The Demon Core, a sphere of plutonium created during the Manhattan Project, symbolizes the risks taken at the dawn of the nuclear age. Intended to be the heart of a third atomic bomb, the core was repurposed after World War II for scientific experiments at Los Alamos Laboratory. These tests aimed to precisely measure the conditions necessary to initiate a nuclear chain reaction, pushing the material to the brink of an uncontrolled fission event. The core gained its infamous name after two separate, fatal accidents underscored the inherent danger of working on the razor’s edge of nuclear physics.
The Plutonium Sphere
The Demon Core was a softball-sized sphere of plutonium-gallium alloy, weighing 6.2 kilograms (14 pounds) and measuring 8.9 centimeters (3.5 inches) in diameter. It was manufactured in two interlocking hemispheres. A small amount of gallium stabilized the plutonium’s crystal structure, allowing it to be hot-pressed into the spherical shape required for an implosion-type weapon. The core was inherently subcritical, meaning it could not sustain a chain reaction on its own, but its mass was extremely close to the necessary threshold.
Its post-war purpose was to help scientists determine the minimum amount of material required to achieve criticality, known as the critical mass. Testing involved placing the subcritical core near materials that could reflect escaping neutrons back into the plutonium. By measuring the increased neutron flux, scientists calculated the exact conditions under which the core would become fully reactive. This hands-on experimentation into nuclear weapon design proved devastatingly unforgiving.
Understanding Nuclear Criticality
Nuclear criticality is defined by the neutron multiplication factor, known as k. This factor is the ratio of neutrons produced by fission in one generation to the number produced in the preceding generation. The system is subcritical when k is less than one, meaning the neutron population and the chain reaction are dying out. A system achieves criticality when k equals one, indicating a self-sustaining chain reaction where the neutron population remains stable.
The danger arises when the system becomes supercritical (k greater than one), causing the neutron population to grow exponentially. The most dangerous state is prompt supercriticality, which occurs when the chain reaction is sustained solely by neutrons released instantaneously from fission, without relying on delayed neutrons. The Demon Core accidents involved brief, uncontrolled excursions into this prompt supercritical state.
These experiments relied on neutron reflectors, such as tungsten carbide or beryllium, to reduce the required critical mass. When reflective material surrounds the plutonium, it scatters neutrons that would normally escape back into the fissile material. This action effectively shrinks the size needed for the chain reaction to sustain itself. By slowly manipulating the reflector, scientists could precisely measure the core’s proximity to the dangerous k=1 threshold.
Mechanism of Accidental Supercriticality
The first accident, involving physicist Harry Daghlian in August 1945, centered on using tungsten carbide bricks as a neutron reflector. Daghlian was manually stacking the bricks around the plutonium sphere, with each additional brick pushing the core closer to the critical point. His neutron-monitoring equipment signaled that the assembly was dangerously close to criticality just as he was about to place the final brick.
In an attempt to reverse the process, Daghlian withdrew his hand, but the final brick accidentally slipped and dropped directly onto the core. This action completed the neutron reflector shell, instantly driving the core into a prompt supercritical state. Daghlian immediately knocked the brick off the assembly to halt the reaction. The momentary spike released a catastrophic burst of radiation, and the core returned to a subcritical state only when the accidental reflector was removed.
The second accident, involving Louis Slotin in May 1946, was part of a risky procedure nicknamed “tickling the dragon’s tail.” This experiment involved manually lowering a nine-inch diameter hemispherical shell of beryllium over the core, which rested inside a second beryllium hemisphere. Beryllium served as the neutron reflector, and the halves had to be kept slightly separated to prevent full enclosure and subsequent supercriticality.
Slotin was using a flat-blade screwdriver as a manual spacer to maintain the separation between the two beryllium halves. The failure occurred when the screwdriver slipped, allowing the upper hemisphere to fall completely over the plutonium core. This full enclosure maximized neutron reflection, instantly triggering a prompt supercritical reaction. Slotin quickly separated the beryllium shells, stopping the reaction after a fleeting but intense burst of fission.
The Immediate Physical Effects
The moment the Demon Core plunged into the prompt supercritical state, a distinct physical phenomenon was observed in both accidents. The massive surge of highly energetic charged particles ionized the surrounding air, creating a visible blue flash of light. This momentary phenomenon is Cherenkov radiation, which occurs when particles travel faster than the speed of light in a specific medium, such as air.
An intense wave of heat was also reported by the scientists present, accompanying the visible light flash. This heat was a physical manifestation of the rapid, enormous burst of neutron and gamma radiation released during the few hundred milliseconds of the supercritical excursion. The scientists immediately deactivated the core by physically separating the reflective material, causing the chain reaction to cease.
Though the core did not explode like a nuclear weapon, the brief, uncontrolled reaction released a lethal dose of radiation. The core itself became highly radioactive from the fission products created during the spike. Due to the two fatal accidents and its high level of induced radioactivity, the Demon Core was melted down and the plutonium material was later recycled for use in other cores.