On the morning of April 26th, 0986 1: 30, a catastrophic accident occurred in Unit 4 of Chernobyl nuclear power plant, located in the eastern part of Belarus-Ukraine forest region of the former Soviet Union. The reactor core was destroyed and some workshops collapsed. The leakage of radioactive pollution not only affected a large area of the former Soviet Union, but also affected Sweden, Finland, Poland and other countries, becoming a nuclear power plant accident that shocked the world. Chernobyl nuclear accident-overview of nuclear power plant The Chernobyl nuclear power plant (5 1.23 minutes north latitude and 30.6 minutes 4 1 second east longitude) is located in Pripyat, Ukraine, which is 0/mile (18 km) northwest of Chernobyl. 0? 70 miles north of Kiev (1 100 km). The nuclear power plant consists of four reactors, each of which can generate 1 GW of electricity (3200 GW of thermal power). During the nuclear accident, four reactors provided 50% of Ukraine's electricity. The workshop project started in 1960s, and the 1 reactor was put into operation in 1977, followed by No.2 (1978), No.3 (198 1) and No.4 (1988). At the time of the accident, two reactors (No.5 and No.6 reactors, each with a power generation capacity of 65.438+0 billion watts) were still under construction. The four reactors in this plant belong to the same type and are called RBMK- 1000. According to the analysis after the accident, the accident of Chernobyl Unit 4 was an accident with excessive reactivity. There is an important defect in the design of ρ μ б κ reactor in the former Soviet Union, that is, there may be a positive cavitation reactivity coefficient due to phase change in coolant. When the reactor power is lower than 20% of the rated value, this positive reactivity effect cannot be offset by the negative reactivity effect of Doppler coefficient, which may lead to the spontaneous increase of reactor power. In order to remedy this design defect, the operating regulations of ρ μ б κ type reactor stipulate that the excess reactivity of the reactor shall not be lower than the value equivalent to 30 control rods in operation; When it is lower than 15 control rod, the reactor must be stopped immediately. The direct cause of the accident was a series of operations that seriously violated the operating rules. Due to the long-term low-power operation of the reactor before the start of the test, the reactor was seriously "poisoned" and was out of control. At the beginning of the test, the operator began to replenish water to maintain the water level in the steam-water separator. At this time, due to the arrival of cold water in the core, the steam production began to decrease greatly, which led to the rise of the automatic adjusting rod. Because the automatic adjusting lever rises to the top, the operator must use the manual lever to control it. By 1: 22, the excess reactivity of the reactor is only 6 ~ 8 control rods, and the power level is only 6 ~ 7% of the rated value. At this time, the turbine emergency protector was removed and the idling test began, and the steam flow rate dropped sharply, resulting in an increase in steam pressure. At this time, all four circulating pumps stopped and the water flow began to decrease again. The combined action of these two factors makes the increase rate of volume steam mass much faster than that under rated conditions, which leads to a sharp increase in reactivity and a large number of steam and nucleate boiling. The fuel was damaged by overheating. Under the action of broken fuel particles and boiling cooling water, the pressure in the fuel pipeline suddenly rises, leading to destruction and explosion. Some measures to reduce safety are also the cause of the accident. In order to complete the test, before the test, the emergency core cooling system was disconnected, and the reactor protection system of turbine generator shutdown signal and water level pressure of steam-water separator was locked, which made the reactor run in unprotected state and lost the possibility of automatic shutdown. Therefore, the Chernobyl nuclear accident not only exposed the design defects of the ρ μ б κ reactor, but also exposed many problems in operation and management. For example, the test outline does not stipulate the safety measures that should be taken, and it has not even been reviewed; During the whole test, a series of violations of operating procedures were not corrected in time.