The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in the Ukrainian SSR (now Ukraine). An explosion and fire released large quantities of radioactive contamination into the atmosphere, which spread over much of Western Russia and Europe. It is considered the worst nuclear power plant accident in history, and is one of only two classified as a level 7 event on the International Nuclear Event Scale (the other being the Fukushima I nuclear incident, which is considered far less serious and has caused no direct deaths). The battle to contain the contamination and avert a greater catastrophe ultimately involved over 500,000 workers and cost an estimated 18 billion rubles, crippling the Soviet economy.
The disaster began during a systems test on 26 April 1986 at reactor number four of the Chernobyl plant, which is near the town of Pripyat. There was a sudden power output surge, and when an emergency shutdown was attempted, a more extreme spike in power output occurred, which led to a reactor vessel rupture and a series of explosions. These events exposed the graphite moderator of the reactor to air, causing it to ignite. The resulting fire sent a plume of highly radioactive smoke fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union and Europe. From 1986 to 2000, 350,400 people were evacuated and resettled from the most severely contaminated areas of Belarus, Russia, and Ukraine. According to official post-Soviet data, about 60% of the fallout landed in Belarus.
The effects of radiation exposure fall into two main classes: deterministic effects, where the effect is certain to occur under given conditions (e.g. individuals exposed to several grays over a short period of time will definitely suffer Acute Radiation Syndrome); and stochastic effects, where the effect may or may not occur (e.g. an increase in radiation exposure may or may not induce a cancer in a particular individual but if a sufficiently large population receive a radiation exposure above a certain level, an increase in the incidence of cancer may become detectable in that population). UNSCEAR, 2011.