Comment by dongobongo

Meltdown accident: basically, reactor is turned off, however heat continues to be generated because of a thing called decay heat which is when isotopes generated by the fission reactions decay to more stable isotopes and release energy. It's about 7% of a fission reactor's power and continues for a few hours until it's negligible. 7% of a gigawatt reactor is like having a couple of jet engines going full blast inside the core. This heat has to be removed, and meltdowns happen when people fail to do so - basically pumps break, coolant leaks, or coolant is blocked from cooling down the core. Recent micro reactors get around this because they don't need active coolant or people to cool down the reactor - they just cool off by conduction or simple heat rejection systems. I read recently that fusion reactor will also generate decay heat from all the activated components and this is comparable to a fission reactor. The difference is there's a lot less radioactive crap in a fusion reactor - but the fusion reactor will still meltdown and they are expensive...