Demon Core: The 3.5-Inch Nuclear Orb That Killed 2 Physicists

Following the end of World War 2 and the devastating impacts of the Hiroshima and Nagasaki nuclear bombings, the Cold War was looming. The immense destruction and power promised by atomic bombs pushed world superpowers into a nuclear research frenzy, with the USA prepared to drop a third on Japan if necessary, and the remaining nations creating their own arsenal as a deterrent or defense.  

Enter the “demon core”. Sitting at a sizeable 6.2 kilograms (13.7 pounds) and 8.9 centimeters (3.5 inches) in diameter, this spherical mass of radioactive plutonium (at the time named “Rhufus”) was designed in nuclear research to be a fissile core for early iterations of the atomic bomb. Throughout 1945 and 1946, the demon core was experimented on and could have found itself in an atomic bomb intended for Japan had it not have been for their surrender. 

As expected from its’ ominous title, the demon core was not kind to the nuclear physicists involved. Designed as a bomb core, it had just a tiny margin before it would increase radioactivity and become supercritical (once the fission reaction has begun, it increases in rate). Therefore, any external factors that could increase reactivity, for example, compression of the core (which is how the fission bomb detonates), must be carefully monitored around the demon core.

Tickling the dragon’s tail 

Despite the danger, researchers used the core as an experimental piece on supercriticality, using neutron reflectors to push it to its’ limits. Neutron reflectors are used to surround the core, and as the nuclear fission reaction occurs, they reflect neutrons back at the nuclear material to increase the amount of fission taking place.

In 1945, alone in his laboratory, physicist Harry Daghlian was performing a neutron reflector experiment on the demon core when he mistakenly dropped a brick of reflective tungsten carbide onto the core, pushing it into supercriticality and releasing a deadly burst of neutron radiation. After a three-week battle with acute radiation sickness, Daghlian succumbed to his wounds, leading to tighter legislation around nuclear research in the Manhattan Project – although it would not be strict enough.

Unperturbed by the gruesome death of his predecessor and colleague, Louis Slotin would continue the work of Daghlian to work out how close the core was to supercriticality. To do so, the procedure was to slowly lower a neutron reflector over the core whilst measuring the activity within. As we learned above, contact between the reflector and the demon core would end horribly, so spacers were used to keep the separation between the two. 

A talented physicist and nuclear daredevil, Louis Slotin was not privy to protocol. Instead, Slotin devised his own method that required less time, less effort but a whole lot more risk. Doing away with the spacers that kept the core from the reflectors, Slotin would use a flathead screwdriver to maintain separation and he would manipulate it with one hand throughout the experiment. He quickly became known for his experiment, which was coined “tickling the dragon’s tail” after famous physicist Richard Feynman likened the daring act to provoking a dragon, and despite warnings by prominent experts, Slotin repeated it multiple times.  

Unfortunately, the bravado would be Slotin’s undoing. On May 21, 1946, Slotin was demonstrating his experiment to several people in Los Alamos Scientific Laboratory when his screwdriver slipped by just a tiny amount, allowing the neutron reflector to surround the core and push it into supercriticality. A flash of blue light burst from the core, followed by intense heat that blasted Slotin and some of his colleagues. Slotin quickly flicked the reflectors off of the core and stopped the reaction, but the damage was done.  

That burst of radiation would kill Slotin within nine days of exposure. Stood right beside him during the accident, Alvin Graves would also receive a huge dose of radiation but would survive the ordeal and live for another 20 years before death. Owing to Slotin’s quick thinking and body position, which absorbed most of the radiation, the remaining onlookers were shielded from the blast and survived to tell the tale.  

Following the accidents, the core would finally gain its’ name as the demon core, before being recycled down into other fissile cores.

An earlier version of this article was published in May 2021.

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