S1E5: The Screwdriver That Killed

Picture a monster. Fangs, claws, fire, something you could at least see coming. The men in this story had a monster too, and they had a name for it. They called it the dragon. But this dragon was the size of a grapefruit. It sat perfectly still on a laboratory bench, cold and silent and ordinary to the eye. And it killed two men with something no eye can see, no ear can hear, and no hand can feel. Until it is far too late.

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This is the story of the Demon Core. One ball of plutonium, two dead men, and a laboratory that watched the same experiment kill once, changed nothing, and watched it kill again.

The core was a sphere of plutonium mixed with a little gallium, fourteen pounds of it, three and a half inches across. About the size of a grapefruit. It was the third atomic bomb core ever built, the one meant to be dropped on Japan after the first two. It was expected to be ready by the sixteenth of August, nineteen forty-five, for a drop on the nineteenth. Japan surrendered on the fifteenth. The war ended, and the core that had been built to end it stayed behind, sitting in a laboratory at Los Alamos, New Mexico.

Los Alamos was the weapons laboratory of the Manhattan Project, a secret town on a mesa, full of some of the smartest physicists on earth. And after the war, the men there kept working. The core never flew. It never fell. But it did not sit locked in a vault, either. It sat on lab benches while physicists ran experiments on it by hand. Because the question they were studying, the whole field of criticality research, was this. How close can you get to a chain reaction without starting one.

The core itself was built to live near that edge. It was designed to sit just below criticality, with only a thin margin of safety standing between it and a chain reaction. That was the whole point of the work. The physicists were mapping exactly how much reflector, how much mass, how much of everything it took to cross the line. To map the edge, you have to stand on it.

To understand what happened, you need to understand the experiment. A chain reaction runs on neutrons. Every fission releases neutrons, and if enough of them strike other atoms and cause more fissions, the reaction sustains itself. That is criticality. A reflector is a shell of material that bounces escaping neutrons back into the core, so the assembly needs less plutonium to reach the edge. The criticality experiments at Los Alamos were about finding that edge exactly. The physicists would build the reflector up, piece by piece, measuring all the way, stopping just short. Just short is a place you can only find by almost not finding it.

The physicists had a phrase for this work. Tickling the dragon's tail. That was their slang for edging a plutonium assembly closer and closer to going critical, feeling for the exact point where the chain reaction begins to feed on itself. Richard Feynman, the brilliant young physicist with a gift for naming things, is credited with the phrase. It made the work sound playful. It was not playful. The dragon's tail was a burst of radiation that could kill a man standing in the room.

The dragon had a keeper, of sorts. Louis Slotin, a Canadian physicist, was one of the laboratory's criticality experts, a man who performed these demonstrations so often that colleagues would gather to watch. He had a particular technique. He would lower one half of a beryllium shell over the core and hold the two halves apart with the blade of an ordinary flathead screwdriver, letting the gap close slowly, feeling the assembly approach criticality. The written procedure called for safety shims, solid blocks that physically could not slip. Slotin had taken the shims off. Enrico Fermi, who knew more about chain reactions than almost anyone alive, had reportedly warned him that if he kept doing it that way, he would be dead within a year.

But Slotin was not the first. Nine months earlier, on the twenty-first of August, nineteen forty-five, a Tuesday evening, another physicist was working with the same core. His name was Harry Daghlian, a young man of twenty-four, part of the critical assembly group. He had helped prepare this very plutonium for the Trinity test, the first atomic explosion in history. That night he was at the remote Omega Site, after dinner, working alone, building a neutron reflector around the core by hand.

The experiment was simple in shape. Stack tungsten carbide bricks around the plutonium, one brick at a time, each brick bouncing neutrons back into the core, and measure how close the assembly comes to criticality with every brick added. Each brick weighed almost ten pounds. A security guard, Private Robert Hemmerly, sat at a desk ten to twelve feet away. Daghlian was working alone with the core, which was against the safety rules. The rules existed because everyone knew what was at stake, and the rules were the kind that get followed until a quiet evening when nothing has gone wrong in a while.

That night the Omega Site was quiet. One physicist, one guard, one sphere of plutonium, and a stack of heavy bricks. Daghlian worked the way he always worked, carefully, one brick at a time, the neutron counters climbing with every addition. The instruments did exactly what they were built to do. They warned him. The final brick would make the assembly supercritical. He knew it. And then his fingers slipped.

The neutron counters, the instruments that warned when the assembly was getting too hot, told him that adding the final brick would push the whole thing over the edge into supercriticality. He was withdrawing his hand when the brick slipped from his fingers. It fell straight onto the center of the assembly. In that instant, the core went prompt critical. A burst of neutron radiation, invisible and silent, poured out of the pile. Daghlian tried to knock the brick off and could not. He had to pull the whole stack apart with his hands to stop the reaction. By then he had absorbed five hundred ten rem of neutron radiation. That is a lethal dose, several times over.

What followed was slow. His mother and sister were flown out to Los Alamos to be with him. For twenty-five days he lived with acute radiation poisoning, and then he fell into a coma, and he died on the fifteenth of September, nineteen forty-five. He was the first person in history known to have been killed by a criticality accident.

Louis Slotin had spent many hours at Daghlian's bedside during those twenty-five days. He had watched, up close, day after day, what a lethal dose of neutrons does to a human body. There is no version of this story in which he did not understand the stakes. He understood them better than almost anyone. And then the laboratory went back to work. The hand-stacked criticality experiments continued. The same core sat on the same benches. Nothing about the way the work was done had changed.

The demonstrations were routine by then. Slotin performed them for visiting scientists the way a professor might demonstrate a chemistry experiment, except the apparatus was a plutonium core and the margin for error was measured in fractions of an inch. Colleagues would gather to watch. It was impressive. It was also, in hindsight, astonishing. The most dangerous object in the building was being handled the way a watchmaker handles a watch. Except a watchmaker's slip costs a gear. This slip cost lives.

Nine months later, on the twenty-first of May, nineteen forty-six, Slotin was demonstrating his screwdriver experiment. It was a Tuesday. The twenty-first, again. The occasion was a demonstration for Alvin Graves, the scientist who was about to take over Slotin's work. Graves had said he had never actually seen the assembly demonstrated, so Slotin was showing him how it was done. Seven other colleagues stood in the room. Eight men in all. One of them, Raemer Schreiber, watching from across the room, told Slotin to take it slow.

Slotin lowered the top half of the beryllium shell over the core, holding it away from the bottom half with nothing but the blade of his screwdriver. The safety shims, the blocks that could not slip, were off to the side. At twenty minutes past three in the afternoon, the screwdriver slipped. It moved a fraction of an inch outward, and the upper shell dropped into place around the core.

The core went prompt critical in a fraction of a second. The men in the room saw a blue flash. That was the air itself glowing, ionized by the burst of radiation. They felt a wave of heat. Slotin tasted something sour. His left hand, the one holding the screwdriver, felt an intense burning. And then, faster than thought, he twisted his wrist and flipped the top shell off and onto the floor, breaking the geometry, stopping the reaction. The whole excursion had lasted a fraction of a second.

He knew at once what had happened. He reportedly said, well, that does it. And then, while he still could, he began sketching the positions of everyone in the room, drawing a careful diagram so the medical teams could calculate how much radiation each man had received. He put his colleagues' survival ahead of his own, in the minutes after he had already taken a lethal dose.

The dose he had absorbed was massive and unsurvivable. Different accounts put the number in the thousands of rem, and the exact figure is genuinely disputed, so this story will not pretend to settle it. What is not disputed is that it was far beyond what a human body can survive. The other seven men received doses estimated from around thirty-five to around three hundred sixty rem. Graves, standing partly behind Slotin, was partly shielded by Slotin's own body. He survived, but was hospitalized for weeks with severe radiation poisoning. He died in nineteen seventy-one, at fifty-five, of heart failure. Whether the radiation contributed is something the records leave uncertain. His father had also died of heart failure.

The other six men lived. Their doses were serious, and some of them carried health effects for the rest of their lives. They lived because one man, in the fraction of a second after the flash, had the presence of mind to break the assembly apart, and because his body, standing between the core and at least one of them, absorbed radiation that would otherwise have been theirs.

The nine days that followed were the same slow road Daghlian had walked. Acute radiation poisoning does its work on a schedule no doctor can change. Slotin's parents came to Los Alamos. He had known from the first minute that he was dying, and he spent what strength he had on the diagram, on the measurements, on making sure the seven men in that room would get the best treatment the doctors could give them.

Slotin died nine days later, on the thirtieth of May, nineteen forty-six, with his parents at his side. He had spent his last conscious hours helping the doctors map the radiation that was killing him, so they could save the seven men who had been standing in that room.

And here is the detail that stays with you. Both accidents happened on the twenty-first of the month. Both happened on a Tuesday. Nine months apart. The same core. The same kind of experiment, done by hand, in a room with men who had buried the first victim. The laboratory had watched it kill once. It had changed nothing. It watched it kill again.

After Slotin's death, the core was melted down, in the summer of nineteen forty-six, and its plutonium was recycled into other weapon cores. It was never assembled again. The screwdriver experiments stopped. The era of tickling the dragon's tail by hand was over. It had taken two dead men to end it.

The core got its nickname after the accidents. The Demon Core. It is worth saying plainly that the demon was never the metal. Plutonium does not scheme or hunger or hate. It does exactly what physics says it will do, every time, without exception. The demon, if there was one, was the arrangement around it. The bench. The bricks. The screwdriver. The culture that called a lethal experiment a game and kept playing after the first death.

It is worth being clear about where the fault lies here. Harry Daghlian was twenty-four, skilled, careful enough to be trusted with the work, and he was working alone at night in violation of the rules. Louis Slotin was a criticality expert who had been warned, by name, by Enrico Fermi, that the screwdriver would kill him. The men were not fools. The institution was. After Daghlian died, no one took the core off the bench. No one made the shims mandatory. No one stopped the hand-stacked experiments. The laboratory let a flathead screwdriver be the only safety device between eight men and a prompt-critical core, and it let that arrangement continue after it had already killed a man.

The men did their jobs. Daghlian stacked his bricks. Slotin gave his demonstration. The machine did what its design allowed, and the institution did what institutions do. It protected the routine. It is easy, from eighty years away, to call them reckless. It is harder, and more honest, to see the shape of it. A workplace where the dangerous thing is done by hand, every day, by experts, becomes a workplace where the dangerous thing feels ordinary. Ordinary is the most dangerous word in this story.

The Manhattan Project had ended, but the laboratory's habits had not. This was a place that had built the atomic bomb in under three years, a place where brilliance was the currency and caution could feel like timidity. The smartest people on earth had taught themselves to treat the most dangerous physics known to science as a bench-top demonstration. Tickling the dragon's tail. The phrase itself tells you everything about the culture. It made a lethal experiment sound like a game, and games do not get safety shims.

That is the horror of this one. Not a monster with fangs. Not a dragon you could see coming. A grapefruit-sized ball of metal on a lab bench, a system that treated a chain reaction like a parlor trick, and two men who paid for the trick with their lives. The dragon never moved. It never had to. The men kept walking up to it.

This is Nuclear Shadows. I am Morgan Miller. Thank you for listening, and I will see you next time.

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