The "Demon Core"

For my first criticality accident I’m going to talk about what is lovingly referred to as the “demon core.” This is due to the fact that it’s been a part of two accidents at Los Alamos National Lab.

The demon core consisted of two hemispheres of plutonium coated in nickel. The core, when the hemispheres are put together, was designed to be 5% below a critical mass meaning that by itself it could not become critical. In order to become critical, the demon core would require a reflector, a material that would bounce neutrons back into the core, or additional nuclear material.

The first incident occurred on August 21, 1945. A physicist was conducting an experiment where he was placing reflector bricks made of tungsten carbide in order to gauge how many it would take to reach criticality. The core was on a stack of bricks and each brick placed around the core put it closer and closer to criticality. The physicist was working alone (always a bad idea) and he accidentally dropped a tungsten carbide brick on top of the core. The brick caused the core to become supercritical and the physicist received a lethal dose of radiation. He unfortunately died 25 days later from acute radiation poisoning.

I wish I could say that the world learned more from this poor man’s death but unfortunately you see physicists and engineers bypassing safety systems, working alone, and thinking that they could do things without help all the time. You’ll definitely hear more about those men from me later.

The second incident occurred on May 21, 1946. It’s popularly portrayed in the 1989 movie Fat Man and Little Boy. A group of scientists were conducting an experiment where they placed the demon core within a beryllium sphere that acted as a neutron reflector. The beryllium sphere was also comprised of two hemispheres and as the hemispheres got closer and closer together around the core, the core became more critical. The experiment consisted of changing the gap between the two hemispheres in order to measure the activity of the core. However, a standard screwdriver inserted in the gap between the hemispheres was manipulating the gap width. If this sounds stupid, it’s because it was, and Slotin, the head physicist, was warned that if he continued to do the experiment he would be “dead within a year”. On the day of the accident, the screwdriver slipped out and the two beryllium hemispheres met. The core became supercritical instantly. Slotin knocked the two spheres apart stopping the reaction. Slotin, four other scientists, a technician, an engineer, and one guard received radiation doses. Slotin died 9 days later from acute radiation poisoning. All others were far enough that they did not receive lethal doses.

I guess the moral of the story today is no one is a god and experiments, especially those with radioactive materials, should be conducted with the utmost respect for the lethality of the material.