Context: https://en.wikipedia.org/wiki/Demon_core https://en.wikipedia.org/wiki/Demon_core tl;dr: prototype warhead core, on which people were doing very risky

Context: https://en.wikipedia.org/wiki/Demon_core https://en.wikipedia.org/wiki/Demon_core tl;dr: prototype warhead core, on which people were doing very risky experiments, which underwent some criticality events by accident, releasing lots of radiation which killed some of them. It was eventually detonated in a weapons test in 1946.