1986 – The Chernobyl disaster occurs in the Ukrainian Soviet Socialist Republic. On 26 April 1986, reactor no. 4 of the Chernobyl Nuclear Power Plant, located near Pripyat, Ukrainian SSR, Soviet Union (later Ukraine), exploded. With dozens of direct casualties and thousands of health complications stemming from the disaster, it is one of only two nuclear 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 US$84.5 billion 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. A design issue when attempting to shut down the reactor in those conditions resulted in a dramatic power surge. The reactor components ruptured and lost coolant, and the resulting steam explosions and meltdown destroyed the reactor building. This was followed by a reactor core fire that spread radioactive contaminants across the Soviet Union and Europe. A 10 km exclusion zone was established 36 hours after the accident, initially evacuating around 49,000 people. This was later expanded to 30 km, resulting in the evacuation of approximately 68,000 more people. Following the explosion, which killed two engineers and severely burned two others, an emergency operation began to put out the fires and stabilize the reactor. Of the 237 workers hospitalized, 134 showed symptoms of acute radiation syndrome (ARS); 28 of them died within three months. Over the next decade, 14 more workers (nine of whom had ARS) died of various causes mostly unrelated to radiation exposure. It is the only instance in commercial nuclear power history where radiation-related fatalities occurred. As of 2005, 6000 cases of childhood thyroid cancer occurred within the affected populations (15 of them fatal), "a large fraction" being attributed to the disaster. The United Nations Scientific Committee on the Effects of Atomic Radiation estimates fewer than 100 deaths have resulted from the fallout. Predictions of the eventual total death toll vary; a 2006 World Health Organization study projected 9,000 cancer-related fatalities in Ukraine, Belarus, and Russia.
