On the afternoon of 21 May 1946, in a low wooden building at Los Alamos, a Canadian physicist named Louis Slotin was holding two half-spheres of beryllium apart with the tip of a flathead screwdriver. Between them sat a 6.2-kilogram plutonium sphere the size of an orange. The screwdriver slipped, the hemispheres closed, the room flooded with a blue glow and a wave of heat, and Slotin — knowing instantly what had happened — yanked the top shell off with his bare hand. He had absorbed a dose that would kill him in nine days. His colleagues in the room lived because his body had been between them and the core.
The plutonium at the center of the accident already had a nickname among the scientists who handled it: the demon core. Nine months earlier, it had killed another physicist, Harry Daghlian, in almost exactly the same way.
The experiment they called “tickling the dragon’s tail”
The setup was crude by the standards of any modern reactor lab. A 6.2-kilogram sphere of plutonium sat on a base. Around it, Slotin was lowering a hollow beryllium half-shell. Beryllium reflects neutrons back into a fissile core, meaning that as the two hemispheres closed, more and more of the neutrons the plutonium was spitting out got bounced back in, triggering fresh fissions, which spat out more neutrons, which bounced back again.
Bring the shells close enough and the chain reaction becomes self-sustaining. Bring them a hair closer and it runs away. The gap between “critical” and “prompt critical” — the point where the reaction accelerates faster than any human can react — was measured in millimeters.
Enrico Fermi, under whom Slotin had worked since the Chicago days, had already told him what he thought of the procedure. As the American Physical Society recounts, a dismayed Fermi warned Slotin that he would be dead within a year if he kept flouting the safety protocols. The whole class of experiment was nicknamed “tickling the dragon’s tail.”
The screwdriver
The safety protocol on paper called for shims — small metal spacers that physically prevented the two beryllium hemispheres from ever touching. Slotin had been running the experiment without them, using the blade of a standard flathead screwdriver wedged between the shells as his only barrier.
He had done it this way many times. On 21 May, he was giving a tour to Alvin Graves, the scientist who was about to take over his role. Graves mentioned he’d never seen the critical assembly demonstrated. Slotin agreed to run through it. Raemer Schreiber, working at a desk across the room, glanced up and told him to go slowly.
What happened next took, by Schreiber’s own account, a few tenths of a second. The screwdriver slipped. The top beryllium shell dropped. The plutonium went prompt critical. According to his official report, Schreiber described turning in response to a noise or movement. Schreiber’s official report described witnessing a blue flash accompanied by a heat wave.
Slotin flipped the top shell to the floor with his hand. The reaction stopped. The room went silent.
What the blue flash actually was
The color came from ionized air. When the core went supercritical, the burst of gamma rays and neutrons stripped electrons from the nitrogen and oxygen molecules around the assembly. As those electrons recombined, they released photons in the blue and ultraviolet part of the spectrum — the same physical effect that gives Cherenkov radiation in reactor pools its ghostly cobalt glow, though the mechanism here was air ionization rather than particles moving faster than light in water.
The heat wave Schreiber felt was real. Slotin, standing closest, was later estimated to have absorbed a lethal dose in less than a second. The lethal whole-body dose for a human is generally considered to be around four to five sieverts. He had received more than that, delivered in a flash.
The nine days
Slotin walked out of the building under his own power. Outside in the corridor, he drew a diagram on a piece of paper showing exactly where each of the eight men in the room had been standing at the moment the shells closed. That diagram, along with the geometry of the assembly, later allowed physicians to estimate individual doses with unusual precision — a grim data set that would inform radiation medicine for decades.
His left hand, the one that had gripped the shell, swelled and blistered. Within days it turned black. His white blood cell count crashed. His internal organs began to fail in sequence, a pattern later doctors would recognize as acute radiation syndrome running through its predictable phases: the prodromal nausea, the deceptive latent period of apparent recovery, the collapse.
He died on 30 May 1946. Accounts of the demon core’s second victim describe a death from radiation nine days after the flash, at the age of 35.
The core had killed before
This same 6.2-kilogram sphere of plutonium had been machined originally for a third atomic bomb, intended for use against Japan if the war had continued past August 1945. When Japan surrendered, the core was kept at Los Alamos for experiments.
On 21 August 1945, physicist Harry Daghlian had been working alone with it, stacking tungsten carbide bricks around the sphere to study how a neutron reflector affected reactivity. He dropped a brick onto the core. It went briefly supercritical. Daghlian pulled the brick off with his hand and died 25 days later.
Nine months after that, the same core killed Slotin. The plutonium sphere had originally been prepared as the pit for a third bomb intended for Japan, which is why the physicists at Los Alamos had it in the first place. After Slotin’s death, staff at the lab began calling it the demon core in earnest. Its plutonium was eventually melted down, in the summer of 1946, and folded back into the U.S. weapons stockpile to be recast into other cores.
The body as shield
The most striking detail in every account of the accident is what Slotin did in the fractions of a second between the flash and the silence. He did not run. He reached forward, into the source of the radiation, and physically separated the two shells with his hand.
Slotin absorbed the highest dose in the room. Graves, standing directly behind him, received enough radiation to make him gravely ill but not to kill him. The five other men in the room, further away and partly shielded by Slotin’s body and the equipment cabinet, survived with lower doses.
Contemporary accounts of the screwdriver setup note that the entire safety of the experiment rested on Slotin’s grip on a single hand tool. There were no interlocks, no remote actuators, no automatic scram. There was a man, a screwdriver, and a mass of fissile metal.
What Los Alamos changed afterward
Schreiber, who had been furthest from the core and received the lowest dose of the group, was tasked with rewriting the criticality protocols. The reforms he helped introduce were straightforward, and read now like a list of things that should have been obvious in 1945.
Hands-on criticality experiments were banned. All future work of that kind would be done remotely, with the assembly on one side of a heavy shield wall and the operators on the other, watching through mirrors or closed-circuit television. Physical shims, not screwdrivers, would hold reflectors apart. Neutron detectors and audible alarms would monitor reactivity in real time. Any experiment approaching criticality would be run with the operators at least a quarter of a mile away.
The lab also began quietly consolidating what would become the modern discipline of criticality safety — the field that governs how fissile material is stored, transported, and handled everywhere from fuel fabrication plants to spent fuel pools. More than any single reform, Slotin’s death forced a shift in the institutional culture at Los Alamos: hands-on brinkmanship with fissile metal, tolerated as the price of wartime speed, was no longer acceptable once the war was over.
The physics that made it possible
To understand why a screwdriver mattered, it helps to think about the plutonium sphere as sitting just below the threshold of a self-sustaining nuclear chain reaction. On its own, in open air, it wasn’t critical. It gave off some spontaneous neutrons, warmed slightly to the touch — Daghlian and Slotin both noted that the core felt warm to the touch — and did nothing dramatic.
Wrap it in a neutron reflector, though, and the geometry changes. Neutrons that would have escaped into the air are bounced back into the plutonium, where they can strike another nucleus and induce another fission. Each fission releases two or three more neutrons. If enough of those neutrons find another nucleus before escaping, the reaction sustains. If more than enough do, it accelerates.
Beryllium is one of the best neutron reflectors known. Closing a beryllium shell around a subcritical plutonium core is the fastest way to make it critical without adding any mass. And because prompt neutrons are released in microseconds, a supercritical reaction ramps up faster than any mechanical response can stop it. The blue flash was over before Slotin’s arm finished moving.
Nuclear chain reactions of this kind are not unique to laboratories. As Nuclear Power Daily has covered in the piece on the Oklo natural reactor in Gabon, uranium deposits in West Africa spontaneously went critical roughly 1.7 billion years ago and cycled for hundreds of thousands of years. What made Slotin’s accident different was that the geometry, the reflector, and the timing were all being controlled — barely — by a human hand.
What happened to the other men in the room
Alvin Graves, the scientist standing behind Slotin, spent months in the hospital. His hair fell out. His white blood cell count dropped to levels his doctors thought incompatible with survival. His sperm count didn’t recover for years. His vision in his left eye was permanently damaged.
He returned to Los Alamos and eventually became its chief of testing during the Cold War. Under his direction, the United States conducted many of the largest thermonuclear tests in history, including the Castle Bravo shot at Bikini Atoll in 1954, which vaporized part of the reef, contaminated the Marshallese island of Rongelap with fallout, and left a legacy of cancers, birth defects, and displaced communities that persists eight decades later.
Schreiber lived until 1998. He rarely spoke about the accident publicly. The others in the room — Marion Cieslicki, Dwight Young, Theodore Perlman, S. Allan Kline, and the security guard Patrick Cleary — mostly returned to civilian life. Cleary was killed in the Korean War four years later. Kline, who was standing near the door, was one of the few who spoke on the record about the experience decades afterward, and disputed some details of the official narrative.
The core itself
The demon core never became a bomb. It had been slated for the third detonation of Operation Crossroads, the United States’ 1946 nuclear tests at Bikini Atoll, but Slotin’s accident had driven up its radioactivity, the core needed time to cool, and the planned third shot was canceled after the underwater Baker test contaminated the target fleet. In the summer of 1946 the plutonium was melted down and recast into the weapons stockpile. For the second time, the core was denied its detonation. The atoms that killed Slotin were never vaporized over a Pacific lagoon; they were quietly folded into other cores back in New Mexico.
The screwdriver, according to Los Alamos archival records, was preserved. So was the room, Building 1 of the Pajarito laboratory site, though it was later demolished. What survived was the protocol change: the quarter-mile rule, the remote handling, the physical shims. Every criticality experiment conducted in the world since 1946 has been done, in some sense, in the shadow of a slipped screwdriver and a nine-day death.
Slotin was 35. His hometown of Winnipeg named a park after him. Thousands attended his funeral. The official memo noted that Graves was later reluctant to discuss what had happened. The core is gone. The blue flash lasted a fraction of a second. The rules it wrote are still in every fuel cycle facility on Earth.
