The Chernobyl disaster was a Nuclear incident that occurred on Saturday 26 April 1986, at the No. 4 reactor in the Chernobyl Nuclear Power plant, near the city of Pripyat in the north of the Ukraine.
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 reactor cooling water circulation until the back-up electrical generators could provide power. This gap was about one minute and had been identified as a potential safety problem that could cause the nuclear reactor core to overheat. It was hoped to prove that the residual rotational energy in a turbine generator could provide enough power to cover the gap. Three such tests had been conducted since 1982, but they had failed to provide a solution. On this fourth attempt, an unexpected 10-hour delay meant that an unprepared operating shift was on duty. During the planned decrease of reactor power in preparation for the electrical test, the power unexpectedly dropped to a near-zero level. The operators were able to restore the specified test power only partially, which put the reactor in a potentially unstable condition. This risk was not made evident in the operating instructions, so the operators proceeded with the electrical test. 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.
A large amount of energy was suddenly released, vaporizing super heated cooling water, and rupturing the reactor core in a highly destructive swam explosion. This was immediately followed by an open-air reactor core fire that released considerable airborne radioactive contamination for about nine days that precipitated onto parts of the USSR and western Europe, before being finally contained on 4 May 1986. Official statement consisted of 29 deaths but in fact thousands of people died or got Cancer due to this accident.