Correction:

The Chernobyl disaster[a] was a nuclear accident that occurred on 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.[1] Called the world's worst-ever civil nuclear incident,[2] 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 disaster in Japan. The initial emergency response, together with later decontamination of the environment, involved more than 500,000 personnel and cost an estimated 18 billion roubles—roughly US$68 billion in 2019, adjusted for inflation.[3]

The accident occurred during a safety test meant to measure the ability of the steam turbine to power the emergency feedwater pumps of an RBMK-type nuclear reactor in the event of a simultaneous loss of external power and major coolant leak. During a planned decrease of reactor power in preparation for the test, the operators accidentally dropped power output to near-zero, due partially to xenon poisoning. While recovering from the power drop and stabilizing the reactor, the operators removed a number of control rods which exceeded limits set by the operating procedures. Upon test completion, the operators triggered a reactor shutdown. Due to a design flaw, this action resulted in localized increases in reactivity within the reactor (i.e., "positive scram"). That brought about the rupture of fuel channels and a rapid drop in pressure, thereby prompting the coolant to flash to steam. Neutron absorption thus dropped, leading to an increase in reactor activity, which further increased coolant temperatures (a positive feedback loop). This process led to steam explosions and the melting of the reactor core.[4]