the presence of a supercharger, a special high-pressure vessel. Most commercial RVD reactors and naval reactor installations 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 water with immersion heaters. In normal mode, the supercharger is connected to the high-pressure reactor vessel (HRVV) 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.

• High pressure heavy water reactors belong to a type of pressurized water reactor (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, in the form of uranium dioxide, as fuel. The fuel is assembled into rods placed in a steel vessel, which in turn is immersed in water. The process of nuclear fission 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 into 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 earlier generation reactors, mercury.
• Sodium-cooled fast neutron reactor.
• Fast neutron reactor with lead coolant.
• Gas-cooled reactors cooled by circulating inert gas, conceived by helium in high-temperature structures. At the same time, carbon dioxide was previously used 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 create steam for a steam turbine.
• Molten salt reactors(MSRs) are cooled by circulating molten salt (usually eutectic mixtures of fluoride salts such as FLiBe). In a typical MSR, the coolant is also used as a 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 still under development, unknown start date, possibly 2030)

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

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

"Gen IV" was mentioned 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 fuel reactor
• Homogeneous water cooled reactor
• Molten salt reactor
• Gas-fueled reactors (theoretically)

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
• Different types of rocket motors offered
• Other forms of heat use
• Desalination
• Heat generation for domestic and industrial heating
• Hydrogen production for use in hydrogen energy
• Production reactors for element conversion
• Breeder reactors capable of producing more fissile material than they consume during 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 refilled with natural or even depleted uranium. In turn, the thorium breeder reactor can be refilled 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, the Lady Godiva pulse reactor) and positron radiation (for example, neutron activation analysis and potassium-argon dating)
• Research Reactor: Typically reactors are used for scientific research and training, testing materials or producing radioisotopes for medicine and industry. They are much smaller than power reactors or ship reactors. Many of these reactors are located on university campuses. There are about 280 such reactors operating in 56 countries. Some work with highly enriched uranium fuel. International efforts are underway to replace low-enriched fuels.

Modern nuclear reactors

These reactors use a high-pressure vessel to hold 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 high pressure vessel passes through a steam generator circuit, which in turn heats the secondary (non-radioactive) circuit. These reactors make up the majority of modern reactors. This is a neutron reactor heating structure device, the newest of which are the VVER-1200, the Advanced Pressurized Water Reactor and the European Pressurized Water Reactor. US Navy reactors are of this type.

Boiling water reactors are similar to 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 the water to boil inside a boiler, creating steam that turns turbines. Unlike a pressurized water reactor, there is no primary or secondary circuit. The heating capacity of these reactors may be higher, and they may be simpler in design, and even more stable and safe. This is a thermal neutron reactor device, the newest of which are the Advanced Boiling Water Reactor and the Economical Simplified Boiling Water Nuclear Reactor.

A Canadian design (known as CANDU), these are heavy water moderated, pressurized coolant reactors. Instead of using a single pressure vessel, as in pressurized water reactors, the fuel is contained in hundreds of high-pressure passages. These reactors operate on natural uranium and are thermal neutron reactors. Heavy water reactors can be refueled while operating at full power, making them very efficient at using uranium (this allows the flow in the core to be precisely controlled). 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 ended its nuclear relationship with India following the 1974 Smiling Buddha nuclear weapons test.

A Soviet development, designed to produce plutonium as well as electricity. RBMKs use water as a coolant and graphite as a neutron moderator. RBMKs are similar to CANDUs in some respects, as they can be recharged during operation and use pressure tubes instead of a high-pressure vessel (as in pressurized water reactors). However, unlike CANDUs, they are very unstable and bulky, making the reactor hood expensive. A number of critical safety flaws were also identified in RBMK designs, although some of these flaws were corrected after Chernobyl disaster. Their main feature is the use of light water and unenriched uranium. As of 2010, 11 reactors remain open, largely due to improved safety levels 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.

They typically use a graphite neutron moderator and CO2 coolant. Because of their high operating temperatures, they can be more efficient at producing 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. The older developments (i.e. Magnox Station) are either closed or will be closed in the near future. However, improved gas-cooled reactors have an expected operating life of another 10 to 20 years. Reactors of this type are thermal neutron reactors. The monetary costs of decommissioning such reactors can be high due to the large volume of the core.

This reactor is designed to be cooled by liquid metal, without a moderator, and produces more fuel than it consumes. They are said to be fuel "breeders" because they produce fissionable fuel through neutron capture. Such reactors can function in the same way as pressurized water reactors in terms of efficiency, but they require compensation for increased pressure because they use liquid metal that does not create excess pressure even at very high temperatures. BN-350 and BN-600 in the USSR and Superphoenix 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 operation in May 2010. All of these reactors use/have used liquid sodium. These reactors are fast neutron reactors and do not belong to thermal neutron reactors. These reactors are of two types:

The use of lead as a liquid metal provides excellent protection against radioactive radiation, and allows operation at very high temperatures. Additionally, lead is (mostly) transparent to neutrons, so fewer neutrons are lost to 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 problems from a toxicity and waste disposal perspective. Lead-bismuth eutectic mixtures can often be used in this type of reactor. In this case, bismuth will present little interference to radiation because it is not completely transparent to neutrons, and can mutate into another isotope more easily than lead. The Russian Alpha-class submarine uses a lead-bismuth-cooled fast reactor as its main power generation system.

Most liquid metal breeder reactors (LMFBRs) are of this type. Sodium is relatively easy to obtain and easy to work with, and it helps prevent corrosion of 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, leaks of superheated liquid from a SCWR or RWD reactor. EBR-I is the first reactor of its type where the core consists of a melt.

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, standardized fuel. The prototype was the AVR reactor.

In them, fuel is dissolved in fluoride salts, or fluorides are used as a coolant. Their various security systems, high efficiency and high energy density are suitable for vehicles. It is noteworthy that they do not have parts exposed to high pressures or flammable 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 transuranic elements, leaving only 0.1% of the transuranium waste compared to conventional once-through uranium light water reactors currently in operation. A separate issue is radioactive fission products, which are not reprocessed and must be disposed of in conventional reactors.

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

Innovative nuclear systems and projects

Advanced Reactors

More than a dozen advanced reactor projects are in development various stages development. Some have evolved from RWD, BWR and PHWR reactor designs, some differ more significantly. The former include the Advanced Boiling Water Reactor (ABWR) (two of which are currently operating and others under construction), as well as the planned Economy Simplified Boiling Water Reactor (ESBWR) and AP1000 plants (see Nuclear Energy Program 2010).

Integrated fast neutron nuclear reactor(IFR) was built, tested and tested during the 1980s, and then retired after the Clinton Administration left office in the 1990s due to nuclear nonproliferation policies. Reprocessing spent nuclear fuel is built into its design and therefore 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 power output due to Doppler broadening of the cross-section of the neutron beam. The reactor uses a ceramic type of fuel, so its safe operating temperatures exceed temperature Range power reduction. Most structures are cooled with inert helium. Helium cannot cause an explosion due to vapor expansion, is not a neutron absorber that would cause radioactivity, and does not dissolve contaminants that could 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 ensure safety is that the fuel balls actually form the 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 designed as a fast neutron reactor, with a passive protection system that could be shut down remotely if problems were suspected.

Clean and environmentally friendly advanced reactor (CAESAR) is a concept for 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 improved boiling water reactor (ABWR) currently in operation. It is not a full fast neutron reactor, but uses mainly epithermal neutrons, which have velocities intermediate between thermal and fast.

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

Subcritical nuclear reactors are intended to be safer and more stable, but are difficult to engineer and economic relations. One example is the Energy Booster.

Thorium based reactors. It is possible to convert thorium-232 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 U-233-based nuclear fuel. U-233 is believed to have favorable nuclear properties compared to conventionally used U-235, particularly better neutron efficiency and a reduction in the amount of long-lived transuranium waste produced.

Improved Heavy Water Reactor (AHWR)- a proposed heavy water reactor that will represent the development of the next generation PHWR type. 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 Center for Nuclear Research (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 completed by 2030. Current reactors in operation are generally considered second or third generation systems. First generation systems have not been used for some time. Development of this fourth generation of reactors was officially launched at the Generation IV International Forum (GIF) based on eight technology goals. The main objectives were to improve nuclear safety, increase security against proliferation, minimize waste and use natural resources, as well as to reduce the costs of construction and launch of such stations.

• Gas-cooled fast neutron reactor
• Fast reactor with lead cooler
• Liquid salt reactor
• Sodium-cooled fast reactor
• Supercritical water-cooled nuclear reactor
• Ultra-high temperature nuclear reactor

What are fifth generation reactors?

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

• Liquid phase reactor. A closed circuit with liquid in the core of a nuclear reactor, where the fissile material is in the form of molten uranium or a uranium solution cooled by a working gas injected into through holes in the base of the holding vessel.
• Gas phase reactor in the core. Closed cycle option for rocket with nuclear engine, where the fissile material is gaseous uranium hexafluoride located in a quartz container. The working gas (such as hydrogen) will flow around this vessel and absorb ultraviolet radiation resulting from the nuclear reaction. Such a 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 reactors. In practice, a reactor operating at such high power densities would produce an uncontrolled flow of neutrons, weakening the strength properties of much 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 materials that are similar to those used in the framework. International project on the implementation of a facility for irradiating materials under thermonuclear reaction conditions.
• Gas-phase electromagnetic reactor. Same as a gas-phase reactor, but with photovoltaic cells that convert ultraviolet light directly into electricity.
• Fragmentation reactor
• Hybrid nuclear fusion. The neutrons emitted during the fusion and decay of the original or "substance in the breeding zone" are used. For example, the transmutation of U-238, Th-232 or spent fuel/radioactive waste from another reactor into relatively benign isotopes.

Reactor with a gas phase in the core. A closed-cycle option for a nuclear-powered rocket, where the fissile material is uranium hexafluoride gas located in a quartz container. The working gas (such as hydrogen) will flow around this vessel and absorb ultraviolet radiation resulting from the nuclear reaction. Such a 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 reactors. In practice, a reactor operating at such high power densities would produce an uncontrolled flow of neutrons, weakening the strength properties of much 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 materials that are similar to the materials used within the framework of the International Project for the Implementation of a Facility for Irradiation of Materials under Thermonuclear Reaction Conditions.

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

Fragmentation reactor

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

Fusion reactors

Controlled nuclear fusion can be used in fusion power plants to produce electricity without the complications associated with working with actinides. However, significant scientific and technological obstacles remain. Several fusion reactors have been built, but only recently have the reactors been able to release more energy than they consume. Although research began in the 1950s, it is expected that a commercial fusion reactor will not operate until 2050. Efforts to harness fusion energy are currently underway within the ITER project.

Nuclear fuel cycle

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

Up to 1% of uranium in nature is the easily fissile isotope U-235. Thus, the design of most reactors involves the use of enriched fuel. Enrichment involves increasing the proportion of U-235 and is usually carried out by gaseous diffusion or in a gas centrifuge. The enriched product is further converted into uranium dioxide powder, which is pressed 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 approximately 4% U-235. In addition, some industrial reactors with high neutron savings 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 using highly enriched fuel (weapons grade/90% uranium enrichment). The risk of theft of this type of fuel (possible for use in nuclear weapons production) has led to a campaign calling for a switch to reactors using low-enriched uranium (which poses less of a proliferation threat).

Fissile U-235 and non-fissile, fissionable U-238 are used in the nuclear transformation process. U-235 is fissioned by thermal (i.e., slow-moving) neutrons. A thermal neutron is one that moves at approximately the same speed as the atoms around it. Since the vibrational frequency of atoms is proportional to their absolute temperature, a 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 to form plutonium-239, which itself is a fuel. Pu-239 is a valuable fuel and must be taken into account even when using highly enriched uranium fuel. Plutonium decay processes will dominate 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 existing reactors are thermal reactors, which typically use water as a neutron moderator (moderator means it slows down a neutron to thermal speed) and also as a coolant. However, a fast neutron reactor uses a slightly different type of coolant that will not slow down the neutron flow too much. This allows fast neutrons to predominate, which can be effectively used to constantly replenish the fuel supply. Simply by placing cheap, unenriched uranium in the core, spontaneously non-fissionable U-238 will turn into Pu-239, “breeding” the fuel.

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

Nuclear Reactor Maintenance

The amount of energy in a nuclear fuel reservoir is often expressed in terms of "full power days", which is the number of 24-hour periods (days) the reactor operates at full power to produce thermal energy. The days of full power operation in a 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 operating cycle, the fuel in some assemblies is "worked out", unloaded and replaced in the form of new (fresh) fuel assemblies. Also, this reaction of accumulation of decay products in nuclear fuel determines the service life of nuclear fuel in the reactor. Even long before the final process of fuel fission occurs, long-lived neutron-absorbing decay byproducts have accumulated in the reactor, preventing the chain reaction from occurring. The proportion of the reactor core replaced during reactor refueling is typically one quarter for a boiling water reactor and one third for a pressurized water reactor. Disposal 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 extremely radioactive and its toxicity poses a risk for thousands of years.

Not all reactors need to be taken out of service for refueling; for example, nuclear reactors with ball fuel cores, RBMK reactors, molten salt reactors, Magnox, AGR and CANDU reactors allow fuel elements to be moved during plant operation. In a CANDU reactor, it is possible to place individual fuel elements in the core in such a way as to adjust the U-235 content of the fuel element.

The amount of energy extracted from a nuclear fuel is called its burnup, which is expressed in terms of the thermal energy produced by the original unit weight of the fuel. Burnup is usually expressed in terms of thermal megawatt days per ton of parent heavy metal.

Nuclear Energy Safety

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

Nuclear and radiation accidents

Some serious nuclear and radiation accidents have occurred. Nuclear power plant accidents include the SL-1 incident (1961), the Three Mile Island accident (1979), the Chernobyl disaster (1986), and the Fukushima Daiichi nuclear disaster (2011). Accidents on nuclear powered ships include reactor accidents on the K-19 (1961), K-27 (1968), and K-431 (1985).

Nuclear reactor plants have been launched into orbit around the Earth at least 34 times. A series of incidents involving the Soviet nuclear-powered unmanned RORSAT satellite resulted in the release of spent nuclear fuel into the Earth's atmosphere from orbit.

Natural nuclear reactors

Although fission reactors are often thought to be a product of modern technology, the first nuclear reactors are found in natural conditions. A natural nuclear reactor can be formed under certain conditions that mimic those in a constructed reactor. To date, up to fifteen natural nuclear reactors have been discovered within three separate ore deposits Oklo uranium mine in Gabon (West Africa). First discovered the well-known "dead" Okllo reactors in 1972 French physicist Francis Perrin. A self-sustaining nuclear fission reaction occurred in these reactors approximately 1.5 billion years ago, and was maintained for several hundred thousand years, producing an average of 100 kW of power output during this period. The concept of a natural nuclear reactor was explained in theoretical terms back in 1956 by Paul Kuroda at the University of Arkansas.

Such reactors can no longer be formed 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 the chain reaction.

Natural nuclear reactors formed when rich uranium mineral deposits began to fill up groundwater, which acted as a neutron moderator and the onset of a significant chain reaction. The neutron moderator, in the form of water, evaporated, causing the reaction to speed up, and then condensed back, causing the nuclear reaction to slow down and meltdown prevented. The fission reaction persisted for hundreds of thousands of years.

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

Environmental problems of nuclear energy

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.

Built under the west bleachers of the University of Chicago football field and turned on on December 2, 1942, Chicago Pile-1 (CP-1) was the world's first nuclear reactor. It consisted of graphite and uranium blocks, and also had cadmium, indium and silver control rods, but had no radiation protection or cooling system. The project's scientific director, physicist Enrico Fermi, described CP-1 as "a damp pile of black bricks and wooden logs."

Work on the reactor began on November 16, 1942. Difficult work has been done. Physicists and university staff worked around the clock. They built a lattice of 57 layers of uranium oxide and uranium ingots embedded in graphite blocks. A wooden frame supported the structure. Fermi's protégé, Leona Woods - the only woman on the project - took careful measurements as the pile grew.

On December 2, 1942, the reactor was ready for testing. It contained 22,000 uranium ingots and used 380 tons of graphite, as well as 40 tons of uranium oxide and six tons of uranium metal. It took $2.7 million to build the reactor. The experiment began at 09:45. It was attended by 49 people: Fermi, Compton, Szilard, Zinn, Heberry, Woods, a young carpenter who made graphite blocks and cadmium rods, doctors, ordinary students and other scientists.

Three people made up the “suicide squad” - they were part of the security system. Their job was to put out the fire if something went wrong. There was also control: control rods that were controlled manually and an emergency rod that was tied to the railing of the balcony above the reactor. In the event of an emergency, the rope had to be cut by a person specially on duty on the balcony and the rod would extinguish the reaction.

At 15:53, for the first time in history, a self-sustaining nuclear chain reaction began. The experiment was a success. The reactor operated for 28 minutes.

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 were seen as both a miracle and a threat.

When the first commercial U.S. reactor came online at Shippingport, Pennsylvania, in 1956, the technology was hailed as the energy source of the future, and some believed the reactors would make generating electricity too cheap. Currently, 442 have been built worldwide. nuclear reactor, about a quarter of these reactors are in the United States. The world has become dependent on nuclear reactors, producing 14 percent of its electricity. Futurists even fantasized about nuclear cars.

When the Unit 2 reactor at the Three Mile Island Power Plant in Pennsylvania experienced a cooling system failure and partial meltdown of its radioactive fuel in 1979, the warm feelings about reactors changed radically. Even though the destroyed reactor was contained and no serious radiation emitted, many people began to view the reactors as too complex and vulnerable, with potentially catastrophic consequences. People were also concerned about 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 demands and dwindling supplies of fossil fuels, as well as growing concerns about climate change resulting from carbon dioxide emissions.

But in March 2011, another crisis occurred - this time the Fukushima 1 nuclear power plant in Japan was badly damaged by an earthquake.

Use of nuclear reaction

Simply put, a nuclear reactor splits atoms and releases the energy that holds their parts together.

If you have forgotten physics high school, we will 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, which are bound together. The force that binds the elements of the core is difficult to even imagine. It is many billions of times stronger than the force of gravity. Despite this enormous force, it is possible to split a nucleus—by shooting neutrons at it. When this is done, a lot of energy will be released. When atoms decay, their particles crash into nearby atoms, splitting them, and those, in turn, are next, and 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 the particles of its nucleus is relatively weak compared to other elements. Nuclear reactors use a specific isotope called Uran-235 . Uranium-235 is rare in nature, with ore from uranium mines containing only about 0.7% Uranium-235. This is why reactors are used enrichedUwounds, which is created by separating and concentrating Uranium-235 through a gas diffusion process.

A chain reaction process can be created in atomic bomb, similar to those 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 that absorb some of the neutrons. This still allows the fission process to release enough energy to heat the water to about 270 degrees Celsius and turn it into steam, which is used to spin the power plant's turbines and generate electricity. Basically, in this case, a controlled nuclear bomb works instead of coal to create electricity, except that the energy to boil the water comes from splitting atoms instead of burning carbon.

Nuclear Reactor Components

There are several different types of nuclear reactors, but they all have some General characteristics. They all have a supply of radioactive fuel pellets - usually uranium oxide - which are arranged in tubes to form fuel rods in active zonesereactor.

The reactor also has the previously mentioned managerserodAnd- made of a neutron-absorbing material such as cadmium, hafnium or boron, which is inserted to control or stop a 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 ordinary water, but reactors in other countries sometimes use graphite, or heavywowwaterat, 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 Iliquidb, usually ordinary water, which absorbs and transfers heat from the reactor to create steam to spin the turbine and cools the reactor area so that it does not reach the temperature at which the uranium will melt (about 3815 degrees Celsius).

Finally, the reactor is enclosed in shellsat, a large, heavy structure, usually several meters thick, made of steel and concrete that keeps radioactive gases and liquids inside where they can't 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 with unpressurized water in the steam generator, which turns into steam, which rotates the turbines. There is also a design high-power channel-type reactor (RBMK) with one water circuit and fast neutron reactor with two sodium and one water circuits.

How safe is a nuclear reactor?

Answering this question is quite difficult and depends on who you ask and how you define “safe”. Are you concerned 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 an acceptable trade-off for the benefits of nuclear power? And to what extent do you trust the government and nuclear energy?

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

Proponents of nuclear power, however, point out that we are all regularly exposed to radiation from a variety of sources, including cosmic rays and natural radiation emitted by the Earth. The average annual radiation dose is about 6.2 millisieverts (mSv), half of it from natural sources and half from man-made sources ranging from chest X-rays, smoke detectors and luminous watch dials. How much radiation do we get from nuclear reactors? Only a tiny fraction of a percent of our typical annual exposure is 0.0001 mSv.

While all nuclear plants inevitably leak small amounts of radiation, regulatory commissions hold plant operators to stringent requirements. They cannot expose people living around the plant to more than 1 mSv of radiation per year, and workers at the plant have a threshold of 50 mSv per year. That may seem 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's important to note that not everyone agrees with this complacent assessment of 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 plants had double the risk of contracting leukemia compared to those living further from nuclear power plants.

Nuclear reactor waste

Nuclear power is touted by its proponents 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 something else environmental problem— disposal of nuclear waste. Some of the spent fuel from the reactors still releases radioactivity. Other unnecessary material that should be saved is radioactive waste high level , a liquid residue from the reprocessing of spent fuel, in which some of the 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 produced by the spent fuel and help shield workers from radiation exposure.

One of the problems with spent nuclear fuel is that it has been altered by the fission process. When large uranium atoms are split, 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 Pperiodohmhalf-life. Other radioactive elements will have different half-lives. In addition, some uranium atoms also capture neutrons, forming heavier elements such as Plutonium. These transuranium elements do not create 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 radioactiveewastes high level from reactors are dangerous to humans and other life forms because they can release huge, lethal dose radiation even from short exposure. Ten years after removing the remaining fuel from a reactor, for example, they are emitting 200 times more radioactivity per hour than it would take to kill a person. And if waste ends up in groundwater or rivers, it can end up in food chain and put a large number of people at risk.

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

What can go wrong with a nuclear reactor?

With government regulators looking back on their experience, engineers have spent a lot of time over the years designing reactors for optimal safety. It's just that they don't break down, work properly, and have backup safety measures if something doesn't go according to plan. As a result, year after year, nuclear power plants appear to be fairly safe compared to, say, air travel, which regularly kills between 500 and 1,100 people a year worldwide.

However, nuclear reactors suffer major breakdowns. On the International Nuclear Event Scale, which rates reactor accidents from 1 to 7, there have been five accidents since 1957 that rate from 5 to 7.

The worst nightmare is a cooling system failure, which leads to overheating of the fuel. The fuel turns to liquid and then burns through the containment, releasing radioactive radiation. In 1979, Unit 2 at the Three Mile Island nuclear power plant (USA) was on the verge of this scenario. Fortunately, a well-designed containment system was strong enough to stop the radiation from escaping.

The USSR was less fortunate. Heavy 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 test, the reaction suddenly intensified and the control rods jammed, preventing an emergency shutdown. The sudden buildup of steam caused two thermal explosions, throwing the reactor's graphite moderator into the air. In the absence of anything to cool the reactor fuel rods, they began to overheat and completely collapse, as a result of which the fuel took on a liquid form. Many station workers and accident liquidators died. 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.

Nuclear reactor manufacturers provide guarantees based on probabilistic assessmente, in which they try to balance the potential harm of an event with the likelihood with which it actually occurs. But some critics say they should prepare instead for rare, unexpected but highly dangerous events. A case in point is the March 2011 accident at the Fukushima 1 nuclear power plant in Japan. The station was reportedly designed to withstand a strong earthquake, but not one as catastrophic as the 9.0 magnitude quake that sent a 14-meter tsunami wave above dikes designed to withstand a 5.4-meter wave. The onslaught of the tsunami destroyed the backup diesel generators that were intended to power the cooling system of the plant's six reactors in the event of a power outage. So even after the Fukushima reactors' control rods stopped fission, the still-hot fuel allowed temperatures to rise dangerously inside the destroyed ones. reactors.

Japanese officials resorted to a last resort - flooding the reactors with huge amounts of sea ​​water with the addition of boric acid, which was able to prevent a disaster, but destroyed the reactor equipment. Eventually, with the help of fire trucks and barges, the Japanese were able to pump fresh water into the reactors. But by that time, monitoring had already shown alarming levels of radiation in surrounding land and water. In one village 40 km from the plant, the radioactive element Cesium-137 was found at levels much higher than after the Chernobyl disaster, raising doubts about the possibility of human habitation in the area.