The Chernobyl disaster was a nuclear accident that occurred on Saturday 26 April 1986, at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR in the Soviet Union. It is considered the worst nuclear disaster in history both in terms of cost and casualties, and 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 Daiichi nuclear disaster in Japan. The initial emergency response, together with later decontamination of the environment, ultimately involved more than 500,000 personnel and cost an estimated 18 billion Soviet rubles—roughly US$68 billion in 2019, adjusted for inflation.
The accident occurred during a safety test on the steam turbine of an RBMK-type nuclear reactor. During the planned decrease of reactor power in preparation for the electrical test, the power unexpectedly dropped to a near-zero level. The operators were unable to fully restore the power level specified by the test program, which put the reactor in an unstable condition.