The Demon Core was a 6.2 kg plutonium sphere used in nuclear weapon research during the Manhattan Project. It earned its ominous nickname after two criticality accidents at Los Alamos in 1945 and 1946. These incidents, caused by missteps during experiments to test critical mass, resulted in fatal radiation exposure to physicists Harry Daghlian and Louis Slotin. The core was intended for a third atomic bomb but was repurposed for tests after Japan’s surrender.