What was the infamous Demon Core?
The 'Demon Core' was a plutonium sphere that killed two scientists in separate nuclear accidents.
During experiments at Los Alamos in 1945 and 1946, this 6.2-kilogram core accidentally went critical. These incidents released massive radiation bursts and a distinct blue glow. In the second accident, physicist Louis Slotin's screwdriver slipped while he was testing the core. He heroically pushed the materials apart to save his colleagues but died nine days later from radiation poisoning.
Nerd Mode
The Demon Core was a 6.2-kilogram (13.7-pound) subcritical mass of plutonium-239 manufactured during World War II for a potential third atomic bomb. It measured 8.9 centimeters (3.5 inches) in diameter and was designed to be pushed to the edge of criticality to test nuclear thresholds. The first fatal accident occurred on August 21, 1945, when physicist Harry Daghlian dropped a tungsten carbide brick onto the core, causing it to go prompt critical.The second and more famous incident happened on May 21, 1946, involving physicist Louis Slotin. Slotin was performing a procedure known as 'tickling the dragon's tail,' where he used a flat-head screwdriver to manually lower a beryllium hemisphere over the core. The screwdriver slipped, allowing the shell to fully enclose the plutonium and trigger a massive release of neutron radiation. This event created a flash of blue light caused by the ionization of air particles.Slotin immediately flipped the top shell off the core with his hand, stopping the reaction and likely saving the lives of seven other scientists in the room. However, he absorbed a lethal dose of approximately 1,000 rads of neutron and gamma radiation. He died nine days later at the age of 35 from acute radiation syndrome. Following these tragedies, Los Alamos National Laboratory ended all hands-on criticality testing and the core was eventually melted down in 1946.
Verified Fact
FP-0008450 · Feb 20, 2026