Chernobyl disaster was a nuclear accident.
The Chernobyl disaster , also referred to as the Chernobyl accident , was a catastrophic nuclear accident . It occurred on 26 April 1986 in the No. 4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant near Pripyat , in what was then part of the Ukrainian Soviet Socialist Republic of the Soviet Union ( USSR ) . The Chernobyl accident dominates the Energy accidents sub-category , of most disastrous nuclear power plant accident in history , both in terms of cost and casualties . It is one of only two nuclear energy accidents classified as a level 7 event ( the maximum classification ) on the International Nuclear Event Scale , the other being the Fukushima Daiichi nuclear disaster in Japan in 2011 .