The "Demon Core" was a subcritical mass of plutonium involved with the deaths of two scientists
The demon core was a 6.2-kilogram (14 lb), 3.5-inch-diameter (89 mm) subcritical mass of plutonium which was involved in two criticality accidents. It briefly went supercritical in two separate accidents at the Los Alamos laboratory in 1945 and 1946, and resulted in the acute radiation poisoning and subsequent deaths of scientists Harry K. Daghlian, Jr., and Louis Slotin. After these incidents the spherical plutonium core was referred to as the "demon core". It was eventually used in the Gilda bomb detonated during Operation Crossroads, the first nuclear test to be conducted after World War II, five weeks after the second fatal accident. It performed normally and with the same explosive yield as the other core used in this set of two tests.@Curionic
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