The Kyshtym disaster was a radiological contamination accident that occurred on 29 September 1957 at Mayak, a plutonium production site for nuclear weapons and nuclear fuel reprocessing plant in the Soviet Union. It measured as a Level 6 disaster on the International Nuclear Event Scale, making it the third most serious nuclear accident ever recorded, behind the Fukushima Daiichi nuclear disaster and the Chernobyl disaster (both Level 7 on the INES). The event occurred in the town of Ozyorsk, Chelyabinsk Oblast, a closed city built around the Mayak plant.