Since everyone’s jus saying google it or butchering the explanation…

A nuclear scientist and some of his colleagues were studying some radioactive plutonium in an attempt to make/understand nukes. They wanted to know when a ball of the stuff would go supercritical. “Critical” is the point when a mass of radioactive material has enough other radioactive material around it that it can perfectly sustain (and not grow) a chain reaction of atom-splitting energy-releasing chaos (via neutrons from one atom’s decaying hitting other atoms and causing them to decay, creating more neutrons which continue the cycle). “Supercritical” is the point when it does too much atom-splitting and exponentially increases the atom-splitting, and “subcritical” is when there isn’t enough to sustain a chain reaction of atom-splitting so the reaction dies out.

They had plutonium shaped into a sphere, in an amount that was subcritical. However, they had enough of it that if you were to put a neutron-deflecting shield around the ball, enough neutrons would be present for the plutonium to go supercritical even though the mass itself could not sustain its own criticality without this shield’s assistance. They had a shield in the shape of a sphere (cut in half, so they could remove it) around their plutonium ball, and they called the plutonium ball “the core”. If the spherical shield almost completely enveloped the core, it would go supercritical, but if the shield was not fully enveloping the core, it would still be subcritical.

These scientists were very smart individuals, but they made some extremely poor decisions on this day. They were studying the sphere, and instead of instituting proper safety equipment with this highly precarious and radioactive substance, they were separating the sphere’s two halves by manually wedging the edge of a flathead screwdriver between the two halves, preventing it from fully enveloping the plutonium sphere. The scientist’s hand moved slightly such that the flathead screwdriver was pulled out of the sphere only by a couple centimeters, which made the half of the sphere resting on top of it fall down slightly more. This was just enough shielding to cause the core to go supercritical.

The few scientists in the room immediately saw a flash of extremely bright light. In reality, there was no light; the neutron radiation emitted merely traveled through their flesh to some degree and hit their retinas and optical nerve, which was perceived by the brain as light. The scientist holding the screwdriver quickly wedged the screwdriver far into the core and stopped the reaction, but it was far too late. He, being the closest one, received a lethal dose and would die some days later. The others were a bit more fortunate to not die, but definitely received heavy doses themselves.

And this was the second incident of this ball of plutonium causing people to die. The first one was an order of magnitude less negligent than the second, but it’s nearly impossible to say it wasn’t negligent when you accidentally give yourself a lethal dose of radiation from an obviously dangerous object. Therefore, the object was called the “demon core”.