21st August 1945: The Demon Core fatally irradiates physicist Harry Daghlian during an experiment at Los Alamos Laboratory

**Scott Allsop** (0:00)
On the 21st of August 1945, physicist Harry Daghlian suffered fatal irradiation during an experiment with The Demon Core at Los Alamos Laboratory. Welcome to HistoryPod.
Following the successful tests and wartime deployment of nuclear bombs, scientists at Los Alamos continued to study the behaviour of fissile materials in order to improve weapon design and safety. One experimental assembly, later known as The Demon Core, involved a subcritical mass of plutonium surrounded by neutron-reflecting materials, which was designed to measure how close the configuration was to a critical state. On the evening of the 21st of August, Daghlian was working alone with the plutonium core. He was stacking blocks of tungsten carbide around the core to reflect neutrons and bring it closer to criticality for measurement purposes. During the process, he accidentally dropped a block onto the assembly, which increased the reflection of neutrons and pushed the system into a supercritical state. Daghlian immediately attempted to disassemble the structure with his bare hands.
Although he successfully stopped the reaction, Daghlian suffered severe radiation poisoning and was taken to hospital for treatment. He died approximately three weeks later as a result of complications caused by acute radiation exposure. The incident was classified as a criticality accident, demonstrating the dangers associated with experimental nuclear research. Although Daghlian's death led to increased safety measures in nuclear research facilities, the Demon Core was involved in a second fatal accident involving physicist Louis Slotin the following year. Together, these incidents became key case studies in nuclear safety, and in the development of stricter protocols for handling radioactive materials.