https://en.wikipedia.org/wiki/Demon_core
The demon core was a 6.2-kilogram (14 lb)
subcritical mass of
plutonium measuring 89 millimetres (3.5 in) in diameter, which was involved in two
criticality accidents, on August 21, 1945 and May 21, 1946. The core was intended for use in a third World War II nuclear bomb, but remained in use for testing after Japan's surrender. 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
On August 21, 1945, the plutonium core produced a burst of
neutron radiation that led to physicist
Harry Daghlian's death. Daghlian made a mistake while performing
neutron reflector experiments on the core. He was working alone; a security guard, Private Robert J. Hemmerly, was seated at a desk 10 to 12 feet (3 to 4 m) away.
[8] 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. While attempting to stack another brick around the assembly, Daghlian accidentally dropped it onto the core and thereby caused the core to go well into supercriticality, a self-sustaining critical chain reaction. He quickly moved the brick off the assembly, but received a fatal dose of radiation. He died 25 days later from acute
radiation poisoning.
[9]
On the day of the accident, Slotin's screwdriver slipped outward a fraction of an inch while he was lowering the top reflector, allowing the reflector to fall into place around the core. Instantly there was a flash of blue light and a wave of heat across Slotin's skin; the core had become supercritical, releasing an intense burst of neutron radiation estimated to have lasted about a half second.
[6] Slotin quickly twisted his wrist, flipping the top shell to the floor. The heating of the core and shells stopped the criticality within seconds of its initiation,
[15] while Slotin's reaction prevented a recurrence and ended the accident.