Nuclear reactor: principle of operation, device and circuit

The design and operation of the nuclear reactor are based on the initialization and control of a self-sustaining nuclear reaction. It is used as a research tool, for the production of radioactive isotopes and as an energy source for nuclear power plants.

Nuclear reactor: the principle of operation (briefly)

Here, the nuclear fission process is used, in which the heavy nucleus breaks up into two smaller fragments. These fragments are in a very excited state and emit neutrons, other subatomic particles and photons. Neutrons can cause new fissions, as a result of which they are radiated even more, and so on. Such a continuous self-sustaining series of splittings is called a chain reaction. At the same time, a large amount of energy is allocated, the production of which is the purpose of using nuclear power plants.

The principle of operation of a nuclear reactor and an atomic power station is such that a colony of 85% of the cleavage energy is released within a very short period of time after the start of the reaction. The rest is produced as a result of the radioactive decay of the fission products, after they have emitted neutrons. Radioactive decay is a process in which an atom reaches a more stable state. It continues after the completion of the division.

In an atomic bomb, the chain reaction increases its intensity until most of the material is split. This happens very quickly, producing extremely powerful explosions, typical for such bombs. The design and operation of the nuclear reactor are based on maintaining a chain reaction at a regulated, almost constant level. It is designed in such a way that it can not explode like an atomic bomb.

Chain reaction and criticality

The physics of the nuclear fission reactor is that the chain reaction is determined by the probability of splitting the nucleus after the emission of neutrons. If the population of the latter decreases, then the fission rate eventually falls to zero. In this case, the reactor will be in a subcritical state. If the population of neutrons is maintained at a constant level, then the fission rate will remain stable. The reactor will be in a critical condition. And, finally, if the neutron population increases with time, the fission rate and power will increase. The state of the core becomes supercritical.

The operating principle of a nuclear reactor is as follows. Before its launch, the neutron population is close to zero. The operators then remove the control rods from the core, increasing the fission of the nuclei, temporarily shifting the reactor to a supercritical state. After reaching the rated power, operators partially return the control rods, regulating the number of neutrons. In the future, the reactor is maintained in a critical state. When it needs to be stopped, operators insert the rods completely. This suppresses division and moves the active zone to a subcritical state.

Types of Reactors

Most of the existing nuclear installations in the world are energy generating heat necessary for the rotation of turbines, which set in motion electric power generators. There are also many research reactors, and some countries have submarines or surface ships driven by the energy of the atom.

Power plants

There are several types of reactors of this type, but the construction on light water has found wide application. In turn, it can use water under pressure or boiling water. In the first case, the liquid under high pressure is heated by the heat of the core and enters the steam generator. There, heat from the primary circuit is transferred to the secondary circuit, also containing water. Generated in the final analysis, steam serves as a working fluid in the cycle of a steam turbine.

The boiling-type reactor operates on the principle of a direct energy cycle. Water, passing through the active zone, is brought to a boil at an average pressure level. Saturated steam passes through a series of separators and dryers located in the reactor vessel, which leads to a superheated state. Superheated water vapor is then used as a working fluid rotating the turbine.

High-temperature with gas cooling

High-temperature gas-cooled reactor (HTGR) is a nuclear reactor, the principle of which is based on the use of a mixture of graphite and fuel microspheres as a fuel. There are two competing designs:

• German "backfill" system that uses spherical fuel cells with a diameter of 60 mm, which is a mixture of graphite and fuel in a graphite shell;
• American variant in the form of graphite hexagonal prisms, which adhere, creating an active zone.

In both cases, the cooling liquid consists of helium under a pressure of about 100 atmospheres. In the German system, helium passes through gaps in a layer of spherical fuel cells, and in the American system, through holes in graphite prisms located along the axis of the central zone of the reactor. Both variants can work at very high temperatures, since graphite has an extremely high sublimation temperature, and helium is completely chemically inert. Hot helium can be used directly as a working fluid in a gas turbine at high temperature, or its heat can be used to generate steam of the water cycle.

Liquid Metal Reactor: Scheme and Principle of Operation

Reactors on fast neutrons with a sodium coolant were given great attention in the 1960s and 1970s. Then it seemed that their ability to reproduce nuclear fuel in the near future is necessary for the production of fuel for the rapidly developing nuclear industry. When in the 1980s it became clear that this expectation was unrealistic, enthusiasm was extinguished. However, a number of reactors of this type have been built in the USA, Russia, France, Great Britain, Japan and Germany. Most of them work on uranium dioxide or its mixture with plutonium dioxide. In the United States, however, the greatest success was achieved with metallic fuels.

CANDU

Canada focused its efforts on reactors that use natural uranium. This eliminates the need for its enrichment to resort to the services of other countries. The result of this policy was the deuterium-uranium reactor (CANDU). Control and cooling in it is produced by heavy water. The device and principle of operation of a nuclear reactor consists in the use of a tank with cold D ₂ O at atmospheric pressure. The active zone is permeated with pipes of zirconium alloy with fuel from natural uranium, through which heavy water cooling it circulates. Electricity is produced by transferring the heat of fission in heavy water to a cooling fluid that circulates through the steam generator. The steam in the secondary circuit then passes through an ordinary turbine cycle.

Research installations

To conduct scientific research, the most commonly used nuclear reactor, the principle of which is to use water cooling and plate-like uranium fuel cells in the form of assemblies. Is able to operate in a wide range of power levels, from a few kilowatts to hundreds of megawatts. Since electricity generation is not the main task of research reactors, they are characterized by the generated thermal energy, density and the nominal energy of the core neutrons. It is these parameters that help to quantify the ability of the research reactor to conduct specific investigations. Low-power systems tend to function in universities and are used for training, and high power is needed in research laboratories for testing materials and characteristics, and for general research.

The most common research nuclear reactor, the structure and principle of operation of which are as follows. Its active zone is located in the lower part of a large deep basin with water. This simplifies the observation and placement of channels through which neutron beams can be directed. At low power levels, it is not necessary to pump the coolant, since in order to maintain a safe working condition, the natural convection of the coolant ensures sufficient heat removal. The heat exchanger, as a rule, is located on the surface or in the upper part of the basin, where hot water is accumulated.

Ship installations

The initial and main application of nuclear reactors is their use in submarines. Their main advantage is that, unlike fossil fuel combustion systems, they do not need air to generate electricity. Consequently, an atomic submarine can remain immersed for a long time, and a conventional diesel-electric submarine must periodically rise to the surface in order to start its engines in the air. Nuclear power gives strategic advantage to naval ships. Due to it there is no need to refuel in foreign ports or from easily vulnerable tankers.

The principle of operation of a nuclear reactor on a submarine is classified. However, it is known that in the US it uses highly enriched uranium, while slowing and cooling is produced by light water. The design of the first USS Nautilus nuclear submarine reactor was strongly influenced by powerful research facilities. Its unique features are a very large reserve of reactivity, providing a long period of operation without refueling and the ability to restart after a stop. The power station in the submarines must be very quiet to avoid detection. To meet the specific needs of different classes of submarines, different models of power plants were created.

US Navy aircraft carriers use a nuclear reactor, the operating principle of which is believed to be borrowed from the largest submarines. Details of their design were also not published.

In addition to the United States, nuclear submarines are available in Britain, France, Russia, China and India. In each case, the design was not disclosed, but it is believed that they are all very similar - this is the result of the same requirements for their technical characteristics. Russia also has a small fleet of nuclear icebreakers, on which the same reactors were installed, as on Soviet submarines.

Industrial plants

For the production of weapons-grade plutonium-239 , a nuclear reactor is used, the operating principle of which is high productivity with a low level of energy production. This is due to the fact that a prolonged stay of plutonium in the core leads to the accumulation of undesirable ²⁴⁰ Pu.

Production of tritium

At present, the main material obtained by such systems is tritium ( ³ H or T) - charge for hydrogen bombs. Plutonium-239 has a long half-life of 24,100 years, so countries with nuclear weapons arsenals that use this element tend to have more than necessary. Unlike ²³⁹ Pu, the half-life of tritium is approximately 12 years. Thus, in order to maintain the necessary reserves, this radioactive isotope of hydrogen must be produced continuously. In the USA, in Savannah River (South Carolina), for example, there are several heavy water reactors that produce tritium.

Floating power units

Nuclear reactors have been built that can provide remote isolated areas with electricity and steam heating. In Russia, for example, small power plants specially designed for servicing arctic settlements were used. In China, a 10-MW HTR-10 facility supplies heat and electricity to the research institute in which it is located. The development of small, automatically controlled reactors with similar capabilities is under way in Sweden and Canada. Between 1960 and 1972, the US Army used compact water reactors to provide remote bases in Greenland and Antarctica. They were replaced by black oil power stations.

The conquest of space

In addition, reactors were developed for power supply and movement in outer space. Between 1967 and 1988, the Soviet Union installed small nuclear installations for satellites of the Cosmos series for powering equipment and telemetry, but this policy became the target of criticism. At least one of these satellites entered the Earth's atmosphere, resulting in radioactive contamination in remote areas of Canada. The United States launched only one satellite with a nuclear reactor in 1965. However, projects for their use in long-range space missions, manned studies of other planets or on a permanent lunar base continue to be developed. This will necessarily be a gas-cooled or liquid-metal nuclear reactor, the physical principles of which will ensure the highest possible temperature necessary to minimize the size of the radiator. In addition, the reactor for space technology should be as compact as possible in order to minimize the amount of material used for shielding and to reduce weight during launch and space flight. The fuel reserve will ensure the operation of the reactor for the entire period of space flight.