When a nuclear power accident occurs:

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)