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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Nuclear reactor on fast neutrons. History of invention and production
Directory / The history of technology, technology, objects around us The world's first nuclear power plant (NPP), built in the city of Obninsk near Moscow, gave current in June 1954. Its power was very modest - 5 MW. However, it played the role of an experimental facility, where experience was accumulated in the operation of future large nuclear power plants. For the first time, the possibility of generating electrical energy based on the fission of uranium nuclei, and not by burning fossil fuels and not by hydraulic energy, was proved.
Nuclear power plants use nuclei of heavy elements - uranium and plutonium. During nuclear fission, energy is released - it "works" in nuclear power plants. But you can use only nuclei that have a certain mass - the nuclei of isotopes. The atomic nuclei of isotopes contain the same number of protons and different numbers of neutrons, which is why the nuclei of different isotopes of the same element have different masses. Uranium, for example, has 15 isotopes, but only uranium-235 is involved in nuclear reactions. The fission reaction proceeds as follows. The uranium nucleus spontaneously disintegrates into several fragments; among them there are particles of high energy - neutrons. On average, there are 10 neutrons for every 25 decays. They hit the nuclei of neighboring atoms and break them, releasing neutrons and a huge amount of heat. The fission of a gram of uranium releases as much heat as the combustion of three tons of coal. The space in the reactor where the nuclear fuel is located is called the core. Here the atomic nuclei of uranium are fissioning and thermal energy is released. To protect the operating personnel from the harmful radiation that accompanies the chain reaction, the walls of the reactor are made sufficiently thick. The speed of a nuclear chain reaction is controlled by control rods made of a substance that absorbs neutrons (most often it is boron or cadmium). The deeper the rods are lowered into the core, the more neutrons they absorb, the fewer neutrons are involved in the reaction and the less heat is released. Conversely, when the control rods are lifted from the core, the number of neutrons involved in the reaction increases, an increasing number of uranium atoms fission, releasing the thermal energy hidden in them. In case the core overheats, an emergency shutdown of the nuclear reactor is provided. Emergency rods quickly fall into the core, intensely absorb neutrons, the chain reaction slows down or stops. Heat is removed from a nuclear reactor using a liquid or gaseous coolant, which is pumped through the core by pumps. The heat carrier can be water, metallic sodium or gaseous substances. It takes heat from the nuclear fuel and transfers it to the heat exchanger. This closed system with a coolant is called the primary circuit. In the heat exchanger, the heat of the primary circuit heats the water of the secondary circuit to boiling. The resulting steam is sent to a turbine or used for heating industrial and residential buildings.
Before the catastrophe at the Chernobyl nuclear power plant, Soviet scientists confidently said that in the coming years two main types of reactors would be widely used in the nuclear power industry. One of them, VVER, is a water-cooled power reactor, and the other, RBMK, is a high-power reactor, channel. Both types are related to slow (thermal) neutron reactors. In a pressurized water reactor, the active zone is enclosed in a huge, 4 meters in diameter and 15 meters high, steel cylinder case with thick walls and a massive lid. Inside the case, the pressure reaches 160 atmospheres. The heat carrier that removes heat in the reaction zone is water, which is pumped through by pumps. The same water also serves as a neutron moderator. In the steam generator, it heats and turns the secondary water into steam. The steam enters the turbine and rotates it. Both the first and second circuits are closed. Once every six months, the burnt-out nuclear fuel is replaced with fresh one, for which the reactor must be stopped and cooled. In Russia, Novovoronezh, Kola and other nuclear power plants operate according to this scheme. In RBMK, graphite serves as the moderator, and water is the coolant. The steam for the turbine is produced directly in the reactor and returned there after being used in the turbine. The fuel in the reactor can be replaced gradually, without stopping or dampening it. The world's first Obninsk nuclear power plant belongs to this type. The Leningrad, Chernobyl, Kursk, Smolensk stations of high power were built according to the same scheme. One of the serious problems of nuclear power plants is the disposal of nuclear waste. In France, for example, this is done by a large firm, Cogema. Fuel containing uranium and plutonium, with great care, in special transport containers - sealed and cooled - is sent for processing, and waste - for vitrification and burial. “We were shown the individual stages of processing fuel brought from nuclear power plants with the greatest care,” I. Lagovsky writes in the journal Science and Life. “Unloaders, an unloading chamber. You can look into it through the window. The thickness of the glass in the window is 1 meter 20 centimeters "A manipulator at the window. Unimaginable cleanliness around. White overalls. Soft light, artificial palm trees and roses. A greenhouse with real plants for relaxing after work in the zone. Cabinets with control equipment of the IAEA - the international atomic energy agency. The operator's room - two semicircles with displays ", - from here they control unloading, cutting, dissolution, vitrification. All operations, all movements of the container are sequentially reflected on the displays of the operators. The halls of work with high-activity materials themselves are quite far away, on the other side of the street. Vitrified waste is small in volume. They are enclosed in steel containers and stored in ventilated shafts until they are taken to the final burial site ... The containers themselves are a work of engineering art, the purpose of which was to build something that cannot be destroyed. Railway platforms loaded with containers were derailed, rammed at full speed by oncoming trains, and other conceivable and unthinkable accidents during transportation were arranged - the containers withstood everything. After the Chernobyl disaster in 1986, scientists began to doubt the safety of nuclear power plants and, in particular, RBMK-type reactors. The VVER type is more prosperous in this regard: the accident at the American station Three Mile Island in 1979, where the reactor core partially melted, the radioactivity did not go beyond the vessel. The long trouble-free operation of Japanese nuclear power plants speaks in favor of VVER. And, nevertheless, there is one more direction, which, according to scientists, is able to provide humanity with heat and light for the next millennium. This refers to fast neutron reactors, or breeder reactors. They use uranium-238, but not for energy, but for fuel. This isotope absorbs fast neutrons well and turns into another element - plutonium-239. Fast neutron reactors are very compact: they do not need any moderators or absorbers - their role is played by uranium-238. They are called breeder reactors, or breeders (from the English word "breed" - multiply). The reproduction of nuclear fuel makes it possible to use uranium dozens of times more fully, therefore fast neutron reactors are considered one of the promising areas of nuclear energy. In reactors of this type, in addition to heat, secondary nuclear fuel is also produced, which can be used in the future. Here, neither in the first nor in the second circuits there is high pressure. The coolant is liquid sodium. It circulates in the primary circuit, heats up itself and transfers heat to sodium in the second circuit, which, in turn, heats the water in the steam-water circuit, turning it into steam. The heat exchangers are isolated from the reactor. One of these promising stations - it was given the name Monju - was built in the Shiraki region on the coast of the Sea of Japan in a resort area four hundred kilometers west of the capital. “For Japan,” says K. Takenouchi, Head of the Department of the Kansai Nuclear Corporation, “the use of breeder reactors means the ability to reduce dependence on imported natural uranium through the repeated use of plutonium. Therefore, our desire to develop and improve “fast reactors” and achieve a technical level is understandable. capable of competing with modern nuclear power plants in terms of efficiency and safety. The development of breeder reactors should be the main power generation program in the near future." The construction of the Monju reactor is already the second stage in the development of fast neutron reactors in Japan. The first was the design and construction of the 50-100 MW Joyo (Japanese for "eternal light") experimental reactor, which began operation in 1978. It investigated the behavior of fuel, new structural materials, components. The Monju project started in 1968. In October 1985, they began to build a station - to dig a foundation pit. During the development of the site, 2 million 300 thousand cubic meters of rock were dumped into the sea. The thermal power of the reactor is 714 MW. The fuel is a mixture of plutonium and uranium oxides. The active zone has 19 control rods, 198 fuel blocks, each of which has 169 fuel rods (fuel elements - TVELs) with a diameter of 6,5 millimeters. They are surrounded by radial fuel-producing units (172 units) and neutron shield units (316 units). The entire reactor is assembled like a nesting doll, only it is no longer possible to disassemble it. The huge reactor vessel, made of stainless steel (diameter - 7,1 meters, height - 17,8 meters), is placed in a protective casing in case sodium spills during an accident. “The steel structures of the reactor chamber,” A. Lagovsky reports in the journal Science and Life, “the shells and wall blocks are filled with concrete as protection. The primary sodium cooling systems, together with the reactor vessel, are surrounded by an emergency shell with stiffeners - its inner diameter is 49,5, 79,4 meters high and 13,5 meters high.The ellipsoidal bottom of this bulk rests on a solid concrete pad 1 meters high.The shell is surrounded by a one and a half meter annular gap, and then follows a thick layer (1,8-0,5 meters) of reinforced concrete.The dome of the shell also protected by a layer of reinforced concrete XNUMX meters thick. Following the anti-emergency shell, another protective building is arranged - an auxiliary one - 100 by 115 meters in size, which meets the requirements of anti-seismic construction. Why not a sarcophagus? Secondary sodium cooling systems, steam-water systems, fuel loading and unloading devices, and a storage tank for spent fuel are located in the auxiliary reactor vessel. In separate rooms there are a turbogenerator and standby diesel generators. The strength of the emergency shell is designed both for an overpressure of 0,5 atmospheres and for a vacuum of 0,05 atmospheres. A vacuum can form when oxygen burns out in the annular gap if liquid sodium spills. All concrete surfaces that may come into contact with sodium spills are completely lined with steel sheets thick enough to withstand thermal stresses. This is how they protect themselves in case that may not happen at all, since there should be a guarantee both for pipelines and for all other parts of a nuclear installation. Author: Musskiy S.A.
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