What Was the Demon Core and Why Was It So Deadly?

The Demon Core was a 6.2-kilogram plutonium sphere that caused two fatal radiation accidents at Los Alamos Laboratory in 1945 and 1946. Originally intended as the fissile core for a third atomic bomb against Japan, it became infamous for killing two brilliant physicists in separate criticality experiments.

The Third Bomb That Never Was

The Demon Core began life with a singular purpose: to serve as the heart of America’s third atomic weapon in World War II. This softball-sized sphere of plutonium was manufactured as part of the Manhattan Project, designed to deliver devastating force against Japan. However, Japan’s surrender in August 1945 left the core without its intended target, relegating it to laboratory experiments at Los Alamos.

The plutonium sphere represented cutting-edge nuclear science, but also unprecedented danger. When brought to criticality—the point where a nuclear chain reaction becomes self-sustaining—the core would emit intense radiation and the characteristic blue flash of Cherenkov radiation.

Harry Daghlian’s Fatal Mistake

On August 21, 1945, physicist Harry Daghlian was conducting a late-night experiment alone in the Los Alamos laboratory. He was carefully stacking tungsten carbide bricks around the plutonium core to bring it close to criticality. As he positioned the final brick, it slipped from his grasp and fell onto the core.

The core went supercritical instantly. A brilliant blue flash filled the room as the plutonium released a massive burst of radiation. Daghlian quickly knocked the brick away, but the damage was done. He had absorbed a lethal dose of radiation—approximately 510 rems. Despite medical treatment, acute radiation syndrome claimed his life 25 days later.

Louis Slotin’s Heroic Sacrifice

Nine months later, on May 21, 1946, physicist Louis Slotin was demonstrating the same type of criticality experiment to seven colleagues. Using only a screwdriver to maintain separation between two beryllium hemispheres surrounding the plutonium core, Slotin was showing how close the assembly could come to criticality.

The screwdriver slipped. The beryllium shells snapped shut around the core, instantly achieving supercriticality. Another blue flash erupted, flooding the room with deadly radiation. Slotin’s immediate response saved seven lives—he threw himself between his colleagues and the core while manually separating the hemispheres.

Slotin absorbed approximately 1,000 rads of radiation in mere seconds, while his quick thinking limited his colleagues’ exposure to survivable levels. All seven witnesses lived, but Slotin died nine days later from acute radiation poisoning.

The End of Hands-On Nuclear Experiments

The second tragedy finally convinced Los Alamos officials to ban all hands-on criticality experiments permanently. The Demon Core had claimed two of America’s brightest nuclear physicists, demonstrating that even experts could fall victim to the unforgiving nature of fissile materials.

In 1946, the infamous plutonium sphere was melted down and recycled into other nuclear weapons. Though it never detonated as originally intended, the Demon Core’s legacy profoundly shaped nuclear safety protocols worldwide, establishing remote-controlled procedures that remain standard practice today.