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 the north of the Ukrainian SSR, close to the border with the Byelorussian SSR, in the Soviet Union. It is one of only two nuclear energy accidents rated at seven—the maximum severity—on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear accident in Japan. The initial emergency response and subsequent mitigation efforts involved more than 500,000 personnel and cost an estimated 18 billion roubles—roughly US$68 billion in 2019, adjusted for inflation. It is considered the worst nuclear disaster in history. The accident occurred during a test of the steam turbine's ability to power the emergency feedwater pumps in the event of a simultaneous loss of external power and coolant pipe rupture. Following an accidental drop in reactor power...