The Demon Core, a subcritical plutonium sphere, was developed during the Manhattan Project. Intended for a third atomic bomb, its deployment became unnecessary after Japan’s 1945 surrender. Instead, it was retained for scientific experimentation at Los Alamos to understand fissile materials. The core gained notoriety from its involvement in two fatal criticality accidents that claimed the lives of two scientists.
Anatomy of the Demon Core
The Demon Core was a sphere of plutonium-gallium alloy, weighing 6.2 kilograms (about 13.7 pounds) and measuring 8.9 centimeters (3.5 inches) in diameter. This alloy stabilized plutonium’s delta phase, allowing hot-pressing into a spherical shape. It was also nickel-coated to prevent corrosion and block alpha particles.
The core consisted of three parts: two hemispheres and an anti-jet ring. This design managed neutron flux for weapon implosion. For experiments, neutron-reflective materials like tungsten carbide or beryllium hemispheres surrounded the core, reflecting neutrons back into the plutonium.
The Science of Criticality
Nuclear criticality refers to the state where a nuclear chain reaction becomes self-sustaining. This begins when a neutron strikes a fissile atom, splitting it and releasing more neutrons and energy. If enough newly released neutrons strike other fissile atoms, the reaction continues and amplifies.
A subcritical state means not enough neutrons are sustained for a chain reaction, leading to a fizzle. In a critical state, the number of neutrons produced exactly balances those lost, resulting in a stable, self-sustaining reaction. Conversely, a supercritical state occurs when the rate of neutron production rapidly increases, leading to an exponential increase in energy release.
Achieving criticality involves manipulating factors such as the mass, density, and shape of the fissile material, and the presence of neutron reflectors. Reflectors, like beryllium or tungsten carbide, surround the core and bounce escaping neutrons back into the material. This reduces the fissile material needed to reach criticality by conserving neutrons. Compression also increases density, bringing fissile atoms closer and enhancing neutron capture.
Dangerous Experiments: Pushing the Limits
The experiments with the Demon Core were known as “tickling the dragon’s tail.” This phrase described the perilous nature of bringing the fissile core to the brink of criticality without allowing it to become fully supercritical. The goal was to precisely measure how close the core was to initiating a self-sustaining chain reaction.
Scientists adjusted the spacing between the core and its neutron-reflective shell. This was done by hand, using tools like shims or screwdrivers to manipulate the reflective hemispheres or tungsten carbide bricks. Each small adjustment brought the assembly closer to the point where the neutron population would rapidly multiply. The procedure required precision and steady hands, as even a slight deviation could trigger an uncontrolled burst of radiation.
The experiments gathered data on neutron multiplication and critical mass configurations for atomic weapons development. However, the manual nature of these procedures introduced human risk. The fine line between a controlled subcritical state and a dangerous supercritical excursion was narrow, demanding concentration and flawless execution from researchers.
The Fatal Incidents
The Demon Core was involved in two fatal accidents that highlighted the hazards of criticality experiments. The first occurred on August 21, 1945, involving physicist Harry Daghlian. Daghlian was manually stacking tungsten carbide bricks around the plutonium core to reflect neutrons. While positioning the final brick, a neutron detector alarm sounded, indicating the core was approaching supercriticality.
In his haste, the brick slipped and fell onto the core, causing a momentary supercritical excursion and a burst of radiation. Daghlian received a lethal dose of radiation and died 25 days later from acute radiation syndrome.
Less than a year later, on May 21, 1946, a second tragedy involved physicist Louis Slotin. Slotin was demonstrating a criticality experiment, using a screwdriver to keep two beryllium hemispheres, acting as neutron reflectors, slightly separated around the core. The procedure required the hemispheres to be brought close together without touching.
During the demonstration, the screwdriver slipped, allowing the hemispheres to close completely around the core, instantly making it supercritical. A bright blue flash and burst of heat filled the room, indicating neutron radiation release. Slotin quickly separated the hemispheres, ending the reaction, but he had absorbed a lethal dose of radiation. He succumbed to acute radiation poisoning nine days later.