On 21 August 1945, Los Alamos scientist Harry K. Daghlian, Jr. suffered fatal radiation poisoning after dropping a tungsten carbide brick onto a sphere of plutonium. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.
On 21 May 1946, another Los Alamos scientist, Louis Slotin, accidentally irradiated himself during a similar incident, when a critical mass experiment with the very same sphere of plutonium (see demon core) took a wrong turn. Immediately realizing what had happened he quickly disassembled the device, likely saving the lives of seven fellow scientists nearby. Slotin succumbed to radiation poisoning nine days later.
http://en.wikipedia.org/wiki/Criticality_accident
