The Chernobyl disaster was a 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 Soviet Socialist Republic.
The accident started during a safety test on an RBMK-type nuclear reactor, which was commonly used throughout the Soviet Union. A large amount of energy was suddenly released, vapourising 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.
Due to the often long incubation periods for radiation exposure to induce cancer, fewer than 100 documented deaths are likely to be attributable to increased exposure to radiation from the Chernobyl disaster. The number of exposure related deaths is 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.
The Chernobyl disaster is considered the worst nuclear power plant accident in history, both in terms of cost and casualties. The struggle to safeguard against hazards immediately after the accident, together with later decontamination efforts of the surroundings, ultimately involved more than 500,000 liquidators and cost an estimated 18 billion Soviet rubles—roughly US$68 billion in 2019. Ten 10 RBMK reactors continue to be operational as of 2019.