The Chernobyl disaster was a nuclear accident that occurred on Saturday 26 April , at the No. The accident started during a safety test on an RBMK-type nuclear reactor. The test was a simulation of an electrical power outage to help create a safety procedure for maintaining reactor cooling water circulation until the back-up electrical generators could provide power. Three such tests had been conducted since , but they had failed to provide a solution. On this fourth attempt, an unexpected hour delay meant that an unprepared operating shift was on duty. The operators were able to only partially restore the specified test power, which put the reactor in an unstable condition.
It is the most significant accident in U. The accident began with failures in the non-nuclear secondary system, followed by a stuck-open pilot-operated relief valve in the primary system. This allowed large amounts of nuclear reactor coolant to escape. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident due to inadequate training and human factors , such as human—computer interaction design oversights relating to ambiguous control room indicators in the power plant's user interface. In particular, a hidden indicator light led to an operator manually overriding the automatic emergency cooling system of the reactor because the operator mistakenly believed that there was too much coolant water present in the reactor and causing the steam pressure release. The accident crystallized anti-nuclear safety concerns among activists and the general public and resulted in new regulations for the nuclear industry. It has been cited as a contributor to the decline of a new reactor construction program, a slowdown that was already underway in the s.