The first two accidents I will examine are both because of the demon core. The demon core was a 6.2-kilogram sphere made of an alloy of plutonium subsequently coated in nickel to prevent corrosion. The core was produced at the Hanford Site in Washington State and shipped to Los Alamos for scientists to experimentally determine how close to criticality the core was. The core was designed to contain 5% less plutonium than would be required to achieve a critical fission reaction. The two incidents were caused by unintended super criticality of this core.

The first incident occurred on August 21, 1945. The core was situated within a stack of tungsten carbide, a material that reflects neutrons. Harry Daghlian, the physicist working with the core, accidentally dropped a tungsten carbide brick onto the core, causing the core to instantly go supercritical. A mass of nuclear material is supercritical if each fission reaction on average causes more than one new fission reaction. In milliseconds, the demon core went from being an innocuous mass of metal to a nuclear reactor operating at full power. Daghlian received an estimated exposure of 20,000 rem in his right hand, the part of his body that was closest to the supercritical core. For comparison, the United States Nuclear Regulatory Commission mandates workers receive no more than 5 rem exposure annually. Daghlian died 28 days after the accident. As Daghlian’s condition deteriorated he allowed himself to be photographed and these photographs were used in medical reports documenting the types of injuries associated with high dose radiation exposure.

The second incident with the demon core occurred on May 21, 1946 when physicist Louis Slotin was demonstrating to his successor how to perform the criticality experiment. Slotin had the demon core situated between two hemispheres of neutron reflective beryllium. Keeping the hemispheres from closing around the core was a standard screwdriver wedged between them. The incident occurred when the screwdriver Slotin was holding slipped from between the two hemispheres allowing them to close and reflect neutrons back into the core, causing it to go super critical. Slotin received internal radiation burns, what a medical expert called a 3-dimensional sunburn. Both of these incidents helped medical researchers learn about the short term and long term effects of exposure to nuclear radiation. After these incidents, Los Alamos wisely stopped manual criticality testing.

The SL-1 reactor was an experimental portable low power reactor designed to power remote Army radar stations in the Arctic. The requirements for the reactor mandated a small core, which gave the center control rod a disproportionate amount of control over the criticality in the reactor. On January 3, 1961 the primary control rod for the SL-1 reactor was withdrawn approximately 26 inches, 3 inches past what would have brought the reactor prompt critical, a level of criticality characterized by rapid uncontrollable fission, like a nuclear weapon. Within 4 milliseconds the water in the reactor vessel explosively vaporized. The energy released was so high that the entire 26,000 pound reactor vessel jumped 9 feet vertically. This incident brought about the “one stuck rod” criterion that mandated total removal of one control rod could not bring a shutdown reactor to criticality. This design is still used in modern reactors to prevent one simple mistake from causing a nuclear disaster. Little radiation was released to the environment because the reactor was contained in a building that functioned similarly to a containment building in a modern reactor. The SL-1 incident is the only incident with a power generating reactor in the United States that directly caused fatalities.

Next week I will cover the story of the graphite moderated Chernobyl number 4 reactor that caused the first global scale nuclear power disaster.