Los Alamos Scientific Laboratory, 21 August 1945

A critical assembly was being created by hand stacking 4.4 kg tungsten carbide bricks around the plutonium core. Figure shows a reenactment of the configuration with about half of the tungsten blocks in place. The lone experimenter was moving the final brick over the assembly for a total reflector of 236 kg when he noticed from the nearby neutron counters that the addition of this brick would make the assembly supercritical. As he withdrew his hand, the brick slipped and fell onto the center of the assembly, adding sufficient reflection to make the system superprompt critical. A power excursion occurred. He quickly pushed off the final brick and proceeded to unstack the assembly. His dose was estimated as 510 rem from a yield of 10*16 fissions. He died 28 days later.
An Army guard assigned to the building, but not helping with the experiment, received a radiation dose of approximately 50 rem.
The nickel canning on the plutonium core did not rupture.