From the Wikipedia article: > The demon core was a sphere of plutonium that was involved in two fatal radiation accidents when scientists tested it as a fissil

From the Wikipedia article: > The demon core was a sphere of plutonium that was involved in two fatal radiation accidents when scientists tested it as a fissile core of an early atomic bomb > In both cases, an experiment was intended to demonstrate how close the core was to criticality with a tamper, but the core was accidentally put into a critical configuration That doesn’t sound very reassuring