26 April 1986,
In the Soviet Union there was the Chernobyl nuclear accident. During a test at the nuclear power plant in the Ukrainian city of Pripiat, overheating of the reactor core caused an explosion causing a huge toxic and radioactive cloud. Following the accident management, the Soviet authorities had to report on what happened at an international conference on nuclear weapons, and EE.UU pressured them to indicate that the damage was less than it was.