I have a slighty different take on the dirty bomb issue. I did environmental cleanup for a while, mostly groundwater but a bit of nuke work.

If the contaminate is very finely divided and air dispersed, cleanup of the old porous concrete and brick is going to take more than a firehose and coat of paint. Even if it's just an alpha and beta emitter, IMO the affected areas would end up demo'ed - you guys know 'acceptable levels' at the plant will not be acceptable on the street. I don't think it would be a big killer like dropping the WTC, but if half a dozen blocks in midtown Manhattan have to be demo'ed that's a truly effective terrorist attack.