The Demon Core – Both the first and the second person to die from an accidental uncontrolled nuclear reaction were working with the same plutonium core



Since the dawn of the nuclear age there have been at least 60 “criticality accidents”, or uncontrolled nuclear chain reactions due to the unintentional assembly of a critical mass of a particular fissile material. The first 2 deaths due to such a accident occurred at Los Alamos National Laboratory in 1945 and 1946. Amazingly, the two fatal accidents involved the same plutonium core going briefly critical. These unfortunate accidents lead to this particular core being nicknamed “The Demon Core”.

A criticality accident is an uncontrolled nuclear chain reaction. It is sometimes referred to as a critical excursion or a critical power excursion and represents the unintentional assembly of a critical mass of a given fissile material, such as enriched uranium or plutonium, in an unprotected environment. A critical or supercritical fission reaction (one that is sustained in power or increasing in power) generally only occurs inside reactor cores and occasionally within test environments; a criticality accident occurs when the same reaction is achieved unintentionally and in an unsafe environment. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed is still incapable of producing a nuclear detonation of the type seen in fission bombs, as the reaction lacks the many engineering elements that are necessary to induce explosive supercriticality. The heat released by the nuclear reaction will typically cause the fissile material to expand, so that the nuclear reaction becomes subcritical again within a few seconds.

On August 21, 1945, the plutonium core produced a burst of neutron radiation that led to Harry Daghlian’s death. Daghlian, a physicist, made a mistake while working alone performing neutron reflection experiments on the core. 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 critical, a self-sustaining prompt critical chain reaction. Despite quick action in moving the brick off the assembly, Daghlian received a fatal dose of radiation. He died 25 days later from acute radiation poisoning.

On May 21, 1946, physicist Louis Slotin and seven other Los Alamos personnel were in a Los Alamos laboratory conducting an experiment to verify the exact point at which a subcritical mass (core) of fissile material could be made critical by the positioning of neutron reflectors. The test was known as “tickling the dragon’s tail” for its extreme risk. 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 via 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. Allowing them to close completely could result in the instantaneous formation of a critical mass and a lethal power excursion. Under Slotin’s unapproved protocol, the only thing preventing this was the blade of a standard flathead screwdriver, manipulated by the scientist’s other hand. Slotin, who was given to bravado, became the local expert, performing the test almost a dozen separate times, often in his trademark bluejeans 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 it.

On the day of the accident, Slotin’s screwdriver slipped outward a fraction of an inch while 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 a massive burst of neutron radiation estimated to have lasted about a half second. He quickly flipped the top shell to the floor. The heating of the core and shells stopped the criticality within seconds of its initiation, but Slotin’s reaction prevented a recurrence and ended the accident. Slotin’s body’s positioning over the apparatus also shielded the others from much of the neutron radiation. He received a lethal dose of 1000 rads neutron/114 rads gamma in under a second and died nine days later from acute radiation poisoning. The nearest person to Slotin, Alvin C. Graves, 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, developed chronic neurological and vision problems as a result of the exposure, and died 20 years later of a heart attack. This heart attack may have been caused by hidden complications from radiation exposure, but its cause could also have been genetic, as his father had died from the same cause.

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