In August 1945, the United States dropped two atomic bombs on Japan, marking the only time in history that nuclear weapons have been used in warfare. The destruction caused by these bombs was overwhelming, and a third bomb was ready for deployment. Fortunately, Japan surrendered just days before it could be used, saving tens of thousands of lives.
While this bomb never caused casualties in combat, its plutonium core tragically led to the deaths of two American physicists, earning it the 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 bomb’s core underwent a series of crucial experiments. This plutonium sphere, designed specifically for bomb use, had a narrow safety margin before reaching supercriticality, a threshold that was rigorously tested.
In one key experiment, scientists aimed to determine the core’s criticality – the point at which the fissionable material would sustain a nuclear chain reaction. During these tests, the core was partially surrounded by neutron reflectors, which redirected neutrons back into the core, amplifying the reaction.
Had the core been fully encased in neutron reflectors, it would have rapidly achieved supercriticality, resulting in a massive release of radiation.
At that time, safety protocols were alarmingly lax, permitting scientists to conduct these experiments by hand.
First accident
In 1945, physicist Harry Daghlian was conducting this experiment when it took a tragic turn. While positioning neutron-reflecting tungsten carbide blocks around the core to bring it close to criticality, he accidentally dropped one of the blocks onto it. Although Daghlian quickly removed the block, the damage was already done. In that brief moment, the core became supercritical, emitting a deadly burst of radiation.
Daghlian suffered through three weeks of intense radiation sickness before succumbing to his injuries. Following his death, stricter safety protocols were implemented to prevent similar incidents.
The following year, Daghlian’s colleague, Louis Slotin, carried on with the experiments. Slotin was a brilliant physicist, though his approach to safety was known to be relaxed.
In Slotin’s version of the experiment, two half-sphere neutron reflectors were carefully brought closer around the core to increase its activity. Metal spacers were used to keep the half-spheres from fully covering the core, minimizing 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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