The Demon Core 1945

The Demon Core was the nickname given to a 6.2-kilogram (14 lb) subcritical mass of plutonium that accidentally went critical on two separate instances at the Los Alamos laboratory in 1945 and 1946. Each incident resulted in the acute radiation poisoning and subsequent death of a scientist. After these incidents the mass of plutonium was referred to as the Demon Core.
The Slotin and Daghlian incidents were combined and fictionalized in the film Fat Man and Little Boy.