Database of radiological incidents and related events--Johnston's Archive

Los Alamos criticality accident, 1945

compiled by Wm. Robert Johnston
last modified 14 September 2005

Date: 21 August 1945

Location: Los Alamos Scientific Laboratory, New Mexico, USA

Type of event: criticality accident with plutonium metal assembly


An experimenter was hand-stacking tungsten carbide bricks around a plutonium metal assembly. The plutonium assembly comprised two hemispheres with a total mass of 6.2 kg, just short of (bare) critical mass. While moving a final brick, the experimenter noticed from neutron counters that the final brick would make the assembly supercritical. At this point he accidentally dropped the brick onto the pile, providing sufficent neutron reflection to result in a supercritical power excursion. The experimenter quickly removed the final brick and disassembled the assembly. He sustained a dose of 510 rem and died 28 days later. A nearby Army guard received a dose of 50 rem.

Consequences: 1 fatality (510 rem dose).


© 2004, 2005 by Wm. Robert Johnston.
Last modified 14 September 2005.
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