In August 1945, the United States dropped two atomic bombs on Japan, marking the only time in history that nuclear weapons were used in warfare. The destruction these bombs caused was vast, and a third bomb was ready to be deployed. Fortunately, Japan surrendered just days before it could be dropped, saving tens of thousands of lives.
Although this bomb never took lives in combat, its plutonium core tragically resulted in the deaths of two American physicists, leading to its ominous nickname, the “Demon Core.”
A third bomb
It’s easy to think that the Manhattan Project, the U.S. initiative to create atomic weapons, was only ever meant to produce two bombs. However, this wasn’t the case. The project expanded into a large-scale production operation for nuclear weapons. Most of the resources in this multi-billion-dollar endeavor went toward obtaining enriched uranium and plutonium, which were particularly challenging to produce at the time.
By the summer of 1945, the project had produced enough material for three bombs, with a fourth in the pipeline. This material was designated for the Trinity Test and the deployment of the Little Boy and Fat Man bombs. When Japan didn’t immediately surrender after the two bombs were dropped, the U.S. military began to prepare a third, which was scheduled for release on August 19.
However, Japan surrendered on August 16.
At the time, few involved in the Manhattan Project expected that only a small number of bombs would be used. Many believed more would be required to force Japan to surrender, and some worried that even with a surrender, the war might reignite. In the end, the third device was never used, leaving the U.S. with its 6.2 kg, nine-cm wide plutonium core. This core was later repurposed for testing and incorporated into other projects.
Demon Core
Between 1945 and 1946, the core of the bomb underwent a series of experiments. This plutonium sphere, specifically designed for bomb use, had a narrow safety margin before it reached supercriticality, a threshold that was extensively tested.
In one important experiment, scientists sought to determine the core’s criticality – the point at which the fissionable material could sustain a nuclear chain reaction. During these tests, they partially surrounded the core with neutron reflectors, which redirected neutrons back into the core, intensifying the reaction.
If the core had been fully encased in neutron reflectors, it would have quickly reached supercriticality, unleashing a massive burst of radiation.
At the time, safety protocols were disturbingly relaxed, allowing scientists to perform these experiments by hand.
First accident
In 1945, physicist Harry Daghlian was performing this experiment when it took a tragic turn. He was positioning neutron-reflecting tungsten carbide blocks around the core to bring it near criticality when he accidentally dropped one of the blocks onto it. Though Daghlian quickly removed the block, it was too late. In that split second, the core became supercritical, releasing a lethal burst of radiation.
Daghlian endured three weeks of severe radiation sickness before ultimately passing away. After his death, stricter safety protocols were introduced to prevent similar tragedies.
The following year, Daghlian’s colleague, Louis Slotin, continued the experiments. Slotin was a brilliant physicist, known for his somewhat casual approach to safety.
In Slotin’s version of the experiment, two half-sphere neutron reflectors were gradually brought closer around the core to increase its activity. Metal spacers were used between the half-spheres to prevent them from fully enclosing the core and to reduce the risk of another accident.
Second incident
Quite the risk-taker, Slotin ignored the protocol and did away with the spacers, using his own method instead. His method was faster but was also much more dangerous. Slotin would use a simple flathead screwdriver to maintain the gap between the reflectors, adjusting it by hand as necessary. He became quite proficient at this technique and became known among his colleagues for “tickling the dragon’s tail,” as it was called at the time.
Slotin’s colleagues were aware that this technique was extremely risky, and even tried to warn him, but he continued anyway.
On May 21, 1946, Slotin was performing the experiment in front of a small group of people in a Los Alamos laboratory. Using his usual technique, he lowered the two neutron reflecting half-spheres around the core, using the screwdriver to keep them from fully closing.
However, on this occasion, the screwdriver slipped by a tiny amount, allowing the two neutron reflectors to completely enclose the core. The core immediately entered super criticality, emitting a bright blue flash of light and a powerful blast of radiation.
End of the ‘Demon Core’
Slotin quickly removed the neutron reflectors, but like Daghlian, the damage was already done. He had been showered by an extremely high dose of radiation. As he was leaning over the core at the moment the accident happened, he absorbed much of the radiation, likely saving the lives of the others in the room.
Within minutes of the accident, Slotin was already showing signs of radiation poisoning. He died just 9 days later.
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After the plutonium sphere claimed two lives, it became known as the “demon core.” It was meant to be used Operation Crossroads nuclear tests, but this never happened and it was eventually melted down and recycled into other cores.
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