A nuclear energy accident that occurred on Saturday 26 April 1986, at the No. 4 reactor in the Chernobyl Nuclear Power Plant. 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.
A large amount of energy was released, vaporizing superheated cooling water and rupturing the reactor core in a highly destructive steam 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.
As a result of rising ambient radiation levels off-site, a 10 kilometers radius exclusion zone was created around 35 hours after the accident. About 50,000 people were evacuated from the area, primarily from Pripyat. The exclusion zone was later increased to 30 km radius when a further 68,000 people were evacuated from the wider area.
Determining the total eventual number of exposure related deaths is uncertain based. Model predictions with the greatest confidence values of the eventual total death toll in the decades ahead from Chernobyl releases vary, from 4,000 fatalities when solely assessing the three most contaminated former Soviet states, to about 9,000 to 16,000 fatalities when assessing the total continent of Europe.
To reduce the spread of radioactive contamination from the wreckage and protect it from weathering, the protective Chernobyl Nuclear Power Plant sarcophagus was built by December 1986.