I really doubt...
..that there will be the mass panic that you predict. Any more than the mail system shut down during the anthrax scare. I've never heard of anyone who thinks that an A-bomb going off will kill off the world.
It's a really good idea to be concerned about the NBC forms of warfare, since terrorists are trying so hard (successfully, in some cases) to upscale to that arena.
I hope that if someone who read your post finds themself in an affected area, they are not discouraged (by your post) from rapidly vacating that area.