It depends what aspect of nuclear power you are seeking to learn about: (Not sure about slashdot's policy on posting links) Wikipedia: Nuclear_Safety has a pretty good run down of the safety features. If you're interested in the terminology and general theory behind nuclear reactors, check out Wikipedia: Nuclear_reactor_physics Based on my experience in the Naval Nuclear program, the information presented in those articles should be good enough. If you want information on the physical layout of the reactor systems, go to the NRC website, under "Reactors" --> "Power reactors."

FYI supercritical is NOT worse than prompt critical. Supercritical means power level is going UP (more atoms being split as time goes on). Prompt critical is when the reactor is critical on prompt neutrons alone. The severity of this is affected by the delayed neutron fraction, but it's never a good thing. Look up the difference between prompt and delayed neutrons to learn more about this. The problem with nuclear power is that understanding it requires more learning than most people are willing to do, and so they fear it instead.