Demon core
The demon core was a spherical subcritical mass of plutonium in diameter, manufactured during World War 2 by the US nuclear weapon development effort, the Manhattan Project, as a fissile core for an early atomic bomb, that was involved in two criticality accidents, on August 21, 1945 and May 21, 1946. The core was intended for use in a possible third nuclear weapon to be dropped on Japan, but when Japan's surrender made this unnecessary it was used for testing. It was designed with a small safety margin to ensure a successful explosion of the bomb. The device briefly went supercritical when it was accidentally placed in supercritical configurations during two separate experiments intended to guarantee the core was indeed close to the critical point. The incidents happened at the Los Alamos Laboratory in 1945 and 1946, both resulting in the acute radiation poisoning and subsequent deaths of scientists Harry Daghlian and Louis Slotin. After these incidents the spherical plutonium core was referred to as the "demon core".
Manufacturing and early history
The demon core was a solid sphere measuring 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 core of the device used in the Trinity nuclear test at the Alamogordo Bombing and Gunnery Range in July did not have such a ring.The refined plutonium was shipped from the Hanford Site in Washington state to the Los Alamos Laboratory; an inventory document dated August 30 shows Los Alamos had expended "HS-1, 2, 3, 4; R-1" 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. The metallurgists used a plutonium-gallium alloy, which stabilized the 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 R. Groves, Jr., 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", as President Harry S. Truman was waiting to see the effects of the first two attacks. On August 13, the third bomb 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.
First incident
The core, assembled, was designed to be at "−5 cents". In this state there is only a small safety margin against extraneous factors that might increase reactivity, causing the core to become supercritical, and then prompt critical, a brief state of rapid energy increase. These factors are not common in the environment; they are circumstances like the compression of the solid metallic core, the addition of more nuclear material, or provision of an external reflector which would reflect outbound neutrons back into the core. The experiments conducted at Los Alamos leading to the two fatal accidents were designed to guarantee that the core was indeed close to the critical point by arranging such reflectors and seeing how much neutron reflection was required to approach supercriticality.On August 21, 1945, the plutonium core produced a burst of neutron radiation that led to 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 moved 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 received a fatal dose of radiation. He died 25 days later from acute radiation poisoning.
Second incident
On May 21, 1946, physicist Louis Slotin and seven other Los Alamos personnel were in a Los Alamos laboratory conducting another experiment to verify the closeness of the core to criticality by the positioning of neutron reflectors. Slotin, who was leaving Los Alamos, was showing the technique to Alvin C. Graves, who would use it in a final test before the Operation Crossroads nuclear tests scheduled a month later at Bikini Atoll. It required the operator to place two half-spheres of beryllium around the core to be tested and manually lower the top reflector over the core using a thumb hole on the top. As the reflectors were manually moved closer and farther away from each other, scintillation counters measured the relative activity from the core. The experimenter needed to maintain a slight separation between the reflector halves in order to stay below criticality. The standard protocol was to use shims between the halves, as allowing them to close completely could result in the instantaneous formation of a critical mass and a lethal power excursion. Under Slotin's own unapproved protocol, the shims were not used and the only thing preventing the closure was the blade of a standard straight screwdriver manipulated in Slotin's other hand. 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 flirting with the possibility of a nuclear chain reaction as "tickling the dragon's tail", based on a remark by physicist Richard Feynman, who compared the experiments to "tickling the tail of a sleeping dragon".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 blue light and a wave of heat across Slotin's skin; the core had become supercritical, releasing an intense burst of neutron radiation estimated to have lasted about a half second. Slotin quickly twisted his wrist, flipping the top shell to the floor. The heating of the core and shells stopped the criticality within seconds of its initiation, while Slotin's reaction prevented a recurrence and ended the accident. The position of Slotin's body over the apparatus also shielded the others from much of the neutron radiation, but he received a lethal dose of neutron and gamma radiation in under a second and died nine days later from acute radiation poisoning. The nearest person to Slotin, Graves, who was watching over Slotin's shoulder and was thus partially shielded by him, received a high but non-lethal radiation dose. Graves was hospitalized for several weeks with severe radiation poisoning and developed chronic neurological and vision problems as a result of the exposure. He died 20 years later, at age 55, of a heart attack. It may have been caused by hidden complications from radiation exposure, but could also have been genetic in nature, as his father had died from the same cause.
The second accident was reported by the Associated Press on 26 May 1946: "Four men injured through accidental exposure to radiation in the government's atomic laboratory here 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 fissionable material."
Medical studies
Follow-up research was conducted on the health of the men. An early report was published in 1951. A later report was compiled for the U.S. government and submitted in 1979. A summary of its findings:| Name | Origin | Age at accident | Profession | Dose | Aftermath | |
| Louis Alexander Slotin | Winnipeg, Manitoba, Canada | 35 | physicist | neutron gamma | died 9 days after the accident of acute radiation syndrome, gastrointestinal focus | |
| Alvin C. Graves | Austin, Texas | 34 | physicist | neutron gamma | died in 1965 of myocardial infarction, with aggravating "compensated myxedema and cataracts", while skiing | |
| Samuel Allan Kline | Chicago, Illinois | 26 | physics student, later patent attorney | died in 2001 ; refused to take part in studies and was prevented from obtaining his own medical records from the incident | ||
| Marion Edward Cieslicki | Mt. Lebanon, Pennsylvania | 23 | physicist | neutron gamma | died of acute myelocytic leukemia in 1965 | |
| Dwight Smith Young | Chicago, Illinois | 54 | photographer | neutron gamma | died of aplastic anemia and bacterial endocarditis in 1975 | |
| Raemer Edgar Schreiber | McMinnville, Oregon | 36 | physicist | neutron gamma | died of natural causes in 1998, at the age of 88 | |
| Theodore Perlman | Louisiana | 23 | engineer | neutron gamma | "alive and in good health and spirits" as of 1978; probably died in June 1988, in Livermore, California | |
| Private Patrick Joseph Cleary | New York City | 21 | security guard | neutron gamma | Sergeant 1st Class Cleary was KIA on 3 Sep 1950 while fighting with the 8th Cavalry Regiment, US Army in the Korean War. |
Two machinists, Paul Long and another, unidentified, in another part of the building, 20 to 25 feet 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.