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 – there is a time gap between the moment of power outage and the moment at which the back-up generators reach full 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 they had failed to provide a solution. On this fourth attempt, the test was delayed by 10 hours, so an unprepared operating shift had to perform it. During a gradual decrease of reactor power that was done in preparation for the test, the power unexpectedly dropped to a near-zero level at one moment. The operators were able to partially restore power, 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 the test proceeded even though the power was still lower than prescribed. Upon test completion, the operators triggered a reactor shutdown, but a combination of unstable conditions and reactor design flaws caused an uncontrolled nuclear chain reaction instead.

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