What, do you want to encourage unsafe demon core handling?
The second supercriticality event, as described in the wiki article Tapper linked, was caused because the "cowboy" handling the core dropped the beryllium hemisphere that is a neutron reflector, closing a full reflector sphere, something that causes supercriticality (I.E. a nuclear reaction and the resultant burst of radiation) because the only thing he had propping the two hemispheres apart was a screwdriver, which slipped off. There should have been something solid keeping the hemispheres apart, but it was just a screwdriver because, again, the guy doing it was a self-styled "cowboy" who liked messing with nuclear reactors in jeans and cowboy boots to look cool. His horrible death by radiation poisoning encouraged actual rules be made for this sort of shit, and counts as one of those serious Darwin Award winners.
The key point that artists get wrong is that neutrons striking the fuel will start a chain reaction - it's safe when the reflector sphere is open to let neutrons escape, while neutrons will trigger a nuclear chain reaction if they are reflected by both hemispheres being closed together until they hit more fuel, and the disaster was caused by dropping the top so it closed. A lot of people who reference the image, however, seem to think he was just breaking into something that is supposed to be a sphere by prying apart the core with a screwdriver like they were breaking open something that was safe when closed.