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 (now Ozyorsk) in Chelyabinsk Oblast, Russian SFSR, Soviet Union.

The disaster is the third-worst nuclear incident (by radioactivity released) after the Chernobyl disaster and the Fukushima Daiichi disaster. It measured as a Level 6 disaster on the International Nuclear Event Scale (INES), making it the third-highest on the INES (which ranks by population impact), behind the Chernobyl disaster, which resulted in the evacuation of 335,000 people, and the Fukushima Daiichi disaster, which resulted in the evacuation of 154,000 people; the Chernobyl disaster and the Fukushima Daiichi disaster are both Level 7 disasters on the INES. At least 22 villages were exposed to radiation from the Kyshtym disaster, with a total population of around 10,000 people evacuated. Some were evacuated after a week, but it took almost two years for evacuations to occur at other sites.The disaster spread hot particles over more than 52,000 square kilometres (20,000 sq mi), where at least 270,000 people lived. Since Chelyabinsk-40 (later renamed Chelyabinsk-65 until 1994) was not marked on maps, the disaster was named after Kyshtym, the nearest known town.

The becquerel (English: ; symbol: Bq) is the SI derived unit of radioactivity. One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. For applications relating to human health this is a small quantity, and SI multiples of the unit are commonly used.The becquerel is named after Henri Becquerel, who shared a Nobel Prize in Physics with Pierre and Marie Skłodowska Curie in 1903 for their work in discovering radioactivity.