Well there have been accidents obviously. Japan being the most major.
But ask yourself what happens when something happens that causes a disruption to a nuclear plant on a major scale. It could be war. it could be a natural disaster. What happens when those cooling pumps sudden shut down and there is no way to maintain power or keep them going.
How about a meteor strike. How about if someone drops a nuclear bomb just close enough to prevent normal access to and normal operation of the plant. Lots of possibilities if one thinks about it.
There are actually failsafe BWR designs that can fall back on passive cooling if the pumps die.