Companion piece · 2026-10-19
On May 21, 1946, a screwdriver slipped from its position between two pieces of metal at a laboratory in Los Alamos, New Mexico.
For a fraction of a second, the room was illuminated by a blue flash.
Eight people were present. The scientist closest to the apparatus immediately separated its components, ending the nuclear chain reaction that had begun.
He had already received a fatal radiation dose.
Nine days later, physicist Louis Slotin died at the age of 35.
The plutonium core involved had caused another scientist's death nine months earlier.
It would eventually become known as the Demon Core.
A Third Nuclear Weapon
The core was manufactured in 1945 as part of the Manhattan Project, the American programme responsible for developing the first nuclear weapons.
By August, the United States had detonated the Trinity device in New Mexico and dropped atomic bombs on Hiroshima and Nagasaki.
Another plutonium core was being prepared for a possible third attack against Japan.
Japan's surrender on August 15 eliminated that immediate requirement.
The unused core remained at Los Alamos, where researchers continued investigating the properties of nuclear materials.
It consisted of approximately 6.2 kilograms of plutonium alloy, formed into a sphere roughly 8.9 centimetres in diameter.
The core was capable of sustaining a nuclear chain reaction under appropriate conditions.
By itself, however, it remained below criticality.
Scientists could increase its reactivity by surrounding it with materials that reflected escaping neutrons back into the plutonium.
These experiments helped establish the minimum conditions necessary for a self-sustaining chain reaction.
The work was informally described as tickling the dragon's tail.
The terminology was not part of the formal safety documentation.
August 21, 1945: The First Accident
Harry Daghlian was a 24-year-old physicist assigned to the laboratory's Criticality Assembly Group.
On the evening of August 21, he was conducting an experiment at the remote Omega Site.
His equipment consisted of the plutonium core and tungsten-carbide bricks, each weighing approximately 4.4 kilograms.
Daghlian was positioning the bricks around the core to study neutron reflection.
He was working without another scientist present, although a security guard, Robert Hemmerly, was stationed nearby.
As Daghlian moved another brick toward the assembly, neutron-counting equipment indicated that the configuration was approaching criticality.
He attempted to withdraw the brick.
Instead, it slipped from his hand and fell onto the assembly.
The additional neutron reflection initiated a sudden nuclear excursion.
Daghlian attempted to dislodge the brick and subsequently removed surrounding material, ending the reaction.
He received a substantial dose of neutron and gamma radiation.
Despite medical treatment, he developed acute radiation syndrome.
He died on September 15, 1945, twenty-five days after the accident.
The security guard also received a radiation dose but survived.
Following Daghlian's death, Los Alamos revised its procedures, including requiring another person to be present during criticality experiments.
The same plutonium core remained available for research.
Louis Slotin and the Second Experiment
Louis Slotin was a Canadian physicist from Winnipeg who had joined the Manhattan Project in 1942.
He had worked with Enrico Fermi's research group at the University of Chicago and later joined Robert Bacher's Weapon Physics Division at Los Alamos.
Slotin was particularly experienced in nuclear weapon assembly.
He had participated in assembling the device detonated during the Trinity test in July 1945.
By May 1946, he was preparing to leave Los Alamos. His colleague Alvin Graves was expected to assume some of his responsibilities.
On May 21, Slotin conducted a demonstration involving the same plutonium core that had irradiated Daghlian.
This experiment used two hemispherical beryllium reflectors rather than tungsten-carbide bricks.
The purpose was to measure how neutron multiplication changed as the reflectors approached a closed configuration.
The approved arrangement included spacers to prevent the two hemispheres from fully closing.
Slotin instead used the blade of an ordinary screwdriver to maintain the separation.
Seven other people were present, including Graves, physicist Raemer Schreiber, and security guard Patrick Cleary.
May 21, 1946: The Screwdriver Slips
During the afternoon demonstration, Slotin lowered the upper beryllium hemisphere toward the lower one.
The screwdriver slipped.
The upper reflector fell into place, briefly enclosing the plutonium core.
The assembly became supercritical.
Witnesses reported a blue flash and a sensation of heat.
Schreiber later estimated that the flash lasted no more than a few tenths of a second.
The light was associated with radiation interacting with the surrounding air.
Slotin immediately displaced the upper hemisphere, stopping the excursion.
There was no nuclear detonation.
The incident produced a short, intense burst of ionizing radiation rather than an explosive release comparable to an atomic weapon.
Slotin was positioned closest to the assembly and received the largest exposure.
He subsequently drew a diagram showing where the other people had been standing, assisting efforts to reconstruct their radiation doses.
Nine Days in Hospital
Slotin experienced nausea and vomiting shortly after the accident.
Over the following days, his condition deteriorated.
Medical records described extensive radiation damage, declining blood-cell counts, and progressive failure of normal physiological functions.
Contemporary estimates placed his whole-body radiation exposure far above ordinarily lethal levels.
The exact dose remains subject to reconstruction because reliable individual dosimetry was unavailable.
Slotin died on May 30, 1946.
The other seven people present survived the immediate accident.
Alvin Graves, who had been standing close to Slotin, also developed radiation sickness and required hospitalization.
Some survivors experienced later health problems, although establishing whether particular illnesses resulted from the exposure remains difficult.
Both Daghlian and Slotin died at the Los Alamos hospital. Contemporary correspondence records that, by coincidence, Slotin was placed in the same hospital room where Daghlian had died.
The End of Hands-On Criticality Experiments
Slotin's death prompted significant changes to criticality research at Los Alamos.
Experiments involving dangerous configurations of fissile material were suspended while laboratory procedures were reviewed.
Subsequent work increasingly relied on remotely operated equipment, greater physical separation, and engineered safety mechanisms.
These changes reduced reliance on an operator's reflexes or ability to maintain a delicate arrangement manually.
The accidents demonstrated that familiarity with hazardous equipment was not an adequate substitute for physical safeguards.
In both cases, the scientists had recognized the risks involved.
Neither accident required a previously unknown nuclear phenomenon.
The established properties of neutron reflection were sufficient to explain both events.
What Happened to the Core
Before Slotin's accident, the plutonium core had been considered for use in Operation Crossroads, the American nuclear weapons tests conducted at Bikini Atoll in 1946.
Some later accounts incorrectly stated that it was detonated during the first test, Able, on July 1.
Archival research by nuclear historian Alex Wellerstein established otherwise.
The core was retained following the Slotin accident. It was subsequently considered for a later Crossroads test, which was cancelled.
During the summer of 1946, the plutonium was melted down and recycled into new weapons material.
The original core therefore ceased to exist as an individual assembly.
Its association with two fatal accidents produced the name by which it is remembered.
The Demon Core had not malfunctioned.
The plutonium behaved according to established nuclear physics. The accidents occurred because experimental arrangements allowed those reactions to develop without sufficient protection for the people nearby.
The final safety improvement was to ensure that future experiments no longer depended on a screwdriver remaining in place.
This has been Nuclear Shadows. I'm Morgan Miller.
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