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. It was manufactured by the Manhattan Project, the U.S. nuclear weapon development program during World War II. The core was a solid 6.2-kilogram (14 lb) sphere measuring 89 millimeters (3.5 in) in diameter, consisting of two plutonium-gallium hemispheres and a ring designed to keep neutron flux from "jetting" out of the joined surface between the hemispheres during implosion. The core was originally known as "Rufus" but after the accidents, it was referred to as the "demon core".

The first accident happened less than a week after Japans surrender, and only two days after the date of the demon cores cancelled bombing run. The second accident occurred when a scientist was performing a criticality experiment on the demon core, which involved bringing the exposed core of a nuclear weapon nearly to the point of criticality, a tricky operation known as “tickling the dragon’s tail”. The scientists who were experimenting on the core, Harry Daghlian and Louis Slotin, both died in nearly identical ways after toying with the hunk of plutonium.

After these incidents, hands-on criticality experiments were stopped, and remote-control machines and TV cameras were designed to perform such experiments with all personnel at a quarter-mile distance. The demon core was intended for use in the Operation Crossroads nuclear tests, but after the criticality accident, time was needed for its radioactivity to decline and for it to be re-evaluated for the effects of the fission products it held, some of which were highly neutron poisonous to the desired level of fission. The demon core was denied its detonation and was instead melted down and reintegrated into the US nuclear stockpile, to be recast into other cores as necessary.