4. Kyshtym Disaster-Russia It occurred on 29 September 1957 at Mayak, a nuclear fuel reprocessing plant in the Soviet Union.
The explosion, estimated to have a force of about 70–100 tons of TNT, threw the 160-ton concrete lid into the air.
There were no immediate casualties as a result of the explosion, but it released an estimated 20 MCi (800 PBq) of radioactivity.
Most of this contamination settled out near the site of the accident and contributed to the pollution of the Techa River.
Over a half million people were affected by differing levels of radiation. Many of the towns along the Techa River, who had already been drinking deliberately contaminated water, were evacuated around a week later.