The Chernobyl disaster was a catastrophic nuclear accident that occurred on 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.
The accident occurred during a safety test on a common Soviet reactor type - the RBMK nuclear power reactor. The test was a simulation of an electrical power outage, to develop a safety procedure for keeping reactor cooling water circulating until the emergency generators could provide power. This gap was about one minute and had been identified as a potential safety problem which could cause core overheating. Three such tests had previously been conducted since 1982 but had failed to provide a solution. Unfortunately, on this fourth occasion, the test had been delayed by ten hours and the reactor operating shift that had specifically prepared for the test procedure was replaced by the next shift. The test supervisor then failed to follow the test procedure, creating unstable operating conditions which, combined with inherent RBMK reactor design flaws and the intentional disabling of several emergency safety systems, resulted in an uncontrolled nuclear chain reaction. A huge amount of energy was suddenly released, which vapourised superheated cooling water, rupturing the reactor pressure vessel in a highly destructive steam explosion, which was instantly followed by an open-air reactor core fire.
