The Chernobyl disaster began on 26 April 1986 with the explosion of the No. 4 reactor of the Chernobyl Nuclear Power Plant near the city of Pripyat in northern Ukraine, near the Belarus border in the Soviet Union. It is one of only two nuclear energy accidents rated at the maximum severity on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear accident. The response involved more than 500,000 personnel and cost an estimated 18 billion roubles—around US$68 billion in 2019. It remains the worst nuclear disaster in history, and the costliest disaster in human history, with an estimated cost of US$700 billion.

The accident occurred during a test of the steam turbine's ability to power the emergency feedwater pumps in case of external power loss. Following an accidental drop in reactor power, the operators restarted the reactor with a prohibited control rod configuration. This led to a power surge, causing reactor components to rupture and lose coolant. The resulting steam explosions and meltdown destroyed the containment building, followed by a reactor core fire that spread radioactive contaminants across the USSR and Europe. A 10-kilometre (6.2 mi) exclusion zone was established 36 hours after the accident, initially evacuating around 49,000 people, primarily from Pripyat. The exclusion zone was later expanded to 30 kilometres (19 mi), resulting in the evacuation of approximately 68,000 more people.