The device and principle of operation of a 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: principle of operation (briefly)
Here, the nuclear fission process is used , in which the heavy nucleus decomposes 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 emitted even more, and so on. Such a continuous self-sustaining series of cleavages is called a chain reaction. At the same time, a large amount of energy is released, the production of which is the purpose of using nuclear power plants.
The principle of operation of a nuclear reactor and nuclear power plant is such that about 85% of the fission energy is released within a very short time after the start of the reaction. The rest is produced as a result of the radioactive decay of fission products after they 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, a chain reaction increases in intensity until most of the material is cleaved. This happens very quickly, producing extremely powerful explosions characteristic of such bombs. The device and principle of operation of a nuclear reactor are based on maintaining a chain reaction at a controlled, almost constant level. It is designed in such a way that it cannot explode like an atomic bomb.
Chain reaction and criticality
The physics of a nuclear fission reactor is that a chain reaction is determined by the probability of nuclear fission after the emission of neutrons. If the population of the latter decreases, then the rate of division will eventually drop to zero. In this case, the reactor will be in a subcritical state. If the neutron population is maintained at a constant level, then the fission rate will remain stable. The reactor will be in critical condition. And finally, if the neutron population grows over time, the fission rate and power will increase. The state of the core will become supercritical.
The principle of operation of a nuclear reactor is as follows. Before its launch, the neutron population is close to zero. Then the operators remove the control rods from the core, increasing nuclear fission, which temporarily puts the reactor into a supercritical state. After reaching rated power, operators partially return control rods by adjusting the number of neutrons. Subsequently, the reactor is maintained in critical condition. When it needs to be stopped, operators insert the rods completely. This suppresses division and puts the core in a subcritical state.
Types of Reactors
Most of the world's nuclear installations are power plants that generate the heat necessary to rotate the turbines that drive electric power generators. There are also many research reactors, and some countries have submarines or surface ships driven by atomic energy.
Power plants
There are several types of reactors of this type, but light water construction has found wide application. In turn, it can use pressurized water or boiling water. In the first case, the liquid under high pressure is heated by the heat of the active zone and enters the steam generator. There, heat from the primary circuit is transferred to the secondary, also containing water. The steam ultimately generated serves as the working fluid in the steam turbine cycle.
The boiling type reactor operates on the principle of a direct energy cycle. Water passing through the core 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 its superheated state. Superheated water vapor is then used as the working fluid rotating the turbine.
High Temperature Gas Cooled
A high-temperature gas-cooled reactor (VTGR) is a nuclear reactor whose operating principle is based on the use of a mixture of graphite and fuel microspheres as fuel. There are two competing designs:
- German "filling" system, which uses spherical fuel cells with a diameter of 60 mm, which are a mixture of graphite and fuel in a graphite shell;
- the American version in the form of graphite hexagonal prisms, which coalesce, creating an active zone.
In both cases, the coolant consists of helium at a pressure of about 100 atmospheres. In the German system, helium passes through gaps in the 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 options 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 in the water cycle.
Liquid metal nuclear reactor: scheme and principle of operation
Much attention was paid to fast sodium neutron reactors in the 1960s and 1970s. Then it seemed that their ability to reproduce nuclear fuel in the near future was necessary for the production of fuel for the rapidly developing nuclear industry. When it became clear in the 1980s that this expectation was unrealistic, enthusiasm faded. However, a number of reactors of this type were 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 has focused on natural uranium reactors. 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 carried out with heavy water. The device and principle of operation of a nuclear reactor consists in using a tank with cold D 2 O at atmospheric pressure. The active zone is penetrated by pipes made 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 the heavy water of the coolant, which circulates through the steam generator. The steam in the secondary circuit then passes through a normal turbine cycle.
Research facilities
For scientific research, a nuclear reactor is most often used, the principle of which is the use of water cooling and plate uranium fuel cells in the form of assemblies. It is capable of functioning in a wide range of power levels, from several kilowatts to hundreds of megawatts. Since the production of electricity is not the main task of research reactors, they are characterized by the generated thermal energy, density and nominal neutron energy of the core. It is these parameters that help quantify the ability of a research reactor to conduct specific research. Low-power systems tend to operate in universities and are used for training, and high power is needed in research laboratories for testing materials and characteristics, as well as 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 at the bottom of a large deep pool of water. This simplifies the observation and placement of channels through which neutron beams can be directed. At low power levels, there is no need to pump coolant, since in order to maintain a safe working condition, natural convection of the coolant provides sufficient heat dissipation. The heat exchanger is usually located on the surface or in the upper part of the pool, where hot water collects.
Ship installations
The initial and main use 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 submerged for a long time, and a regular 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. Thanks to it, there is no need to refuel at foreign ports or from easily vulnerable tankers.
The principle of operation of a nuclear reactor in a submarine is classified. However, it is known that in the USA it uses highly enriched uranium, and slowing down and cooling is done with light water. The design of the first USS Nautilus nuclear submarine reactor was heavily influenced by powerful research facilities. Its unique features are a very large reactivity reserve, which ensures a long period of operation without refueling and the possibility of restarting after a stop. The submarine power station must be very quiet to avoid detection. To meet the specific needs of different classes of submarines, different models of power plants were created.
A U.S. Navy aircraft carrier uses a nuclear reactor whose operating principle is believed to be borrowed from the largest submarines. Details of their design have also not been published.
In addition to the United States, nuclear submarines are available from the UK, 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 a consequence of the same requirements for their technical characteristics. Russia also has a small fleet of atomic icebreakers, on which the same reactors were installed as on Soviet submarines.
Industrial installations
For the purposes of the production of weapons-grade plutonium-239 , a nuclear reactor is used, the principle of which is high productivity with a low level of energy production. This is due to the fact that a long stay of plutonium in the core leads to the accumulation of undesirable 240 Pu.
Tritium production
Currently, the main material obtained using such systems is tritium ( 3 H or T) - a charge for hydrogen bombs. Plutonium-239 has a long half-life of 24,100 years, so countries with arsenals of nuclear weapons using this element usually 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 United States, the Savannah River, South Carolina, for example, has several heavy water reactors that produce tritium.
Floating power units
Nuclear reactors have been created that can provide remote isolated areas with electricity and steam heating. In Russia, for example, small power plants specifically designed for servicing Arctic settlements have found application. In China, a 10-MW HTR-10 installation supplies heat and electricity to the research institute in which it is located. Small automated reactors with similar capabilities are being developed in Sweden and Canada. Between 1960 and 1972, the U.S. Army used compact water reactors to provide remote bases in Greenland and Antarctica. They were replaced by fuel oil power plants.
Space conquest
In addition, reactors have been developed for power supply and movement in outer space. Between 1967 and 1988, the Soviet Union installed small nuclear installations on the Cosmos series satellites to power equipment and telemetry, but this policy has become a target of criticism. At least one of these satellites entered the Earth’s atmosphere, which resulted in remote contamination of remote areas of Canada. The United States launched only one nuclear reactor satellite in 1965. However, projects for their use in long-distance space flights, manned explorations 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 provide the highest temperature necessary to minimize the size of the radiator. In addition, the space technology reactor 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 supply will ensure the operation of the reactor for the entire space flight period.