Demon Core - First Incident

First Incident

On August 21, 1945, the plutonium core produced a burst of neutron radiation that caught Harry Daghlian in its path. Daghlian was a physicist who made a mistake while working alone performing neutron reflection experiments on the core. The core was placed within a stack of neutron-reflective tungsten carbide bricks, and the addition of each brick moved the assembly closer to criticality. While attempting to stack another brick around the assembly Daghlian accidentally dropped it onto the core and thereby caused the core to go critical, a self-sustaining chain reaction. Despite quick action in moving the brick off the assembly, Daghlian received a fatal dose of radiation. He died 25 days later from acute radiation poisoning.

Another person who was in the lab at the time of the accident—Private Robert J. Hemmerly, a Special Engineer Detachment (SED) guard—received an exposure of approximately 31 roentgens (0.31 Gy) of soft X-rays (80 kV equivalent) and less than 1 roentgen (0.01 Gy) of gamma rays. Hemmerly died in 1978 (33 years after the accident) from acute myelogenous leukemia at the age of 62.