The Demon Core was the nickname given to a 6.2-kilogram (14 lb) subcritical mass of plutonium that accidentally went critical in two separate accidents at the Los Alamos laboratory in 1945 and 1946. Both incidents resulted in the acute radiation poisoning and subsequent death of a scientist...Via Uncertain Times.
While lowering the top reflector, Slotin's screwdriver slipped a fraction of an inch, allowing the top 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 a massive burst of neutron radiation. He quickly knocked the two halves apart, stopping the chain reaction and likely saving the lives of the other men in the laboratory. Slotin's body positioning over the apparatus also shielded the others from much of the neutron radiation. He received a massively lethal dose in under a second and died nine days later from acute radiation poisoning. The nearest physicist to Slotin, Alvin C. Graves, was watching over Slotin's shoulder and was thus partially shielded by him, receiving a high but non-lethal radiation dose. Graves was hospitalized for several weeks with severe radiation poisoning, developed chronic neurological and vision problems as a result of the exposure, suffered a significant shortening of his lifespan and died of a radiation-induced heart attack 20 years later. The other six people in the room were far enough away from the assembly to avoid fatal injury, however they all suffered other complications as a result of the accident. Two people suffered severe shortening of their lives and died years later from radiation induced complications: leukemia (at age 42, 18 years after the accident) and clinical aplastic anemia.
And a big tip of the proverbial hat to Swift Loris, who offers this scene of John Cusack as Slotin in the film Fat Man and Little Boy: