Chemobyl atomic power plant disaster in 1986 upsc
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The Chernobyl disaster was a nuclear accident that occurred on Saturday, 26 April 1986 at the No. 4 nuclear reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR. It is considered the worst nuclear disaster in history and is one of only two nuclear energy disasters rated at seven—the maximum severity—on the International Nuclear Event Scale, the other being the 2011 Fukushima Daiichi nuclear disaster in Japan.
The accident started during a safety test on an RBMK-type nuclear reactor, which was commonly used throughout the Soviet Union. The test was a simulation of an electrical power outage to aid the development of a safety procedure for maintaining cooling water circulation until the back-up generators could provide power. This operating gap was about one minute and had been identified as a potential safety problem that could cause the nuclear reactor core to overheat.
Three such tests had been conducted since 1982, but had failed to provide a solution. On this fourth attempt, the test was delayed by 10 hours, so the operating shift that had been prepared was not present. During the test preparation in which several nuclear reactor safety systems were disabled and reactor power was manually decreased, the power unexpectedly dropped to a very low level. The operators were able to restore the power level, but this put the reactor in a highly unstable condition. The risks were not made evident in the operating instructions, despite a similar accident occurring years before, and they proceeded with the test even though the power level was still lower than prescribed in the procedure. Upon test completion, they triggered the reactor shutdown, but a combination of reactor design and construction flaws caused an uncontrolled nuclear chain reaction instead.