What is the Demon Core? (Plutonium)

demon core atomic bomb

The demon core was a spherical 6.2kg mass of plutonium, created during WW2 in the US, under the program – the Manhattan Project. This was intended to be the fissile core for the originally developed atomic bomb. Two major, fatal incidents in 1945 and in 1946, linked it to its nickname, the demon core.

What is the demon core?

Originally intended for use in third nuclear bomb but was made redundant after Japan’s surrender. On two occasions the core breached into a supercritical mode, when a manual testing process dropped out of the safety parameters. These both happened at the Los Alamos Laboratory in 1945 and 1946, resulting in the acute radiation poisoning and subsequent deaths of scientists Harry Daghlian and Louis Slotin.

Manufacturing the demon core and early history

The demon core was a sphere measuring 89 mm in diameter. It consisted of three parts: two plutonium-gallium hemispheres and a ring, designed to keep neutron flux from jetting out of the joined surface between the hemispheres during implosion.

The refined plutonium material for HS-7, R-3 was in the Los Alamos metallurgy section, and was prepared for a distribution date by September 5 1945. The metallurgists used a plutonium-gallium alloy, which stabilised the δ phase allotrope of plutonium. The sphere was coated with nickel, preventing any corrosion. Strictly, Marshall said, “It is not to be released on Japan without express authority from the President”. On August 13, the third bomb was scheduled. The third core remained at Los Alamos.


The demon core first incident

The core was designed with a small safety margin against extraneous factors that could cause the core to become supercritical. These factors are circumstances like the compression of the solid metallic core or provision of an external reflector which would reflect outbound neutrons back into the core. The experiments were designed to guarantee that the core was indeed close to the critical point by arranging such reflectors and measuring how much neutron reflection was actually required.

On August 21, 1945, the plutonium core produced a burst of neutron radiation that led to Daghlian’s death. Daghlian was working and a security guard, Robert J. Hemmerly, was seated 4 m away. The core was placed within a stack of neutron-reflective tungsten carbide bricks and the addition of each brick moved the assembly closer to criticality. Daghlian’s progressive addition of the bricks led to one accidentally dropped onto the core, causing it to go into supercriticality, a self-sustaining critical chain reaction. He moved the brick off the assembly, but received a fatal dose of radiation. He lost his life from acute radiation poisoning, within a month.



The Philadelphia Incident

On September 2, 1944, three men entered the transfer room at the Philadelphia Navy Yard to repair a clogged tube. This consisted of two concentric pipes with liquid uranium hexafluoride and the innermost pipe contained high-pressure steam.

They had all volunteered to work in a dangerous environment. Bragg Jr., a chemical engineer was hired by the Navy Research Lab, Meigs was an employee of the H. K. Ferguson Company, the prime contractor for the thermal diffusion project and Kramish was a physicist worked for Oak Ridge.

At 1:20 PM, there was an explosion and the tube disintegrated releasing the liquid uranium hexafluoride and the escaping steam dousing hydrofluoric acid over the engineers. Bragg and Meigs died and Kramish, has horrendous burns. U.S.S. Wisconsin was located within 100m and the sailors were not aware that they had been exposed to the toxic uranium hexafluoride. The manhatten project was unofficial and therefore a press release read “Explosion at Navy Yard.” Years passed before the true nature of the incident was revealed.


Second fatal demon core incident at Los Alamos

On May 21, 1946, physicist Louis Slotin and seven colleagues were conducting another experiment to verify the closeness of the core to criticality by the positioning of neutron reflectors. This was a demonstration to Alvin C. Graves, who would use it in a final test before the Operation Crossroads nuclear tests at Bikini Atoll. It required the operator to place two half-spheres of beryllium (a neutron reflector) around the core to be tested and manually lower the top reflector over the core using a thumb hole on the top. The experimenter needed to maintain a slight separation between the reflector halves in order to stay below criticality.

Louis Slotin’s death

Under Slotin’s own unapproved protocol, shims were not used and the only thing preventing the closure was the blade of a standard flat-tipped screwdriver manipulated in Slotin’s other hand.  Scientists referred to this flirting a nuclear chain reaction, based on a remark by physicist Feynman, commenting that it was the same as “tickling the tail of a sleeping dragon”.


The demon core blue glow

Slotin’s screwdriver slipped outward a fraction of an inch while he was lowering the top reflector, allowing the reflector to fall into place around the core. Instantly, there was a flash of blue light and a wave of heat across Slotin’s skin. The heating of the core and shells stopped the criticality within seconds of its initiation, while Slotin’s reaction ended the accident and position of Slotin’s body shielded the others from the neutron radiation. He absorbed a fatal dose of approximately 1,000 rad (10 Gy) neutron and 114 rad (1.14 Gy) gamma radiation and died after a week.



What happened to the demon core?

The demon core, originally scheduled for use at Crossroads, was actually melted down and likely used in the creation of the development of other military applications.




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