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 a power source for nuclear power plants.
Nuclear reactor: how it works (briefly)
Here, the process of nuclear fission is used, in which a heavy nucleus breaks up into two smaller fragments. These fragments are in a highly excited state and emit neutrons, other subatomic particles and photons. Neutrons can cause new fissions, as a result of which more neutrons are emitted, and so on. Such a continuous self-sustaining series of splits 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 a nuclear power plant is such that about 85% of the fission energy is released within a very short period of time after the start of the reaction. The rest is produced inthe result of the radioactive decay of fission products after they have emitted neutrons. Radioactive decay is the process by which an atom reaches a more stable state. It continues even after the division is completed.
In an atomic bomb, the chain reaction increases in intensity until most of the material is split. This happens very quickly, producing the 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 the chain reaction is determined by the probability of nuclear fission after the emission of neutrons. If the population of the latter decreases, then the fission rate will eventually drop 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 critical condition. And finally, if the population of neutrons grows over time, the fission rate and power will increase. 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. The operators then remove the control rods from the core, increasing nuclear fission, which temporarily translatesreactor to supercritical state. After reaching the nominal power, the operators partially return the control rods, adjusting the number of neutrons. In the future, the reactor is maintained in a critical state. When it needs to be stopped, the operators insert the rods completely. This suppresses fission and brings the core to a subcritical state.
Types of reactors
Most of the world's nuclear installations are energy generating, generating the heat needed to turn turbines that drive electrical power generators. There are also many research reactors, and some countries have nuclear-powered submarines or surface ships.
Power plants
There are several types of reactors of this type, but the light water design 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 core and enters the steam generator. There, the heat from the primary circuit is transferred to the secondary, which also contains water. The eventually generated steam 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 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 brings it tosuperheated state. The superheated water vapor is then used as a working fluid to turn a turbine.
High Temperature Gas Cooled
The High Temperature Gas Cooled Reactor (HTGR) 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 "filler" system that uses 60 mm diameter spherical fuel elements, which are a mixture of graphite and fuel in a graphite shell;
- American version in the form of graphite hexagonal prisms that interlock to form 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 elements, and in the American system, through holes in graphite prisms located along the axis of the central zone of the reactor. Both options can operate at very high temperatures, since graphite has an extremely high sublimation temperature, while helium is completely chemically inert. Hot helium can be applied directly as a working fluid in a gas turbine at high temperature, or its heat can be used to generate water cycle steam.
Liquid metal nuclear reactor: scheme and principle of operation
Fast neutron reactors with sodium coolant received much attention in the 1960s and 1970s. Thenit 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, the enthusiasm faded. However, a number of reactors of this type have been built in the USA, Russia, France, Great Britain, Japan and Germany. Most of them run on uranium dioxide or its mixture with plutonium dioxide. In the United States, however, the greatest success has been with metallic fuels.
CANDU
Canada has 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 carried out by heavy water. The device and principle of operation of a nuclear reactor is to use a tank with cold D2O at atmospheric pressure. The core is pierced by pipes made of zirconium alloy with natural uranium fuel, through which heavy water cools it. Electricity is produced by transferring the heat of fission in heavy water to coolant that is circulated through the steam generator. The steam in the secondary circuit then passes through the normal turbine cycle.
Research installations
For scientific research, a nuclear reactor is most often used, the principle of which is to use water cooling andlamellar uranium fuel elements in the form of assemblies. Capable of operating over a wide range of power levels, from a few kilowatts to hundreds of megawatts. Since power generation is not the main task of research reactors, they are characterized by the generated thermal energy, density and nominal energy of neutrons in the core. It is these parameters that help to quantify the ability of a research reactor to conduct specific surveys. Low power systems are typically used in universities for teaching, while high power systems are needed in R&D labs for material and performance testing and 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 bleed coolant, as the natural convection of the coolant provides sufficient heat dissipation to maintain a safe operating condition. The heat exchanger is usually located on the surface or at the top of the pool where hot water accumulates.
Ship installations
The original and main use of nuclear reactors is in submarines. Their main advantage isthat, unlike fossil fuel combustion systems, they do not require air to generate electricity. Therefore, a nuclear submarine can remain submerged for long periods of time, while a conventional diesel-electric submarine must periodically rise to the surface to start its engines in the air. Nuclear power gives a strategic advantage to the ships of the Navy. It eliminates the need to refuel at foreign ports or from vulnerable tankers.
The principle of operation of a nuclear reactor on a submarine is classified. However, it is known that in the USA it uses highly enriched uranium, and slowing down and cooling is done by light water. The design of the first reactor of the nuclear submarine USS Nautilus was strongly influenced by powerful research facilities. Its unique features are a very large reactivity margin, which ensures a long period of operation without refueling and the ability to restart after a stop. The power station in the subs must be very quiet to avoid detection. To meet the specific needs of different classes of submarines, different models of power plants were created.
The US Navy aircraft carriers use a nuclear reactor, the principle of which is believed to be borrowed from the largest submarines. Details of their design have also not been released.
Besides the USA, the UK, France, Russia, China and India have nuclear submarines. In each case, the design was not disclosed, but it is believed that they are all very similar - thisis a consequence of the same requirements for their technical characteristics. Russia also has a small fleet of nuclear-powered icebreakers that have the same reactors as Soviet submarines.
Industrial installations
For 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 unwanted 240Pu.
Tritium production
Currently, the main material produced by such systems is tritium (3H or T), the charge for hydrogen bombs. Plutonium-239 has a long half-life of 24,100 years, so countries with nuclear weapons arsenals using this element tend to have more of it than they need. Unlike 239Pu, tritium has a half-life of approximately 12 years. Thus, in order to maintain the necessary supplies, this radioactive isotope of hydrogen must be produced continuously. In the US, 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 electricity and steam heating to remote isolated areas. In Russia, for example, have found applicationsmall power plants specifically designed to serve Arctic communities. In China, a 10 MW HTR-10 plant supplies heat and power to the research institute where it is located. Small controlled reactors with similar capabilities are being developed in Sweden and Canada. Between 1960 and 1972, the US Army used compact water reactors to power remote bases in Greenland and Antarctica. They have been replaced by oil-fired power plants.
Space exploration
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 Kosmos satellites to power equipment and telemetry, but this policy became a target for criticism. At least one of these satellites entered the Earth's atmosphere, resulting in radioactive contamination of remote areas of Canada. The United States launched only one nuclear-powered satellite in 1965. However, projects for their use in deep space flights, manned exploration 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 possible temperature necessary to minimize the size of the radiator. In addition, a space reactor should be as compact as possible to minimize the amount of material used forshielding, and to reduce weight during launch and space flight. The supply of fuel will ensure the operation of the reactor for the entire period of the space flight.
