Nuclear energy, also known as atomic energy, is the energy generated when nuclear fission or fusion occurs, which is widely used in industrial and military fields. Nuclear power is the abbreviation of nuclear power generation, which is the electricity generated by nuclear power generation. Using nuclear energy to generate electricity is conducive to optimizing the national or regional energy structure, improving energy security and economy, and playing an increasingly important role in economic and social development. According to the statistics of the International Atomic Energy Agency (IAEA), as of the end of January 2011, there were 442 nuclear power units in operation in the world, and the operating nuclear power plants were mainly distributed in North America, Asia and Europe, accounting for about 16% of the global power generation. There are 65 nuclear power units under construction, including 30 in China. At present, there are 13 nuclear power units in operation in China, ranking 11th in the world, with an installed capacity of 11.169 million kWh, accounting for 1.16% of China’s total installed power capacity, and the annual power generation is equivalent to 31.72 million tons of coal.
First, the working principle of nuclear power plants
A nuclear power station is a power station that uses the energy released by nuclear fission or fusion reaction of atoms to produce electric energy. At present, nuclear power plants in commercial operation all use nuclear fission reaction to generate electricity. Nuclear power plants are generally divided into two parts: nuclear islands (including reactor devices) that generate steam by atomic nuclear fission and conventional islands (including turbine-generator systems) that generate electricity by steam. The fuels used in nuclear power plants are generally the chemical elements uranium and plutonium. At present, the main types of nuclear power plants in operation and under construction are PWR, candu nuclear power plant, boiling water reactor, fast reactor and gas-cooled reactor.
Pressurized water reactor (PWR) nuclear power plant is a thermal neutron reactor which uses pressurized light water as coolant and moderator, and water does not boil in the reactor, and uses thermal neutrons to cause chain reaction. Daya Bay Nuclear Power Plant, Ling ‘ao Nuclear Power Plant, Qinshan No.1 Nuclear Power Plant, Qinshan No.2 Nuclear Power Plant and Tianwan Nuclear Power Plant in China all belong to this type of reactor. Candu nuclear power plant uses light water as coolant and heavy water as moderator, and the water does not boil in the reactor. It is also a thermal neutron reactor that uses thermal neutrons to cause chain reaction. Qinshan No.3 nuclear power station in China belongs to this type of reactor. Boiling water reactor nuclear power plant uses light water as coolant and moderator, but water boils in the reactor, which uses thermal neutrons to cause chain reaction. Fukushima Daiichi nuclear power plant in Japan belongs to this type of reactor. The fast reactor nuclear power plant is a nuclear reactor in which the thermal energy released by the chain reaction caused by fast neutrons is converted into electrical energy. The nuclear power plant imported from Russia and to be built in Sanming, Fujian Province belongs to this type of reactor. Gas-cooled reactor nuclear power plant is a thermal neutron reactor with gas (carbon dioxide or helium) as coolant, which is caused by thermal neutrons. So far, three types of reactors have been developed, such as natural uranium graphite gas-cooled reactor, improved gas-cooled reactor and high temperature gas-cooled reactor. The Shidaowan nuclear power plant to be built in China belongs to high temperature gas-cooled reactor. The nuclear power plants in operation and under construction in China are mainly pressurized water reactor nuclear power plants.
PWR nuclear power plants generally have three loops: primary loop (reactor device), secondary loop (turbine generator system) and tertiary loop (circulating water system). Nuclear fuel burns in a special form in the reactor, generating heat to heat the coolant in the primary circuit. The heated coolant enters the steam generator under the push of the main pump, which heats the water in the secondary circuit and turns it into steam, and then is sent back to the core by the main pump for reheating. This continuous cycle is called a loop. Steam enters the steam turbine through the pipeline, which drives the turbine generator to do work and generate electricity, then enters the condenser, and is cooled into water and returned to the steam generator. This steam-water cycle process is called the secondary loop. The third loop uses seawater or fresh water, and its function is to cool the steam in the second loop in the condenser to change it back into condensed water.
In order to ensure the safety of nuclear power plant, many safety facilities are also considered in the design, including: automatic shutdown system, reactor overpressure protection system, emergency core boron injection system, emergency core cooling system, emergency water supply system, radioactive material containment system and its supporting emergency power supply and cooling ventilation system. On March 11th, 2011, the safety system of Fukushima nuclear power plant in Japan performed related functions, such as automatic shutdown and emergency diesel generator start-up. However, due to the magnitude 9 earthquake and tsunami, the related systems were out of power and lost the final heat sink due to flooding.
According to China’s nuclear safety regulations and the recommendations of the International Atomic Energy Agency, the principle of "defense in depth" is strictly implemented in the design, construction and operation of nuclear power plants in China, and multi-level overlapping protection is provided from equipment and measures to ensure that radioactive materials can be effectively contained without leakage. "Defense in Depth" includes the following five lines of defense: careful design and construction to ensure excellent equipment and safety training of nuclear power plants; Strengthen operation management and supervision, and eliminate faults in time; Design and provide multi-level safety system and protection system to prevent equipment failure and human error from causing accidents; Activate the safety system of nuclear power plants, strengthen the management of power plants in accidents, and prevent accidents from expanding; Start the emergency response plan inside and outside the factory, and strive to reduce the impact of the accident on the surrounding residents.
Two major nuclear accidents and experience feedback
? Due to the complexity of the nuclear power plant system and the limitation of human understanding, there were two major nuclear accidents before the Fukushima nuclear accident. The accident at Three Mile Island in the United States on March 28th, 1979 had little impact on the environment. The Chernobyl accident on April 26, 1986 really made people realize the potential threat of nuclear power plants. In the accident, 237 occupational workers were exposed to clinically effective overdose radiation, of which 134 showed symptoms of acute radiation and 28 died within 3 months. From 1986 to 1987, the average radiation dose of 200,000 people who participated in accident treatment was about 100 millisieverts, of which about 10% people received 250 millisieverts and a few people received more than 500 millisieverts. After the accident, about 10% of the 116,000 residents evacuated from the restricted area with a radius of 30 kilometers received more than 50 millisieverts, and less than 5% received more than 100 millisieverts. Short-term exposure of humans to radiation doses below 100 millisieverts is basically harmless. The investigation shows that the fundamental causes of Chernobyl accident are design defects (positive power coefficient, no containment, etc.) and human errors (serious violation of regulations). The Chernobyl nuclear accident is by far the most serious one (the Fukushima nuclear accident cannot be accurately evaluated at present due to the lack of specific data).
Through the summary and reflection of nuclear accidents, many new safety measures and safety concepts have been developed. After the Chernobyl accident, international nuclear safety group of IAEA put forward the concept of nuclear safety culture, and published the report "Safety Culture" in 1991, which was widely accepted worldwide. At the same time, nuclear operators in the world realize that any nuclear accident will have an impact on other nuclear power plants. Therefore, it is necessary to strengthen exchanges and cooperation among nuclear operating units, promote effective experience feedback, and establish a nuclear safety culture to prevent nuclear accidents. In May 1989, 144 nuclear operators around the world signed the Charter of the World Association of Nuclear Operators (WANO) in Moscow, aiming at improving the safety and reliability of nuclear power plants around the world.
Third, the new generation of nuclear power technology and its prospect
After nearly 60 years of development, nuclear power has formed a complete set of systematic and complete theoretical system and accumulated a lot of operating experience. The newly designed nuclear power units are divided into two directions: one is to increase the configuration of special safety facilities to enhance the safety of the units, represented by EPR nuclear power units in France and VVER nuclear power units in Russia; Second, a large number of passive special safety facilities are used to enhance the safety of the unit, represented by the AP1000 nuclear power unit in the United States. French EPR adopts the design concept of "plus", that is, to increase redundancy to improve safety, and the safety system is increased from two series to four series, which also increases the complexity of the safety system and takes corresponding measures to prevent and alleviate serious accidents. The American AP1000 safety system adopts the "passive" design concept, which better achieves the "simplified" design principle. The safety system uses the natural characteristics of materials (gravity, natural circulation, energy of compressed gas, etc.), and does not need pumps, AC power supplies and corresponding support systems such as ventilation and cooling water, which greatly simplifies the safety system and reduces human errors. The design concept of VVER nuclear power unit in Russia is similar to that of EPR in France.
The accident of Fukushima nuclear power plant in Japan has once again aroused people’s concern about the safety risks of nuclear power, which will promote people’s continuous exploration of new technologies, with the aim of making nuclear energy safer, cleaner and more efficient. (Author: Party Secretary and Deputy Director of the Nuclear and Radiation Safety Center of the Ministry of Environmental Protection)
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