The event at the experimental prototype L-IV nuclear reactor led to an axident consisting of a steam explosion and reactor fire in Leipzig, Nazi Germany on June 23rd, 1942.
The L-IV test, indicated that the spherical geometry, with five metric tons of heavy water and 10 metric tons of metallic uranium, could sustain a fission reaction.
The fire and subsequent explosion was the world's first nuclear accident in history. 4 people died during it and it has a unofficial INES accident rating of 4.
Whilst the accident is not a Cold War era accident it was the first of them all!