The “Demon Core” was a 6.2-kilogram (14 lb) sphere of plutonium-gallium alloy, measuring about 8.9 centimeters (3.5 in) in diameter, developed as part of the United States’ Manhattan Project during World War II. Manufactured in 1945 at Los Alamos, New Mexico, its primary function was to serve as the fissile core for an atomic bomb, specifically intended as the heart of a potential third nuclear weapon for use against Japan. Japan’s surrender in August 1945 meant the core was never deployed for military use. Instead, scientists at Los Alamos used it for experiments to understand criticality, the precise conditions under which a nuclear chain reaction becomes self-sustaining. These experiments aimed to gather data for nuclear weapons development and safety.
The Daghlian Incident
The Demon Core’s first criticality accident occurred on August 21, 1945, involving physicist Harry Daghlian. Daghlian was conducting an experiment to manually build a neutron reflector around the plutonium core using tungsten carbide bricks, aiming to determine how close the assembly was to criticality. As he stacked the bricks, his neutron counter indicated the assembly was nearing a supercritical state. Daghlian tried to remove a brick, but it slipped and fell directly onto the core.
This sudden contact caused the core to go promptly supercritical, releasing an intense burst of neutron radiation. Eyewitnesses described a bright blue flash of light and a wave of heat emanating from the core. Daghlian quickly reacted, disassembling the arrangement to stop the reaction, but he had already received a lethal dose of radiation. He reported a tingling sensation in his hand immediately after the incident. Daghlian endured severe symptoms of acute radiation poisoning, including blistering skin, swelling, and abdominal pain, eventually falling into a coma. He died 25 days later, becoming the first fatality from a criticality accident.
The Slotin Incident
The Demon Core was involved in another accident on May 21, 1946, with physicist Louis Slotin. Slotin was performing a criticality experiment, known as “tickling the dragon’s tail,” which involved bringing two beryllium hemispheres close to the core to reflect neutrons and push it near criticality. He was demonstrating this procedure to colleagues, using a screwdriver to maintain a small gap between the hemispheres, preventing them from fully enclosing the core. This method was known to be highly risky due to its reliance on manual dexterity.
During the experiment, the screwdriver Slotin was using to keep the hemispheres apart slipped. This allowed the upper beryllium hemisphere to fall, briefly enclosing the core and causing it to become promptly supercritical. The room was instantly filled with a blue flash of light and a sensation of heat, indicating a significant burst of neutron radiation. Slotin, closest to the core, instinctively jerked his hand to separate the hemispheres, ending the reaction and likely shielding his colleagues.
Despite his quick actions, Slotin received a lethal dose of radiation, experiencing immediate symptoms like a sour taste in his mouth and a burning sensation in his hand. He suffered severe acute radiation syndrome, including internal burns and organ failure, and died nine days later. Other scientists in the room also received varying degrees of radiation exposure, some experiencing long-term health effects.
The Science of Criticality and Its Immediate Harm
Criticality in nuclear materials refers to the state where a nuclear chain reaction becomes self-sustaining. This occurs when neutrons released by nuclear fission events cause subsequent fission events at a rate that maintains the reaction without external intervention. The accidents happened when the core inadvertently reached “prompt criticality,” a highly unstable state where the chain reaction is sustained solely by immediate neutrons, without relying on delayed neutrons, leading to a rapid and uncontrolled energy release.
The immediate harm from these incidents stemmed from extreme exposure to ionizing radiation, which directly damages the body’s cells and tissues. This rapid, high-dose exposure results in acute radiation syndrome (ARS), also known as radiation sickness or poisoning. ARS manifests with initial symptoms like nausea, vomiting, and fatigue. Damage to rapidly dividing cells, such as those in bone marrow and the gastrointestinal tract, leads to more serious symptoms like infections, bleeding, and organ failure, often proving fatal within days or weeks depending on the radiation dose.
Decommissioning and Lasting Impact
Following the two fatal accidents, the Demon Core was deemed too dangerous for further hands-on experimentation. Its radioactivity levels had also increased due to the fission products generated during the incidents. The core was melted down in the summer of 1946, and its plutonium recycled for use in other nuclear applications. This marked the end of the Demon Core’s existence as a distinct entity.
The deaths of Harry Daghlian and Louis Slotin served as a stark lesson in nuclear safety. These incidents led to stricter protocols for handling fissile materials at Los Alamos and other nuclear facilities. A key change was the cessation of “hands-on” criticality experiments. Scientists subsequently used remote-control machinery and television cameras to manipulate radioactive materials from a safe distance, often up to a quarter-mile away, to prevent direct human exposure. The accidents profoundly influenced nuclear safety principles, emphasizing control of fissile materials, geometry control, and neutron poisoning to prevent uncontrolled chain reactions.