In the chaotic twilight of World War II, as the world teetered between unimaginable destruction and uneasy peace, a small, deadly object was born in secret laboratories.
It wasn’t a bomb. It wasn’t a missile.
It was a single sphere of plutonium, about the size of a grapefruit, weighing just over 14 pounds.
And it would become known as the Demon Core.
This deceptively small object would claim the lives of brilliant scientists, trigger some of the darkest legends of the Manhattan Project, and stand as a chilling symbol of how even the most brilliant human minds can fall prey to catastrophic mistakes.
What Was the Demon Core?
The Demon Core was a subcritical mass of plutonium-239, originally destined to be used in a third atomic bomb after Hiroshima and Nagasaki.
Had Japan not surrendered on August 15, 1945, this core would have likely been loaded into a bomb casing and dropped on another Japanese city.
Instead, the sphere was retained at Los Alamos Laboratory for criticality experiments—studies on how close fissile material could be brought to a chain reaction without actually detonating.
These tests were known as “tickling the dragon’s tail.”
The dragon, of course, was nuclear fission—and if tickled the wrong way, it would awaken in an instant, with deadly results.
The First Fatality: Harry Daghlian (August 21, 1945)
Just days after Japan’s surrender, physicist Harry Daghlian was working late at night, alone, performing criticality experiments with the Demon Core.
His task was to slowly build a structure of neutron-reflecting bricks (made of tungsten carbide) around the core, measuring how close it could get to criticality.
But in a fateful mistake, Daghlian accidentally dropped a brick onto the assembly, triggering a prompt critical reaction.
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A flash of blue light lit up the room—the unmistakable signature of radiation discharge.
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A burst of deadly neutron radiation flooded the area.
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Daghlian, realizing what he had done, desperately disassembled the structure, but the damage was done.
He died 25 days later from acute radiation poisoning, in excruciating pain, becoming the first victim of the Demon Core.
The Second Fatality: Louis Slotin (May 21, 1946)
Nine months later, the Demon Core struck again.
Physicist Louis Slotin—already known for his cavalier attitude toward lab safety—was demonstrating a criticality experiment to a group of scientists.
The test involved using two half-spheres of beryllium to nearly encase the Demon Core, pushing it to the edge of criticality.
Slotin used nothing but a screwdriver to hold the upper hemisphere away, carefully adjusting the gap.
As he manipulated the spheres, the screwdriver slipped.
The top beryllium half dropped into place, fully enclosing the Demon Core, and triggering an instant criticality accident.
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A brilliant blue flash engulfed the room.
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Everyone present felt an instant wave of heat across their skin.
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Slotin instinctively lifted the sphere, ending the reaction—but it was too late.
He had absorbed a lethal dose of radiation. He died nine days later, suffering horrifying radiation burns and systemic organ failure.
Other observers in the room survived, though many suffered health issues later.
Slotin’s death was so gruesome that radiation exposure protocols at Los Alamos were completely overhauled afterward.
Why Was It Called the Demon Core?
After these two deaths—both involving a deceptively small, deceptively silent sphere—the scientists at Los Alamos began to whisper about the sphere as if it were cursed.
The nickname “Demon Core” stuck, a reminder that the forces they had unleashed were not fully under human control, no matter how brilliant or careful they believed themselves to be.
The accidents were seen not just as tragedies, but as warnings from the nuclear dragon they had awakened.
What Happened to the Demon Core?
The Demon Core was scheduled to be used in Operation Crossroads, a series of nuclear tests at Bikini Atoll in 1946.
But after Slotin’s death, the core was deemed too dangerous and unstable for further experimentation.
It was ultimately melted down and recycled into other nuclear weapons stockpiles.
The sphere that had nearly destroyed several scientists in seconds, and that had the potential to level a city, ceased to exist as a singular entity—its deadly power dispersed into other bombs.
The Legacy of the Demon Core
The story of the Demon Core remains one of the most chilling reminders of the lethal power of nuclear technology and the fragile line between brilliance and catastrophe.
It is a story about:
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Human fallibility in the face of incomprehensible forces
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The cost of curiosity
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The seductive danger of believing we can tame powers we barely understand
Today, the accidents are studied as cautionary tales in physics, engineering, and safety courses around the world.
Because the Demon Core wasn’t just a chunk of plutonium.
It was a mirror, reflecting back the hubris, fear, and awe of the atomic age.
And if you listen closely, some say, you can still hear the ticking—
the dragon’s tail, forever twitching, just beneath the surface.