A short version of the story:

America developed nuclear weapons in WWII, and they made more spherical plutonium "cores" than they ended up needing, so some nuclear physicists ended up using one of the spares for a series of experiments in criticality. Two of said scientists, in two separate experiments, negligently gave themselves fatal doses of radiation via accidental criticality incidents.

One was stacking bricks around the core which would reflect free radicals back at the core to bring it close to, but not quite to the point of criticality. As he was placing the last brick, his instruments told him it was about to go critical, so he started moving the brick away, and then dropped it. The core went critical, there was a blast of heat and brilliant blue light from Cherenkov radiation, and he spent the last couple weeks of his life in a hospital as his body turned to mush.

Another physicist was doing a similar experiment, this time enclosing the core in two metal half-spheres, and adjusting the distance between the half spheres with a flathead screwdriver. He had removed the spacers designed to prevent the shell from closing completely and causing criticality so he could get closer. And the screwdriver slipped. Heat, blue light, physicist spends the next couple weeks dying hideously in a hospital.

The plutonium sphere was nicknamed the "demon core" and its fate is...undisclosed. A popular youtuber reported in a popular video on the topic that it was melted down and returned to the US nuclear stockpile where its fate becomes impossible to track, then later reported that someone in that field replied to that along the lines of "Is that what they said happened to it? Hahaha interesting."

The demon core wrapped in its half-sphere criticality experiment hardware has become internet shorthand for something that is egregiously reckless.

TL;DR: Of the plutonium cores that were made for America's nuclear weapons program, the one that was named the Demon Core wasn't one of the ones that blew up a Japanese city, it was the one two white guys killed themselves with.