There is a pretty good "what if" discussion of this issue available in Pulling Through by Dean Ing. It is a scifi story, but the last 1/3 of the book is an indepth discussion by some writers and engineers on this very issue. They discuss what shelter would be necessary, why, and how to provide a lowtech version at home, as well as how to create a positive pressure room. Also how to create a filtered air supply system with on hand materials, as well as a radiation counter (this last item straight from Civil Defense instruction pamphlet). Pretty interesting reading; I keep a copy of it in our disaster library box.