On 26 April 1986, the no. 4 reactor of the Chernobyl Nuclear Power Plant, located near Pripyat, Ukrainian SSR, Soviet Union (now Ukraine), exploded. With dozens of direct casualties, 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 rubles (about $84.5 billion USD in 2025). It remains the worst nuclear disaster and the most expensive disaster in history, with an estimated cost of US$700 billion.

The disaster occurred while running a test to simulate cooling the reactor during an accident in blackout conditions. The operators carried out the test despite an accidental drop in reactor power, and due to a design issue, attempting to shut down the reactor in those conditions resulted in a dramatic power surge. The reactor components ruptured and lost coolants, and the resulting steam explosions and meltdown destroyed the Reactor building no. 4, followed by a reactor core fire that spread radioactive contaminants across the Soviet Union and Europe. A 10-kilometre (6.2 mi) exclusion zone was established 36 hours after the accident, initially evacuating around 49,000 people. The exclusion zone was later expanded to 30 kilometres (19 mi), resulting in the evacuation of approximately 68,000 more people.