The demon core was a spherical 6.2-kilogram (14 lb) subcritical mass of plutonium 89 millimetres (3.5 in) in diameter, manufactured during World War II by the United States nuclear weapon development effort, the Manhattan Project, as a fissile core for an early atomic bomb, that was involved in two criticality accidents, on August 21, 1945, and May 21, 1946. The core was intended for use in a possible third nuclear weapon to be dropped on Japan, but when Japan’s surrender made this unnecessary, it was used for testing.
It was designed with a small safety margin to ensure a successful explosion of the bomb. The device briefly went supercritical when it was accidentally placed in supercritical configurations during two separate experiments intended to guarantee the core was indeed close to the critical point. The incidents happened at the Los Alamos Laboratory in 1945 and 1946, resulting in the acute radiation poisoning and subsequent deaths of scientists Harry Daghlian and Louis Slotin, respectively. After these incidents the spherical plutonium core was referred to as the “demon core”.