Kyshtym disaster
Mayak is the name for a number of nuclear facilities, which are about  from Ekaterinburg, in Russia. The facilities were also known as Chelyabinsk-65, or Chelyabinsk-40. Mayak was built between 1945 and 1948. It was the first plant that could produce nuclear material in the Soviet Union. At times, up to 25.000 people worked at Mayak. They produced the plutonium fuel that was used for the first Soviet atomic bomb, amongst other things.