1950, Mayak: a military accident:
In the 1950’s, the earliest days of the Mayak reactor, the waters of the Kyzyltash lake were directly used to cool the core of the plutonium-producing reactors. Radioactive waste pumped into the Techa river over the following years was responsible for serious environmental contaminations. There is an interesting contrast to be noted here between military and civilian nuclear accidents: on one hand, serious accidents such as Mayak have been covered up for years by secrecy, whereas there was tremendous media hype around the Three Mile Island accident, which caused more fear than actual damage.
DR(Source Richard Wilson et al)