The Nuclear “Demon Core” that Killed 2 Scientists in the 1940s

During WWII, development of nuclear weapons was paramount for many of the world’s top physicists. Each one of these nuclear weapons required a core of plutonium that measured around 3.5 inches in diameter. 2 cores were used in the nuclear bombing of Japan to stop WWII, but there was a third core ready to be used when needed. When the war ended, the core became the main testing subject for physicists as they continued to improve the United States’ nuclear arsenal.

This third core was a 14 pound subcritical mass of plutonium that measured 3.5 inches in diameter. It was also responsible for the direct deaths 2 physicists and many more who died years later from cancer, for which earned the mass its nickname, the “demon core.”

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The demon core was designed with a small safety margin, only about 5 percent. This was to ensure that it went off in the event of its use. The 14-lb radioactive sphere consisted of two plutonium-gallium hemispheres and a center ring designed to keep neutron flux from jetting out of the core during the implosion. This design maximized the destructive power of bomb core.

On August 21, 1945, the first incident occurred. Physicist Harry Daghlian was performing experiments on the neutron reflectors around the core. He was working alone with only a security guard standing watch about 12 feet away. While moving protective tungsten carbide bricks around the core assembly, Daghlian accidentally dropped one onto the core and due to the low safety factor, the core quickly slipped into supercriticality. Daghlian quickly moved the brick off of the assembly, but it was too late. He received a fatal dose of radiation and died 25 days later from radiation poisoning.

The security guard survived but died 33 years later from what was likely radiation-induced Leukemia.

After this incident, testing on the core persisted.

On May 21, 1946, the lead physicist for the project, Louis Slotin and seven other personnel were in the Los Alamos laboratory conducting experiments on the demon core to determine its critical point. Louis Slotin was slated to leave Los Alamos for other work, so he had begun training physicist Alvin C. Graves to take his post. Slotin was preparing Graves to use the core in the Operation Crossroads nuclear tests scheduled in a month at Bikini Atoll.

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Graves needed to know how to place two half-spheres of beryllium acting as neutron reflectors around the core. This action involved manually lowering the top section onto the core using a small thumb hole. If the two neutron reflectors fell into the wrong position allowing them to close completely, instantaneous formation of a critical mass could occur resulting in a rapid power excursion and the release of lethal doses of radiation.

This action was tedious enough as is, but making it worse, Slotin had developed an unapproved protocol of how the process should work. In his process, the physicist would hold the top hemisphere of the neutron reflectors with the thumb of one hand while the other hand held a small screwdriver between the halves to keep them from coming together.

On the day of the accident, Slotin’s screwdriver slipped ever so slightly while lowering the reflector and a sudden flash of blue light and heat was released across his entire body. The core became instantaneously supercritical and everyone in the room was hit with an intense burst of neutron radiation lasting just a half second. Luckily, Slotin acted fast and flicked the top reflector on the floor, keeping the reaction from continuing. His positioning also shielded others in the room from more lethal doses of radiation.

He received a lethal dose of 1,000 rad neutron and 114 gamma radiation in under one second.  He died 9 days later from radiation poisoning.

Graves was watching the process over Slotin’s shoulder and was luckily partially shielded by Slotin’s body. He was hospitalized for several weeks but survived the incident. He later died of a heart attack. It is unclear whether the heart attack that killed him was a result of radiation or if it was simply genetics.

Along with Graves and Slotin, 6 other people were in the room, all suffering minor injuries with only 1 dying of what was likely radiation-caused leukemia 19 years later.

After these 2 incidents, the demon core needed some time for its radioactivity to decline before it could be evaluated for use in testing. Two other cores were used in the Operation Crossroads tests. These tests resulted in unexpected levels of radiation in the testing area which eventually led to the decision to scrap testing altogether. The demon core was soon melted down with its materials recycled for use in other cores.

Trevor is a civil engineer by trade and an accomplished internet blogger with a passion for inspiring everyone with new and exciting technologies. He is also a published children’s book author whose most recent book, ZOOM Go the Vehicles, is aimed at inspiring young kids to have an interest in engineering.


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