Chelyabinsk Oblast is a federal subject of Russia in the Ural Mountains region, on the border of Europe and Asia. Its administrative center is the city of Chelyabinsk.

The Kyshtym disaster, sometimes referred to as the Mayak disaster or Ozyorsk disaster in newer sources, was a radioactive contamination accident that occurred on 29 September 1957 at Mayak, a plutonium production site for nuclear weapons and nuclear fuel reprocessing plant located in the closed city of Chelyabinsk-40 in Chelyabinsk Oblast, Russian SFSR, Soviet Union.