I can't speak for the last two Greg, but it would probably take awhile for a nuclear power plant to become a real danger to the populace once everything shuts down.  Unlike the problems going on in Japan, there wouldn't be any actual damage that would cause any sort of containment breach.  This actually came up in game I (attempted) to get off the ground awhile back.  It could take at least a month before the stations became a real danger to the surrounding area or for a meltdown to occur.  No massive explosions, as power plants are designed<em>not</em>to blow up, but there would definitely be some nasty radioactive material to contend with inside the buildings themselves.