I wonder about the effect of a bomb (nuclear or otherwise) hitting or detonating at the worst possible distance from a nuclear power plant might be? I'm imagining if it was powerful enough it'd pull a lot of that radioactive material up and out...
In general, if a bomb is powerful enough to result in a major radiation release from a nuclear power plant, then it would probably do more damage if used on a major population center. There are measures you can take to prevent people from getting too irradiated by a dirty bomb, from evacuation to simply staying indoors and wearing a face mask to reduce particulate inhalation. Straightforward explosions, in contrast, do not offer quite as many second chances.
(BTW, on the subject of surviving a nuclear blast: duck and cover. It works. There were some crazy-b...
Just a reminder that some of the old threats are still around (and hence that AI is not only something that can go hideously badly, but also some thing that could help us with the other existential risks as well):
http://blog.practicalethics.ox.ac.uk/2012/03/old-threats-never-die-they-fade-away-from-our-minds-nuclear-winter/
EDIT: as should have been made clear in that post (but wasn't!), the existential risks doesn't come from the full fledged nuclear winter directly, but from the collapse of human society and fragmentation of the species into small, vulnerable subgroups, with no guarantee that they'd survive or ever climb back to a technological society.