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The demon core was a sphere of plutonium that was involved in two fatal radiation accidents when scientists tested it as a fissile core of an early atomic bomb. It was manufactured in 1945 by the Manhattan Project, the U.S. nuclear weapon development effort during World War II. It was a critical mass that weighed and was in diameter. The core was prepared for shipment to the Pacific Theater as part of the Third Shot to be dropped on Japan, but when Japan surrendered, the core was retained for testing and potential later use in the case of another conflict.
The two criticality accidents occurred at the Los Alamos Laboratory in New Mexico on August 21, 1945, and May 21, 1946. In both cases, an experiment was intended to demonstrate how close the core was to criticality, using a neutron-reflective tamper (layer of dense material surrounding the fissile material). In both accidents, the core was accidentally put into a critical configuration. Physicists Harry Daghlian (in the first accident) and Louis Slotin (in the second accident) both suffered acute radiation syndrome and died shortly afterward. At the same time, others present in the laboratory were also exposed. The core was melted down during the summer of 1946, and the material was recycled for use in other cores.
The refined plutonium was shipped from the Hanford Site in Washington to the Los Alamos Laboratory; an inventory document dated August 30 shows Los Alamos had expended "HS-1, 2, 3, 4; R-1" (the components of the Trinity and Fat Man) and had in its possession "HS-5, 6; R-2", finished and in the hands of quality control. Material for "HS-7, R-3" was in the Los Alamos metallurgy section and would also be ready by September 5 (it is not certain whether this date allowed for the unmentioned "HS-8s fabrication to complete the fourth core). The metallurgists used a plutonium-gallium alloy, which stabilized the delta (δ) phase allotrope of plutonium so it could be hot pressed into the desired spherical shape. As plutonium was found to corrode readily, the sphere was then coated with nickel.
On August 10, Major General Leslie Groves, wrote to General of the Army George C. Marshall, the Chief of Staff of the United States Army, to inform him that:
Marshall added an annotation, "It is not to be released on Japan without express authority from the President", on President Harry S. Truman's orders. On August 13, Third Shot was scheduled. It was anticipated that it would be ready by August 16 to be dropped on August 19. This was pre-empted by Japan's surrender on August 15, 1945, while preparations were still being made for it to be couriered to Kirtland Field. The third core remained at Los Alamos. Raemer Schreiber being interviewed by Richard Rhodes
On August 21, 1945, the plutonium core produced a burst of neutron radiation that resulted in physicist Harry Daghlian's death. Daghlian made a mistake while performing neutron reflector experiments on the core. He was working alone; a security guard, Private Robert J. Hemmerly, was seated at a desk away. The core was placed within a stack of neutron-reflective tungsten carbide bricks, and the addition of each brick made the assembly closer to criticality. While attempting to stack another brick around the assembly, Daghlian accidentally dropped it onto the core and thereby caused the core to go well into supercriticality, a self-sustaining critical chain reaction. He quickly moved the brick off the assembly, but he received a fatal dose of radiation. He died 25 days later from acute radiation poisoning.
| ! scope=col Name
! scope=col | Age at accident
! scope=col | Profession
! scope=col | Dose Patient numbers in this document have been identified as: 1 – Daghlian, 2 – Hemmerly, 3 – Slotin, 4 – Graves, 5 – Kline, 6 – Young, 7 – Cleary, 8 – Cieleski, 9 – Schreiber, 10 – Perlman ! scope=col | Aftermath |
| Harry Daghlian]] | Died 25 days after the accident of acute radiation syndrome, haematopoiesis focus | |||
| ! scope=row Private Robert J. Hemmerly
| Died in 1978 (33 years after accident) of acute myelogenous leukemia at age 62 |
By Slotin's own unapproved protocol, the shims were not used. The top half of the reflector was resting directly on the bottom half at one point, while 180 degrees from this point a gap was maintained by the blade of a flat-tipped screwdriver in Slotin's hand. The size of the gap between the reflectors was changed by twisting the screwdriver. Slotin, who was given to bravado, became the local expert, performing the test on almost a dozen occasions, often in his trademark blue jeans and cowboy boots in front of a roomful of observers. Enrico Fermi reportedly told Slotin and others they would be "dead within a year" if they continued performing the test in that manner. Scientists referred to this flirtation with a nuclear chain reaction as "tickling the dragon's tail", based on a remark by physicist Richard Feynman.
On the day of the accident, 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 light; the core had become supercritical, releasing an intense burst of neutron radiation. Slotin quickly twisted his wrist, flipping the top shell to the floor. There was an estimated half-second between when the sphere closed to when Slotin removed the top reflector. Slotin received a lethal dose of neutron and gamma radiation in less than a second, while the position of Slotin's body over the apparatus shielded the others from much of the neutron radiation. Slotin died nine days later from acute radiation poisoning.
Graves, the next nearest person to the core, was watching over Slotin's shoulder and was thus partially shielded by him. He received a high but non-lethal radiation dose. Graves was hospitalized for several weeks with severe radiation poisoning. He died 19 years later, at age 55, of heart attack. While this may have been caused by Graves' exposure to radiation, the condition may have been hereditary, as his father also died of heart failure.
The second accident was reported by the Associated Press on May 26, 1946: "Four men injured through accidental exposure to radiation in the government's atomic laboratory here Los have been discharged from the hospital and 'immediate condition' of four others is satisfactory, the Army reported today. Dr. Norris E. Bradbury, project director, said the men were injured last Tuesday in what he described as an experiment with Fissile material."Associated Press, "Several at Atomic Bomb Laboratory Injured", The San Bernardino Daily Sun, San Bernardino, California, Monday 27 May 1946, Volume 52, page 1.
Two machinists, Paul Long and another, unidentified, in another part of the building, away, were not treated.
After these incidents, the core, originally known as "Rufus", was referred to as the "demon core". Hands-on criticality experiments were stopped, and remote-control machines and TV cameras were designed by Schreiber, one of the survivors, to perform such experiments with all personnel at a quarter-mile distance.
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