Demon core - a true story

The demon core was a spherical 6.2-kilogram (14 lb) subcritical mass of plutonium 89 millimeters (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. The core was prepared for shipment as part of the third nuclear weapon to be used in Japan, but when Japan surrendered, the core was retained at Los Alamos for testing and potential later use. It was involved in two criticality accidents at the Los Alamos Laboratory on August 21, 1945, and May 21, 1946, each resulting in a fatality. Both experiments were designed to demonstrate how close the core was to criticality with a tamper, but in each case, the core was accidentally placed into a critical configuration. Physicists Harry Daghlian and Louis Slotin suffered acute radiation syndrome (ARS) and died soon after, while others present in the lab were also exposed.