The “demon core” is one of the most infamous objects from the early atomic age, a highly volatile mass of plutonium used as a test article at the Los Alamos laboratory. It was a key artifact of the Manhattan Project, originally intended for use in an atomic bomb. The core earned its menacing nickname following a pair of tragic accidents that demonstrated the extreme hazards of working with nuclear materials near the threshold of a chain reaction.
Physical Description and Purpose
The core was a solid sphere of fissile material, measuring approximately 8.9 centimeters (3.5 inches) in diameter and weighing 6.2 kilograms (13.7 pounds). It was fabricated from a plutonium-gallium alloy. This composition was chosen because the small percentage of gallium stabilized the plutonium’s delta phase, making the metal ductile and easier to machine. The sphere was composed of three parts: two hemispheres and a small ring, designed to be a subcritical mass of plutonium-239.
This core was intended to be the fissile pit for a third atomic weapon prepared for deployment near the end of World War II. Japan’s surrender intervened before the weapon could be assembled, leaving the core at Los Alamos. It was then repurposed for criticality experiments meant to investigate the precise conditions required to initiate a nuclear chain reaction. The core was also slated for use in the Operation Crossroads nuclear tests scheduled for 1946.
The Concept of Near-Criticality
The core’s danger stemmed from its design state, which was very close to the point of criticality. Criticality describes the moment a mass of fissile material achieves a self-sustaining nuclear chain reaction, where neutrons released by fission cause an average of one more fission event. The core was designed to be subcritical, meaning it could not maintain a chain reaction on its own because too many neutrons escaped into the surrounding environment.
The core was not far below the critical mass threshold, making it highly susceptible to external influences. Adding a neutron reflector around the sphere could instantly push it into the supercritical state, causing a sudden surge of energy. Materials like tungsten carbide or beryllium were used as reflectors because they efficiently bounced escaping neutrons back into the plutonium, increasing the rate of fission. Scientists informally referred to this type of high-risk experimentation as “tickling the dragon’s tail.”
The Accidents That Gave It Its Name
The core’s notoriety was cemented by two fatal accidents that occurred less than a year apart at the Los Alamos laboratory. The first incident took place in August 1945, when physicist Harry Daghlian Jr. was manually assembling a neutron reflector around the core using tungsten carbide bricks. Daghlian was stacking the bricks one by one while monitoring the core’s neutron count to gauge its proximity to criticality.
His monitoring equipment signaled that the next brick would cause the assembly to become supercritical. While attempting to remove a brick, Daghlian accidentally dropped it directly onto the core, instantly creating a supercritical configuration. A brilliant blue flash of light, caused by the ionization of air, accompanied a massive burst of neutron radiation. Daghlian quickly disassembled the stack but received a lethal dose of radiation, succumbing to acute radiation poisoning 25 days later.
The second accident involved the same core in May 1946, during an experiment conducted by physicist Louis Slotin. Slotin was demonstrating a procedure that involved manually lowering a beryllium hemisphere, a powerful neutron reflector, over the core. The two halves were meant to be kept separated by a small gap, which Slotin maintained only with the tip of a screwdriver. This method bypassed standard safety shims and was known as a risky procedure.
During the demonstration, the screwdriver slipped, allowing the beryllium hemisphere to fully enclose the core, resulting in a second prompt criticality event. Slotin quickly reacted, twisting his wrist to knock the top reflector off the core and halt the chain reaction. This action likely saved the lives of the seven other people in the room, but Slotin absorbed a massive dose of radiation in the process. He died nine days later from acute radiation syndrome, solidifying the core’s deadly reputation.
Final Use and Historical Legacy
Following the second tragedy, the core was removed from use in criticality experiments. The plutonium was highly radioactive from the bursts of fission products and considered too dangerous for further hands-on research. It was set aside to allow its radioactivity to decay, and in late 1946, the material was melted down and recast.
The plutonium from the core was repurposed for other, less notorious cores, ending its existence as a single, identifiable artifact. The deaths of the two physicists served as a costly lesson in nuclear safety. This led to immediate, sweeping changes in laboratory protocols, including the requirement that all future criticality experiments be conducted remotely, with researchers separated from the material by significant distance and shielding.