Louis Slotin and the " Demon Core "

Making Nuclear Bombs at Los Alamos in the 1940's was top secret, highly dangerous and safety procedures were almost unknown. One plutonium–gallium alloy bomb core, known as the "Demon Core", was responsible for the deaths of at least two scientists and probably many more.

21 August 1945, Harry Daghlian accidently dropped a tungsten carbide brick on the core releasing a large dose of neutron radiation, he died 24 days later.

21 May 1946, Louis Slotin was doing an experiment where the two halves of the core are brought close together to start a very weak short lived chain reaction that can provide valuable data. Unfortunately the screwdriver he used to keep them apart slipped and the halves slammed together causing a "prompt critical" reaction and a visible burst of hard radiation. He immediately grabbed it and threw it on the floor but everyone in the surrounding area, including five scientists and the guard outside, were heavily irradiated.
The guard and three of scientists were released form hospital a few days later and the others were in hospital for several weeks.
Louis Slotin lasted 9 agonizing days with severe radiation burns before dying.

The core that had been intended for a third nuclear strike on Japan was never used and later melted down.