The Plutonium Sphere That Killed Its Makers

The Demon Core is one of the most chilling stories of the atomic age: a single sphere of plutonium, built to destroy a city, that never got the chance, and so turned instead on the very scientists who made it, killing two of them in two eerily similar accidents nine months apart, before it was quietly melted down and erased from existence.

To understand it, you have to go back to the final days of the Second World War. In the summer of 1945, the top-secret Manhattan Project at Los Alamos, New Mexico, was building atomic bombs. The first was tested in the desert in July. The second and third were dropped on Hiroshima and Nagasaki in August. And a fourth device was being prepared, with a plutonium core at its heart, a sphere ready to be shipped to the Pacific and dropped on a third Japanese city if the war continued. This core was a ball of plutonium about 3.5 inches across, the size of a grapefruit, weighing around 6.2 kilograms, and it was subtly, quietly alive: warm to the touch from its own radioactivity, sitting always just below the threshold of a nuclear chain reaction. Before it earned its darker name, the scientists simply called it “Rufus.” But then, on 15th August 1945, Japan surrendered. The war was over. The bomb was never needed. And so the deadly core was kept behind at Los Alamos, where physicists used it for experiments, pushing it closer and closer to the edge of criticality to measure exactly how little it took to wake the monster inside. They had a grimly appropriate name for this work: “tickling the tail of the dragon.”

The Horror

The first death came just days after the war ended. On the night of 21st August 1945, a 24-year-old physicist named Harry Daghlian was working late, alone but for a security guard, stacking bricks of tungsten carbide around the core. The bricks acted as a “reflector,” bouncing the core’s escaping neutrons back into it and pushing it toward a runaway reaction. Daghlian’s instruments warned him that placing one last brick would tip the core over the edge, so he moved to pull his hand back. And the brick slipped from his fingers and fell directly onto the core. The plutonium surged into a supercritical state, and a blue glow filled the room. Daghlian, in a heartbeat, thrust his bare hand into the deadly radiation to knock the brick away and stop the excursion before it could grow worse, dooming himself in the process. He had absorbed a lethal dose. Over the next 25 days, Harry Daghlian died slowly and horribly: his hand blistered and burned as if by an invisible fire, then came the nausea, then his body failed. He fell into a coma and died on 15th September 1945, aged just twenty-four.

You would think the lab would have learned. And they did tighten some rules. But nine months later, the Demon Core claimed its second victim, and the parallels are almost unbelievable. On 21st May 1946, the 21st of the month again, a charismatic Canadian physicist named Louis Slotin was demonstrating a criticality experiment to a room full of colleagues. His method was notoriously dangerous: he lowered a dome of neutron-reflecting beryllium down over the core, and kept it from closing completely by wedging the blade of an ordinary flathead screwdriver into the gap, a tiny opening that let just enough neutrons escape to keep the core from going critical. Enrico Fermi had reportedly warned Slotin he’d be “dead within a year” if he kept working this way. On that day, Slotin’s screwdriver slipped. The dome dropped shut over the core. And for a fraction of a second, the room was lit by a brilliant blue flash and a wave of heat. Slotin reacted instantly, tearing the dome off the core with his bare hand to stop the reaction, but it was already too late. Standing closest, his own body had absorbed the brunt of the burst, shielding the seven other people in the room. He is said to have calmly told his colleagues that he was sure he would die, and that the others, standing further back, would probably be alright. He was right on both counts. Louis Slotin died nine days later, on 30th May 1946, aged 35, after days of agony that doctors documented hour by hour as his body failed from the inside. The eerie coincidences chilled everyone: two men, both killed by the same sphere, both by a slipped hand, both on the 21st of the month, both dying slowly in the same small hospital. It was after Slotin’s death that the plutonium ball got the name it has carried ever since: the Demon Core.

The Modern Relic

After killing two of its makers, the Demon Core was scheduled to finally fulfil its purpose. It was to be shipped to Bikini Atoll in the Pacific and detonated in the 1946 nuclear weapons tests. But there was a problem, one that reads almost like the core was cursed: after its two violent accidents, it was reportedly judged too radioactive and unstable to use. And so, in the summer of 1946, the decision was made to destroy it. The Demon Core, the sphere built to end a city that had instead killed two brilliant young men, was melted down. Its plutonium was absorbed back into the American nuclear stockpile, recycled into the cores of other weapons, and it ceased, as a distinct object, to exist. The most infamous single piece of plutonium in history was simply erased.

The relic, then, is not the core itself; it’s gone, buried inside the bombs of the Cold War. The relic is the terrible record it left behind. The accidents transformed nuclear safety forever: after Slotin’s death, all hands-on criticality experiments were banned, and from then on such work was done by remote control, machines operating the material from a quarter of a mile away, so that no human hand would ever “tickle the dragon” again. Los Alamos National Laboratory still stands, and the story is studied to this day. And perhaps the most haunting relic of all is a photograph: a careful reconstruction of Louis Slotin’s fatal experimental setup, the beryllium dome, the core, the screwdriver holding the gap, recreated by the laboratory after his death to document exactly how a slip of one small tool killed a man. You can look at that image now: the innocent-looking metal hemisphere and the ordinary screwdriver, frozen forever at the instant before the blue flash. Two scientists died proving how thin the line was between control and catastrophe. And the thing that killed them was quietly melted away, poured into other weapons, and scattered: a death, and a demon, buried inside the bombs themselves.

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