The "demon core"

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...

[the second incident]: On May 21, 1946, physicist Louis Slotin and seven other scientists were in a Los Alamos laboratory conducting an experiment to verify the exact point at which a subcritical mass (core) of fissile material could be made critical by the positioning of neutron reflectors. It required the operator to place two half-spheres of beryllium (a neutron reflector) around the core to be tested and manually lower the top reflector over the core via a thumb hole on the top. As the reflectors were manually moved closer and further away from each other, scintillation counters measured the relative activity from the core. Allowing them to close completely would result in the instantaneous formation of a critical mass and a lethal power excursion, and the only thing preventing this was the blade of a standard flathead screwdriver manipulated by the scientist's other hand. The test was known as "tickling the dragon's tail" for its extreme risk, and was notoriously unforgiving of even the smallest mistake; many scientists refused to perform the test, but Slotin (who was given to bravado) became the local expert, performing the test almost a dozen separate times, often in his trademark bluejeans and cowboy boots in front of a roomful of observers. Enrico Fermi reportedly told Slotin and others they would be "dead within a year" if they continued performing it.

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.

Via Uncertain Times.

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:

YouTube link.