The Demon Core: Tickling the Dragon’s Radioactive Tail

In the autumn shadow of World War II, amid the desert labs of Los Alamos, a dense metallic sphere sat poised at the threshold between science and catastrophe. Nicknamed the “Demon Core,” this 6.2-kilogram plutonium-gallium alloy sphere was meant to be part of a third atomic bomb, following the devastation of Hiroshima and Nagasaki. But the war ended before it could be used in combat. Instead, it became an object of study, one that would twice betray its handlers in fatal flashes of blue light.

Designed nearly identical to the core of the Fat Man bomb, the Demon Core was repurposed for experiments measuring criticality, the point at which a nuclear chain reaction becomes self-sustaining. Scientists at Los Alamos wanted to understand how close they could bring the core to critical mass by surrounding it with neutron-reflective materials like tungsten carbide and beryllium. What followed were two of the most tragic and revealing incidents in early nuclear research.

The first occurred on August 21, 1945. Physicist Harry Daghlian was alone in a lab, stacking tungsten carbide bricks around the core to increase neutron reflection. As he reached for one final brick, it slipped from his grasp and landed atop the core, sending the system momentarily supercritical. A burst of radiation surged outward. Though Daghlian immediately disassembled the setup, he received a lethal dose of radiation and died weeks later, becoming the first known victim of a criticality accident.

Less than a year later, on May 21, 1946, a second tragedy struck. Physicist Louis Slotin was demonstrating a similar test, using two beryllium hemispheres to reflect neutrons back into the core. Instead of a mechanical safety spacer, he used a flathead screwdriver to keep the spheres apart, a dangerously nonchalant approach. During the demonstration, the screwdriver slipped. The hemispheres clamped shut, and the core once again went supercritical. A visible blue glow filled the room as Slotin swiftly pried the spheres apart, but the damage was done. He absorbed a massive radiation dose and died nine days later. Several others in the room were also exposed to dangerous levels of radiation.

These fatal experiments illustrated the brutal edge of nuclear physics. The Demon Core's sudden transitions to supercriticality were a direct consequence of neutron reflection: the more neutrons bounced back into the core, the more likely it was to reach a self-sustaining chain reaction. Such conditions are at the heart of nuclear bombs and reactors alike, but the line between “almost critical” and “too late” is razor-thin.

Slotin’s death marked a turning point in nuclear research ethics and safety. No longer would hands-on experiments near critical masses be permitted. From then on, remote-controlled devices, robotic arms, and strict safety barriers became standard. The idea of “tickling the dragon’s tail,” as these experiments were grimly nicknamed, was retired. Safety protocols matured, including redundant monitoring systems, mechanical fail-safes, and extensive training. The era of cowboy physicists daring fate gave way to systematic risk management and international safeguards.

Today, the legacy of the Demon Core is both a scientific cautionary tale and a historical inflection point. It reminds us how far nuclear science has come, not only in technological sophistication but in moral responsibility. Where secrecy and wartime urgency once prevailed, modern nuclear research operates under global ethical frameworks like those of the IAEA. We study radioactive materials with a respect born from hard-earned lessons written in the lives of Daghlian and Slotin.

The Demon Core was ultimately melted down and repurposed. It never exploded, but in two terrible flashes, it exposed the lethal costs of pushing boundaries without adequate safeguards. In doing so, it helped reshape the culture of scientific inquiry, moving it away from bold risk and closer toward safe research.

Sources

Dunhill, J. (2021, May 23). The demon core: how one man intervened with his bare hands during a nuclear accident. IFLScience. https://www.iflscience.com/the-demon-core-accident-how-one-man-stopped-a-nuclear-detonation-with-his-bare-hands-59736  

Kyle Hill. (2020, November 27). Demon Core - the True story [Video]. YouTube. https://www.youtube.com/watch?v=aFlromB6SnU 

Museum of Radiation and Radioactivity. (2025, April 15). Museum of Radiation and Radioactivity. https://orau.org/health-physics-museum/index.html 

 NUKEFACTS . . . and fictions. (n.d.). https://web.archive.org/web/20160304073037/http://www.pipeline.com/~rstater/nuke1tttt.html 

Wellerstein, A. (2016, May 23). The Third Core’s Revenge. Restricted Data: A Nuclear History Blog. https://blog.nuclearsecrecy.com/2013/08/16/the-third-cores-revenge/ 

Wellerstein, A. (2016, May 21). Demon Core: The Strange Death of Louis Slotin - The New Yorker. The New Yorker. https://www.newyorker.com/tech/annals-of-technology/demon-core-the-strange-death-of-louis-slotin?intcid=mod-latest 

 Science as theater » American Scientist. (n.d.). https://web.archive.org/web/20170520181417/http://www.americanscientist.org/issues/feature/2002/6/science-as-theater/99999