While not a reactor, there's also the criticality accidents with the so-called Demon Core, aka the core for what would have been the third bomb to be dropped on

While not a reactor, there's also the criticality accidents with the so-called Demon Core, aka the core for what would have been the third bomb to be dropped on Japan. https://en.m.wikipedia.org/wiki/Demon_core https://en.m.wikipedia.org/wiki/Demon_core