Basic knowledge of nuclear power

    Nuclear power technology development: Since the United States Experimental Breeder No. 1 (EBR-1) first used nuclear power to generate electricity in December 1951, the world nuclear power has been developing for more than 50 years.By the end of 2018, there were more than 500 nuclear power generating units in operation worldwide, accounting for about 18 percent of the world's total power generation.

 

    Everything in the world is made up of atoms, which in turn are made up of the nucleus and the electrons around it. The fusion of light nuclei and the splitting of heavy nuclei both release energy, which is called fusion energy and fission energy respectively, or nuclear energy for short.

    The nuclear energy you refer to is nuclear fission energy. The fuel for nuclear power plants is uranium. Uranium is a heavy metal element. Natural uranium is made up of three isotopes:

        Uranium-235 has a content of 0.71%

        Uranium-238 contains 99.28%

        0.0058% Uranium-234 content Uranium-235 is the only nuclide found in nature that is prone to fission.

    When a neutron bombards a uranium-235 nucleus, the nuclear energy of the atom splits into two lighter nuclei, producing two or three neutrons and rays at the same time, and giving off energy. If the new neutron hits another uranium-235 nucleus, it can cause a new fission. In a chain reaction, energy is released in an endless stream.

    How much energy is released from uranium-235 fission? The energy released by the fission of 1 kilogram of uranium-235 is equivalent to the energy released by burning 2,700 tons of standard coal.

 

    The reactor is the key design of a nuclear power plant, and the chain fission reaction takes place in it. There are many types of reactors, and the most used reactor in a nuclear power plant is a pressurized water reactor.

    The first thing you have in a pressurized water reactor is nuclear fuel. The nuclear fuel consists of sintered uranium dioxide pellets, the size of a small finger, packed into zirconium tubes, which are assembled together into a fuel assembly of more than three hundred zirconium tubes containing pellets.Most of the assemblies contain a bundle of control rods that control the strength of the chain reaction and the start and end of the reaction.

    The pressurized water reactor uses water as coolant to flow through the fuel assembly under the push of the main pump. After absorbing the heat generated by nuclear fission, it flows out of the reactor and into the steam generator, where it passes the heat to the water on the secondary side, turning them into steam and sending them to generate electricity, while the temperature of the main coolant itself is lowered. The main coolant from the steam generator is then sent back to the reactor by the main pump for heating. This circulating channel of coolant is called the primary circuit, and the primary

pressure is maintained and regulated by a voltage regulator.

 

    Thermal power stations use coal and petroleum to generate electricity, hydroelectric power stations use hydroelectric power, and nuclear power stations are new power stations that use the energy contained in the nucleus to generate electricity. Nuclear power stations can be roughly divided into two parts: one is the nuclear island that uses nuclear energy to produce steam, including the reactor unit and the primary system;The other is a conventional island that uses steam to produce electricity, including a turbo-generator system.

    The fuel used in nuclear power plants is uranium.Uranium is a very heavy metal. Nuclear fuel made of uranium is fission in a device called a reactor, which produces a great deal of heat energy. This heat energy is carried out by water under high pressure, and steam is produced in a steam generator, which drives a gas turbine to spin with a generator. Electricity is produced continuously and sent far and wide through the electric grid. This is how the most common type of pressurized water reactor nuclear power plant works.

    In developed countries, nuclear power has been developed for decades and has become a mature energy source. China's nuclear industry has been developing for over 40 years, and it has established a quite complete nuclear fuel cycle system from geological investigation, mining to component processing and reprocessing. It has built up many types of nuclear reactors, and have many years of safety management and operation experience, as well as a complete professional and technical team.The construction and operation of nuclear power plant is a complex technology. The country is already able to design, build and operate its own nuclear power stations. The Qinshan nuclear Power Station was researched, designed and built by China itself.

 

    Electricity is produced in power plants.We know of coal-fired power plants that run on coal or oil, hydroelectric plants that run on water, and small or experimental plants that produce electricity from wind, solar, geothermal, tidal, wave and methane. Nuclear power plants are new types of power plants that rely on the energy contained in the nucleus to produce electricity on a large scale.

    The fuel used in nuclear power plants is uranium.Uranium is a very heavy metal.Nuclear fuel made from uranium is fission in a device called a reactor and produces a great deal of heat energy. This heat energy is carried out by water under high pressure. It is produced in steam generators and sent far and wide by electric grids. This is how the most common pressurized water reactor nuclear power plants work.

 

    About 100 years ago, scientists discovered that certain substances emit three kinds of radiation: alpha (alpha) rays, beta (beta) rays and gamma (gamma) rays.

    Later studies proved that alpha rays were streams of alpha particles (helium nuclei) and beta rays were streams of beta particles (electrons), collectively known as particle radiation. The same goes for neutron rays, cosmic rays, etc. Gamma rays are very short wavelength electromagnetic waves called electromagnetic radiation. The same is true of X-rays and so on.

    The common characteristics of these rays are:

        1. They have a certain ability to penetrate matter;

        2. people can not perceive the five senses, but can make the photographic plate sensitive;

        3. irradiation to some special substances can emit visible fluorescence;

        4. Ionization occurs when passing through the substance.

    Rays have certain effects on living organisms mainly through ionization.

    Radiation is not to be feared. There are substances in the food we eat, the houses we live in, and even in our bodies that give off radiation. We all receive a certain amount of radiation when we wear luminous watches, get X-rays, fly in an airplane, and smoke. However, too high a dose of radiation can cause harmful effects.

 

    A nuclear reactor is a device that maintains and controls the nuclear fission chain reaction, thereby enabling the conversion of nuclear energy to heat energy.

    The pressurized water reactor for nuclear power plants has a thick steel tubular shell with several water intakes and outtakes at the waist, called the pressure vessel. The pressure vessel of the 900 MW pressurized water reactor is 12 meters high, 3.9 meters in diameter, and the wall is about 0.2 meters thick.

    Inside the pressure vessel is the reactor core, which is composed of fuel assembly and control rod assembly. Water flows through the gaps between them. The water does two things here: it slows down the neutrons so they can be absorbed by the uranium-235 nuclei, and it takes heat out of them. A 900MW PWR typically contains 157 fuel assemblies containing about 80 tonnes of uranium dioxide.

    The top of the pressure vessel is equipped with a control rod drive mechanism, which can realize reactor opening, shutdown (including emergency shutdown) and power regulation by changing the position of the control rod.

 

    Generally speaking, a nuclear accident occurs in a nuclear facility (such as a nuclear power plant), resulting in the release of radioactive materials and exposing workers and the public to exposure exceeding or equivalent to the prescribed limits.Obviously, there is a wide range of severity of nuclear accidents. In order to have a uniform standard of understanding, the international community has classified seven levels of safety-significant events in nuclear facilities.

    As can be seen from the table, only levels 4-7 are referred to as "accidents".An accident above level 5 requires the implementation of off-site emergency plan. There have been three such accidents in the world, namely the Chernobyl accident in the Soviet Union, the Wentzcale accident in the United Kingdom and the Three Mile Island accident in the United States.

 

    Most plants in China are like this

    1) Reactor building: including internal and external containment vessel and internal structure as well as core melt catcher. The reactor building is a double-layer cylindrical structure, which contains and supports the main facilities associated with the primary circuit (including the pressure vessel and the main cooling circuit, including the main pump, evaporator and pressurizer).The reactor refuelling chamber and internal structure. Auxiliary equipment. The main function of the plant is to prevent the impact of external events on internal reactions and ensure that no leaks occur. Including the primary circuit accident loss of water, so that the pressure and temperature in the plant.

        1.1) Containment: The containment is a double wall structure, wherein the inner wall is composed of prestressed concrete barrel and concrete dome, and the inner side is lined with steel to ensure the sealing. The outer containment resists external impact.The outer and inner containment are isolated by a 1.8-meter wide ring area, which is under negative pressure to collect leakage after a leak accident and ensure that leakage is filtered before it is released into the atmosphere. The double containment is considered to be an effective protection of the environment in case of a serious accident.

        1.2) Internal structure: the main function is to provide support for the reactor pressure vessel and support for auxiliary equipment; Biological protection of personnel and equipment;To prevent the impact of pipe blows and projectiles on containment, circuits and safety systems.

        1.3) Structure description: The internal structure is reinforced concrete structure, including primary shielding wall, secondary shielding wall, reactor refueling chamber; Floor and wall.

        1.4) Core melt Trap: Located under the core CVCS and VDS system, it is divided into three parts, consisting of the lower pit, core melt expansion channel and expansion area.The surface is covered with fine stone concrete.At the bottom is a circulating water system to cool the molten material in case of an accident, and the water comes from a refueling tank.

    2) Safety workshop: Safety workshop 1&4 is divided into 9 layers, which are arranged on both sides of the containment;Plant 2&3 is divided into 8 layers, arranged together, using double walls. The external walls are separated from each floor of the workshop, and the doors leading to the workshop should have access control system.

    3) Fuel building: located in the reactor building and safety building 2, 3 opposite position, and the reactor building and safety building located on a raft foundation. 9 floors (0.00-19.5m zone). The west side is the spent fuel pool and related facilities.On the east side is the accident waste gas filter unit. Adopt double wall, the door should have access control system.

    4) Nuclear auxiliary building: Auxiliary systems that are necessary for power plant operation and have nothing to do with safety are set up in the nuclear auxiliary building, and some maintenance areas are set up. It is a reinforced concrete structure, the foundation is separated from the raft foundation of the plant, and the shielding structure is set around the radioactive equipment and the systematic isolation. Adequate biological isolation is provided.

    5) Access to the plant: The basic plant is equipped with the necessary equipment and facilities to ensure the safe access of personnel to the nuclear island.The foundation of entering and leaving the plant is close to the foundation of the nuclear island, and the settlement joint is set to allow the relative displacement.

    6) Radioactive waste plant: it is divided into radioactive waste plant (HQB) and radioactive waste storage plant (HQS), which can collect, store and treat liquid and solid radioactive waste. For the two units, it is directly connected with the nuclear auxiliary plant building of Unit 1, used to store and transport resin waste and collect, temporary storage, transport waste liquid.A heat pipe is connected between the radioactive waste building and the auxiliary building of No. 2 Unit (2HQS) to transport the waste liquid of No. 2 unit.

    7) Emergency diesel engine room: (HD) is a reinforced concrete structure. Its reinforced concrete raft base, underground part and

external wall are waterproof with asphalt insulation material. The floors, walls and ceiling surfaces used to house the diesel fuel storage tanks and diesel fuel tank rooms are plastered with cement mortar mixed with oleophobic materials.

    8) Safety plant water pump room: for the concrete structure, the reinforced concrete structure design, matching ratio and process should have enough durability to ensure that the main body of the structure can prevent the erosion of groundwater and seawater, all the concrete surface in contact with water should use fine formwork, other places can use rough formwork.

 

    Nuclear plants use very little nuclear fuel to produce large amounts of electricity, and the cost per kilowatt-hour of electricity is more than 20 percent lower than that of coal-fired plants. Nuclear power plants can also greatly reduce the amount of fuel transported.For example, a 1 million kilowatt coal-fired power station would consume 3 to 4 million tons of coal a year, whereas a nuclear power station of the same power would require only 30 to 40 tons of uranium.Another advantage of nuclear power is that it is clean, pollution-free and produces virtually zero emissions, which is perfect for China, which is developing rapidly and is under great environmental pressure.

    In 2007, China generated 62.862 billion KWH of nuclear power and 59.263 billion KWH of on-grid electricity, up 14.61 percent and 14.39 percent, respectively, year on year.The Tianwan Nuclear Power Plant with two units of 1.06 million kW was put into commercial operation in May and August 2007 respectively, bringing the total number of nuclear power units in operation in China to 11, with a total installed capacity of 9.078 million kW.

    By the end of 2007, China's installed electric power capacity had reached 713 million kW, and the country's electricity supply and demand remained in an overall balance.Meanwhile, with the operation of two million-kilowatt nuclear power units at the Tianwan Nuclear Power Plant, China's installed nuclear power capacity has reached 8.85 million kilowatts.

    In 2007, the installed capacity of hydropower and thermal power grew by more than 10 percent, reaching 145 million kW and 554 million kW, respectively. Meanwhile, the total installed capacity of grid-connected wind power doubled to 4.03 million kW.

    China has begun to relax its policy on nuclear power, long stressing a "limited" development of the industry. Since 2003, China has experienced a general energy crunch. In this case, the domestic call for vigorously developing the nuclear power industry is increasingly strong. This latest high-level statement on nuclear power development is undoubtedly worthy of affirmation, as it establishes a strategic position for the nuclear power industry, which is not only positive for solving China's long-term energy tensions, but also an ideal way to maintain China's strategic deterrence capability in peacetime, killing two stones with one stone.

    China currently has a total installed capacity of 8.7 gigawatts of nuclear power plants in construction or under construction. It is estimated that China's installed nuclear power capacity will be about 20 gigawatts by 2010 and 40 gigawatts by 2020.By 2050, according to estimates by different departments, China's installed nuclear power capacity can be divided into three scenarios: high, medium and low: The high scenario is 360 gigawatts (about 30 percent of China's total installed power capacity), the medium scenario is 240 gigawatts (about 20 percent of China's total installed power capacity), and the low scenario is 120 gigawatts (about 10 percent of China's total installed power capacity).

    The National Development and Reform Commission of China is formulating a plan for the development of nuclear power in China's civil industry. It is expected that China's total installed power capacity will be 900 million KWH by 2020, and the proportion of nuclear power will account for 4% of the total power capacity, which means that China's nuclear power will be 36-40 GW by 2020. That means that by 2020,

China will have 40 megawatt nuclear power plants equivalent to Daya Bay.

    Judging from the general trend of nuclear power development, the technological and strategic routes of China's nuclear power development have long been clear and are being implemented: pressurized water reactor at present, fast neutron reactor in the medium term and fusion reactor in the long term.Specifically, in the near future, it will develop thermal neutron reactor nuclear power plants. In order to make full use of uranium resources, adopt the technical route of uranium-plutonium cycle, and develop fast breeder reactor nuclear power plants in the medium term.In the long term, fusion reactor nuclear power plants will be developed, so as to basically solve the contradiction of energy demand "forever".

 

    With Japan as the center, the international nuclear power enterprises have formed a tripartite situation: Hitachi of Japan's Fuji Consortium -- GM of the United States, Toshiba of Japan's Mitsui Consortium -- Westinghouse of the United States, Mitsubishi Heavy Industries of Japan's Mitsubishi Consortium -- Areva of France.The embryonic form of Japan's monopoly in nuclear power technology and market has emerged, and China's energy strategy adjustment to accelerate the development of nuclear energy application is bound to be subject to Japan.

 

    Throughout the history of nuclear power development, nuclear

power plant technology programs can be roughly divided into four

generations, namely:

 

    The development and construction of nuclear power plants began in the 1950s. In 1954, the former Soviet Union built an experimental nuclear power plant with an electric power capacity of 5 megawatts, and in 1957, the United States built the prototype shipping port nuclear power plant with an electric power capacity of 90,000 kilowatts. These achievements proved the technical feasibility of using nuclear energy to generate electricity. These experimental and prototype nuclear power plants are internationally referred to as the first generation of nuclear power plants.

 

    In the late 1960s, on the basis of experimental and prototype nuclear power units, pressurized water reactors, boiling water reactors, heavy water reactors, graphite water cooled reactors and other nuclear power units with an electric power capacity of 300,000 kW were built one after another, which further proved the technical feasibility of nuclear power generation while also proving the economic efficiency of nuclear power. In the 1970s, the energy crisis caused by rising oil prices promoted the great development of nuclear power. The vast majority of the world's more than 400 nuclear power plants in commercial operation were built during this period, traditionally known as second-generation nuclear power plants.

 

    In the 1990s, in order to solve the negative impact of the serious accidents at Three Mile Island and Chernobyl nuclear power plants, the world nuclear power industry concentrated its efforts on the prevention and mitigation of serious accidents. The United States and Europe successively issued the "Advanced light water Reactor User Requirements" document. URD (utility requirements document) and European Users' requirements for Light Water Reactor Nuclear Power Plants (EUR), Further clarify the prevention and mitigation of serious accidents, improve safety and reliability, and improve human factors engineering requirements.In the world, nuclear power units that meet the URD or EUR file are usually referred to as the third generation nuclear power units. The third generation nuclear power units are required to be ready for commercial construction by 2010.

 

    In January 2000, under the initiative of the United States Department of Energy, ten countries interested in developing nuclear energy, including the United States, the United Kingdom, Switzerland, South Africa, Japan, France, Canada, Brazil, South Korea and Argentina, jointly formed the "Fourth Generation International Nuclear Energy Forum" (GIF). In July 2001, they signed a contract to cooperate in the research and development of the fourth generation nuclear energy technology. It is envisaged that fourth-generation nuclear energy solutions will be safer and more economical, with minimal waste, no need for off-site emergency response, and inherent nonproliferation capabilities. High-temperature gas-cooled reactors, molten salt reactors, and sodium-cooled fast reactors are the fourth generation reactors.

    The first generation of nuclear power plant is the prototype reactor, whose purpose is to verify the design technology and commercial development prospects of nuclear power plant. The second generation nuclear power plants are commercial reactors with mature technology, and most of the nuclear power plants in operation now belong to the second generation nuclear power plants.The third-generation nuclear power plants are those that meet the requirements of URD or EUR, with improved safety and economy compared with the second-generation nuclear power plants, and belong to the main direction of future development.

    We already know that radioactivity exists everywhere in nature, and we have been receiving radiation from natural background. So where does this natural radiation come from?And to what extent? The "background" of natural radiation comes from two sources: radiation in the form of high-energy particles from outer space, collectively known as cosmic rays; The other source is natural radioactivity, the radioactive radiation that is naturally present in common matter such as air, water, dirt and rocks, and even food. In addition, people in modern society are exposed to all kinds of man-made radiation, such as X-rays, watching TV, using microwave ovens, etc. The following table lists various types of background radiation according to the size of the radiation. It can be seen from the table that human eating, using, living and traveling will receive a small amount of radioactive radiation, among which the radiation from nuclear power plants is very small and can be completely ignored.

 

    The effect of radiation on the human body begins in the cells. It accelerates cell death, inhibits the formation of new cells, or causes cell deformities, or changes in the body's biochemical reactions. At low radiation doses, the human body itself has a certain ability to repair radiation damage and can repair the above reactions without showing harmful effects or symptoms.But if the dose is too high, beyond the repair ability of the organs or tissues in the body, it will cause local or systemic lesions.The following table shows the currently internationally recognized biological effects of radiation. It can be seen that the human body can withstand a concentrated dose of 25 rems without injury. Of course, each person's ability to resist and constitution is different.