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.
so-called Demon Core was used in an atomic bomb test in 1946, five
weeks after the last accident with it, and proved in practice to have a
slightly increased yield over similar cores which had not been subjected
to criticality excursions.