The "Chernobyl disaster", reactor accident at the Chernobyl nuclear power plant was the worst nuclear power plant accident in history and the only instance so far of level 7 on the International Nuclear Event Scale, resulting in a severe nuclear meltdown.

On 26 April 1986 at 01:23:40 a.m. (UTC+3) reactor number four at the Chernobyl Nuclear Power Plant located in the Soviet Union near Pripyat in Ukraine exploded. Further explosions and the resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area.