The Kyshtym disaster was a radiation contamination incident that occurred on 29 September 1957 at Mayak, a nuclear fuel reprocessing plant in the Soviet Union. Making it the third most serious nuclear accident ever recorded behind the Chernobyl disaster, and Fukushima Daiichi nuclear disaster.
The cooling system in one of the tanks containing about 70–80 tons of liquid radioactive waste failed and was not repaired. The temperature in it started to rise, resulting in evaporation and a chemical explosion of the dried waste. At least 22 villages were exposed to radiation from the disaster with a total population of around 10,000 were evacuated. Some were evacuated after a week but it took almost 2 years for evacuations to occur at other sites.
The CIA knew about this since 1959, however, they decided to keep it a secret stateside because they didn't want to spook people into not supporting the growing US nuclear power industry. They started declassifying documents pertaining to this starting in 1989.