Kyshtym (Mayak)
The third most serious radiological accident on record, at the Mayak reprocessing complex. A waste tank exploded and contaminated a long strip of the southern Urals. The Soviet Union concealed it for 32 years.
| Date | 1957-09-29 |
|---|---|
| INES rating | 6 Serious accident |
| Facility | reprocessing |
| Country | Russia |
| Evacuated | 10,000people |
| Deaths at the time | not established |
|---|---|
| Acute radiation deaths | not established |
Casualty figures were never independently established; the accident was concealed by the Soviet authorities and not acknowledged until 1989. Estimates of long-term effects vary widely and none is well supported.
The cooling system on a buried tank of high-activity liquid waste failed. The contents dried, heated, and the nitrate-acetate salts exploded.
What happened
Mayak produced plutonium for weapons, and stored high-activity liquid waste in buried tanks under active cooling. On one tank the cooling failed and was not repaired. The contents evaporated, the residue heated, and roughly 70 to 80 tonnes of ammonium nitrate and acetate salts detonated with a force estimated in the tens of tonnes of TNT.
Consequences
Contamination spread over a strip roughly 300 km long, since called the East Urals Radioactive Trace. About 10,000 people were evacuated, some not for two years. The affected area remains restricted.
Mayak's earlier practice of discharging waste directly into the Techa river had already exposed the downstream population; the 1957 explosion compounded a site that was already among the most contaminated places on earth.
What changed afterwards
The accident is a reminder that the most serious radiological events have not all involved reactors. Two of the three worst — Kyshtym and Windscale, weeks apart in 1957 — happened at military plutonium facilities, and both were kept secret.
Other accidents
| Accident | INES | Date |
|---|---|---|
| Chernobyl | 7 | 1986-04-26 |
| Fukushima Daiichi | 7 | 2011-03-11 |
| Windscale Pile 1 fire | 5 | 1957-10-10 |
| Three Mile Island Unit 2 | 5 | 1979-03-28 |
| SL-1 | 4 | 1961-01-03 |
| Saint-Laurent A2 fuel melt | 4 | 1980-03-13 |
| Tokaimura criticality accident | 4 | 1999-09-30 |
| Vandellòs I turbine fire | 3 | 1989-10-19 |
| Davis-Besse vessel head corrosion | 3 | 2002-03-06 |
Sources
- World Nuclear Association and IAEA INES documentation · retrieved 2026-09-07