In 1946, a plutonium core designated P-239—originally intended for a nuclear bomb—was quietly melted down and reabsorbed into the U.S. nuclear stockpile. This 14-pound sphere had, just months earlier, claimed the lives of two scientists. This is the true story of the Demon Core.
The atomic cores that destroyed Hiroshima and Nagasaki were small and portable but emitted a disconcerting warmth due to their radiation. Although protected by nickel coatings and containers, they only became deadly when forced into a critical state using explosives to compress the radioactive material and start a chain reaction.
The Demon Core was the third plutonium core built during World War II, intended for a third bomb over Japan. After Japan’s surrender, it was kept at Los Alamos National Laboratory for research on nuclear criticality.
Tickling the Dragon’s Tail
Physicist Otto Robert Frisch was assigned to study neutron multiplication in uranium and plutonium. He led the Critical Assembly Group, conducting dangerous experiments to determine how close a core could get to going critical using neutron reflectors. This method—called neutron reflection—could cause a fatal radiation burst without an actual explosion. Richard Feynman famously described these tests as “tickling the tail of a sleeping dragon.”
Despite the risk, the experiments continued—and within two years, two physicists would die under similar circumstances.
The First Accident – Harry K. Daghlian Jr.
On August 21, 1945, 24-year-old Harry Daghlian was conducting a neutron reflection test with tungsten carbide bricks. While working alone after hours, he accidentally dropped a brick onto the core, driving it into a supercritical state. A blue flash and intense heat followed. He quickly knocked the brick off, stopping the reaction—but had already received a lethal dose of radiation.
Daghlian suffered for 25 days before dying on September 15. A guard nearby, Robert Hemmerly, was also exposed and died decades later from radiation-induced leukemia.
The Second Accident – Louis Slotin
Louis Slotin took over the criticality experiments after Daghlian’s death. On May 21, 1946—nine months later—he conducted a similar test using two beryllium hemispheres to enclose the core. Holding the top half with a screwdriver, Slotin accidentally let the spheres close completely. The core went critical, emitting a burst of radiation.
Slotin knocked the hemisphere off and, despite being fatally exposed, calmly directed others to mark their positions for dose calculations. He died nine days later—treated by the same nurse, in the same hospital, on the same day of the week as Daghlian. He was 35.
Slotin had received over 1,000 rads—more than twice the radiation experienced one kilometer from Hiroshima’s ground zero.
The Aftermath and Legacy
Following the incidents, the Demon Core was retired. A different core was used in the 1946 nuclear test at Bikini Atoll. The Demon Core was later melted down, its material repurposed for other nuclear weapons.
Since 1945, there have been over 60 criticality accidents worldwide, with at least 21 fatalities. The blue glow seen during such events is caused by charged particles moving through the air, releasing energy. While air disperses it harmlessly, human bodies do not—leading to molecular damage, radiation sickness, and often death.
Daghlian and Slotin were among the first victims of the atomic age—not casualties of war, but of scientific pursuit. Their tragic deaths remain a sobering reminder of the dangers of nuclear research and the fine line between discovery and disaster.
