Demon core
The demon core was a sphere of plutonium that was involved in two fatal radiation accidents when scientists tested it as a fissile core of an early atomic bomb. It was manufactured by the Manhattan Project, the U.S. nuclear weapon development effort during World War II. It was a subcritical mass that weighed 6.2 kilograms (14 lb) and was 89 millimeters (3.5 in) in diameter. The core was prepared for shipment to the Pacific Theater as part of the third nuclear weapon to be dropped on Japan, but when Japan surrendered, the core was retained for testing and potential later use in the case of another conflict. The two criticality accidents occurred at the Los Alamos Laboratory in New Mexico on August 21, 1945, and May 21, 1946.