Three Mile Island and Chernobyl come to mind. I can't help wonder if there weren't other, lesser accidents that were covered up, if you know what I mean. Not exactly an "accident" but the ground in and around the Hanford Nuclear Reservation in eastern Washington State will be radioactive until further notice. This can't be the only place in the world where this is the case. Nuclear energy is mostly safe. It's the unsafe bits papered over by that mostly that worry me.