Date: 29 September 1957
Location: Mayak Scientific-Production Association, Kyshtym, Chelyabinsk, Russia, USSR
Type of event: chemical explosion in stored nuclear wastes
Description: The accident occurred at a facility involved in processing and storage of nuclear wastes. Following removal of plutonium, liquid high-level nitrate-containing wastes were stored in underground stainless steel tanks in concrete trenches. The tanks were located about 2 km from the plant, each with a volume of about 300 cubic meters. The tanks included heat exchangers to remove heat buildup in the tanks. The heat exchanger on one tank was shut down in 1956 due to a fault, and it was concluded it was safe to leave the tank uncooled. However, water evaporation in the tank allowed a buildup of nitrates and acetates on the surface of the tank's contents, in contact with air, before a spark from monitoring equipment detonated the contents on 29 Sept. 1957. This chemical explosion had an energy release of 5-10 tons of TNT, releasing about 70- 80 tons of wastes with a total activity of about 20 megacuries.