The only INES Level 6 nuclear event, it was the worst nuclear disaster in history until Chernobyl and contaminated over 52,000 km² in the Soviet Union.
Key Facts
- INES Classification
- Level 6 (only event at this level)
- Area Contaminated
- 52,000 km²
- People in Contaminated Zone
- 270,000 people
- Villages Exposed to Radiation
- 22 villages
- People Evacuated
- 10,000 people
- Evacuation Duration
- Up to nearly two years for all sites
By the Numbers
Location
Cause → Event → Consequence
The Mayak facility, a Soviet plutonium reprocessing plant for nuclear weapons located in the secret closed city of Chelyabinsk-40, operated under conditions that led to a failure in the cooling system of a radioactive waste storage tank. This mechanical failure allowed the contents to overheat and ultimately explode.
On 29 September 1957, a chemical explosion occurred in a storage tank at the Mayak plant, releasing a large quantity of radioactive material. The explosion dispersed hot particles across a vast area of the Soviet Union, exposing at least 270,000 people to radiation and necessitating the evacuation of approximately 10,000 residents from at least 22 villages.
The contaminated territory spanning more than 52,000 square kilometres became known as the East Ural Radioactive Trace. Evacuations were delayed, with some populations not relocated for nearly two years. Because Chelyabinsk-40 was a secret city absent from maps, the disaster was named after the nearby town of Kyshtym, and details were suppressed by Soviet authorities for decades.