When a nuclear power accident occurs what happens?

That would vary with reactor design, where the original problem occurred, operator training and response to the problem, and many other variables. An accident can vary from a tiny release of radioactive gasses that is stopped immediately, through steam ejection of a fuel rod whch impales a single worker onto the ceiling, through a massive steam explosion followed by a graphite fire (all three of these have happened), to a worst case complete core meltdown that melts down to the water table causing massive contamination of drinking water for hundreds of miles (this has not happened).