Thedemon core was a sphere ofplutonium–gallium alloy that was involved in two fatal radiation accidents when scientists tested it as a fissilecore of an earlyatomic bomb. It was manufactured in 1945 by theManhattan Project, the U.S.nuclear weapon development effort duringWorld War II. It was asubcritical mass that weighed 6.2 kilograms (14 lb) and was 8.9 centimeters (3.5 in) in diameter. The core was prepared for shipment to thePacific Theater as part of thethird nuclear weapon to be dropped on Japan, but whenJapan surrendered, the core was retained for testing and potential later use in the case of another conflict.
The twocriticality accidents occurred at theLos Alamos Laboratory inNew Mexico on August 21, 1945, and May 21, 1946. In both cases, an experiment was intended to demonstrate how close the core was tocriticality, using a neutron-reflectivetamper (layer of dense material surrounding thefissile material). In both accidents, the core was accidentally put into a critical configuration. PhysicistsHarry Daghlian (in the first accident) andLouis Slotin (in the second accident) both sufferedacute 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 demon core (like the core used in thebombing of Nagasaki) was, when assembled, a solid 6.2-kilogram (14 lb) softball-sized sphere measuring 8.9 centimeters (3.5 in) in diameter. It consisted of three parts made ofplutonium-gallium: two hemispheres and an anti-jet ring, designed to keepneutron flux from "jetting" out of the joined surface between the hemispheres duringimplosion. The core of the device used in theTrinity Test at theAlamogordo Bombing and Gunnery Range in July did not have such a ring.[1][2]
The refined plutonium was shipped from theHanford Site inWashington to theLos 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 andNagasaki bombs) 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-8"'s fabrication to complete the fourth core).[3] The metallurgists used a plutonium-gallium alloy, which stabilized the delta (δ) phaseallotrope 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.[4]
On August 10, Major GeneralLeslie R. Groves Jr., wrote to General of the ArmyGeorge C. Marshall, theChief of Staff of the United States Army, to inform him that:
The next bomb of the implosion type had been scheduled to be ready for delivery on the target on the first good weather after August 24th, 1945. We have gained 4 days in manufacture and expect to ship the final components from New Mexico on August 12th or 13th. Providing there are no unforeseen difficulties in manufacture, in transportation to the theatre or after arrival in the theatre, the bomb should be ready for delivery on the first suitable weather after August 17th or 18th.[3]
Marshall added an annotation, "It is not to be released on Japan without express authority from the President", on PresidentHarry S. Truman's orders.[3] 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.[3] This was pre-empted byJapan's surrender on August 15, 1945, while preparations were still being made for it to be couriered toKirtland Field. The third core remained at Los Alamos.[5]
The core, once assembled, was designed to be at "−5cents".[6] In this state, there is only a small safety margin against extraneous factors that might increase reactivity, causing the core to become supercritical, and thenprompt critical, a brief state of rapid energy increase.[7] These factors are not common in the environment; they are only likely to occur under conditions such as the compression of the solid metallic core (which would eventually be the method used to explode the bomb), the addition of more nuclear material, or provision of an external reflector which would reflect outboundneutrons 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.[6]
On August 21, 1945, the plutonium core produced a burst ofneutron radiation that resulted in physicistHarry Daghlian's death. Daghlian made a mistake while performingneutron reflector experiments on the core. He was working alone; a security guard, Private Robert J. Hemmerly, was seated at a desk 10 to 12 feet (3 to 4 m) away.[8] The core was placed within a stack of neutron-reflectivetungsten 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 criticalchain reaction. He quickly moved the brick off the assembly, but he received a fatal dose of radiation. He died 25 days later fromacute radiation poisoning.[9]
| Name | Age at accident | Profession | Dose[8]: 20 | Aftermath |
|---|---|---|---|---|
| Haroutune "Harry" Krikor Daghlian Jr. | 24 | Physicist | 200 rad (2.0 Gy) neutron 110 rad (1.1 Gy) gamma | Died 25 days after the accident ofacute radiation syndrome,hematopoietic focus[8]: 22 |
| Private Robert J. Hemmerly | 29 | Special Engineer Detachment guard | 8 rad (0.080 Gy) neutron 0.1 rad (0.0010 Gy) gamma | Died in 1978 (33 years after accident) ofacute myelogenous leukemia at age 62[8]: 9–11, 22 |
On May 21, 1946,[10] physicistLouis Slotin and seven other 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 toAlvin C. Graves, who would use it in a final test before theOperation Crossroads nuclear tests scheduled a month later atBikini Atoll. It required the operator to place two half-spheres ofberyllium (a neutron reflector) around the core to be tested and manually lower the top reflector over the core using a thumb hole at the polar point. As the reflectors were manually moved closer and farther away from each other, neutron detectors indicated the core's neutron multiplication rate. The experimenter needed to maintain a slight separation between the reflector halves to allow enough neutrons to escape from the core in order to stay below criticality. The standard protocol was to useshims between the halves, as allowing them to close completely could result in the instantaneous formation of a critical mass and a lethal power excursion.[10]
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-tippedscrewdriver in Slotin's hand. The size of the gap between the reflectors was changed by twisting the screwdriver. Slotin 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.[11] Scientists referred to this flirtation with anuclear chain reaction as "tickling the dragon's tail", based on a remark by physicistRichard Feynman.[12][13]
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 ofneutron radiation. Slotin quickly twisted his wrist, flipping the top shell to the floor.[14] There was an estimated half-second between when the sphere closed to when Slotin removed the top reflector.[6] Slotin received a lethal dose of 1,000 rad (10 Gy) neutron and 114 rad (1.14 Gy) 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 fromacute 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-lethalradiation dose. Graves was hospitalized for several weeks with severe radiation poisoning.[8] He died 19 years later, at age 55, ofheart failure. 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.[15][16]
The second accident was reported by theAssociated Press on May 26, 1946: "Four men injured through accidental exposure to radiation in the government's atomic laboratory here [Los Alamos] 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 withfissionable material."[17]
Later research was performed concerning 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.[8] A summary of its findings:
| Name | Origin | Age at accident | Profession | Dose[8] | Aftermath |
|---|---|---|---|---|---|
| Louis Alexander Slotin | Winnipeg, Manitoba, Canada | 35 | Physicist | 1,000 rad (10 Gy) neutron 114 rad (1.14 Gy) gamma | Died 9 days after the accident ofacute radiation syndrome, gastrointestinal focus.[10] |
| Alvin C. Graves | Austin, Texas | 36 | Physicist | 166 rad (1.66 Gy) neutron 26 rad (0.26 Gy) gamma | Died in 1965 (19 years after the accident) ofmyocardial infarction, with aggravating "compensatedmyxedema andcataracts", while skiing.[8] |
| Samuel Allan Kline | Chicago, Illinois | 26 | Physics student, later patent attorney | Died in 2001 (55 years after the accident) at age 81; refused to participate with studies and was prevented from obtaining his own medical records from the incident.[8] | |
| Marion Edward Cieslicki | Mt. Lebanon, Pennsylvania | 23 | Physicist | 12 rad (0.12 Gy) neutron 4 rad (0.040 Gy) gamma | Died ofacute myelocytic leukemia in 1965 (19 years after the accident).[8] |
| Dwight Smith Young | Chicago, Illinois | 54 | Photographer | 51 rad (0.51 Gy) neutron 11 rad (0.11 Gy) gamma | Died ofaplastic anemia andbacterial endocarditis in 1975 (29 years after the accident) at age 83.[8] |
| Raemer Edgar Schreiber | McMinnville, Oregon | 36 | Physicist | 9 rad (0.090 Gy) neutron 3 rad (0.030 Gy) gamma | Died of natural causes in 1998 (52 years after the accident), at age 88.[8][14] |
| Theodore Perlman | New Orleans, Louisiana[18] | 23 | Engineer | 7 rad (0.070 Gy) neutron 2 rad (0.020 Gy) gamma | "Alive and in good health and spirits" as of 1978; most likely died in June 1988 (42 years after the accident), in Livermore, California.[8] |
| Private Patrick Joseph Cleary | New York City, New York | 21 | Security guard | 33 rad (0.33 Gy) neutron 9 rad (0.090 Gy) gamma | Sergeant 1st Class Cleary waskilled in action on September 3, 1950 (4 years after the accident), while serving with the8th Cavalry Regiment, US Army in theKorean War.[8][19] |
Two machinists, Paul Long and another, unidentified, in another part of the building, 20–25 ft (6–7.5 m) away, were not treated.[20]
After these incidents, the core, originally known as "Rufus", was referred to as the "demon core".[3][21] 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.[14]
The demon core was intended for use in theOperation Crossroads nuclear tests, but after the second criticality accident, time was needed for its radioactivity to decrease and for it to be re-evaluated for the effects of thefission products it held, some of which were veryneutron poisonous to the desired level of fission. The next two cores were shipped for use inAble andBaker, and the demon core was scheduled to be shipped later for the third test of the series, provisionally namedCharlie, but that test was canceled because of the unexpected level of radioactivity resulting from the underwaterBaker test and the inability to decontaminate the target warships. The core was melted down during the summer of 1946, and the material was recycled for use in other cores.[21]
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