The Demon Core

Wiki:

The Demon Core was the nickname given to a 6.2-kilogram (14 lb) subcritical mass of plutonium that accidentally went critical on two separate instances at the Los Alamos laboratory, in 1945 and 1946. Each incident resulted in the acute radiation poisoning and subsequent death of a scientist. After these incidents, the core was referred to as the Demon Core.

On August 21, 1945, the plutonium core produced a burst of neutron radiation that irradiated Harry Daghlian, a physicist who made a mistake while working alone doing neutron reflection experiments on the core. The core was placed within a stack of neutron-reflective tungsten carbide bricks, and the addition of each brick moved the assembly closer to criticality. Daghlian, while attempting to stack another brick around the assembly, accidentally dropped one of the bricks onto the core, thereby making it critical. Despite moving the brick off the assembly quickly, Daghlian received a fatal dose of radiation. He died 25 days later.

On May 21, 1946, physicist Louis Slotin and other scientists were in a Los Alamos laboratory conducting an experiment that involved creating a fission reaction by placing two half-spheres of beryllium (a neutron reflector) around the same plutonium core. Slotin's hand holding a screwdriver separating the hemispheres slipped, the beryllium neutron reflector hemispheres closed, and the core became supercritical, releasing a very high dose of radiation. He quickly pulled the two halves apart, stopping the chain reaction and hence saving the lives of the seven other men in the laboratory. Louis Slotin himself, however, died 9 days later from acute radiation poisoning. The scientist assisting received sufficient radiation dose to cause serious injuries and some permanent partial disability, while the others in the room suffered no permanent injuries from the accident.

The Demon core was put to use for the ABLE detonation test of the Crossroads series on July 1, 1946, demonstrating that the criticality experiments of Daghlian and Slotin increased the efficiency of the weapon.