If I had time to worry about a nuclear event, the number one tool I'd want, short of a bona-fide fallout shelter, is a fallout radiation meter.

You need it to tell how bad the fallout is, how well your improvised shelter is working and if it needs to be improved, how long you can spend outside it, etc.

While those keychain radiation detectors or an improvised "KFM" meter would be better than nothing, they do not respond quickly enough to do an effective radiation survey. Although, it would serve to keep you busy.

If I was writing a novel, I think I'd have some of the characters take shelter in a warehouse. They would use a forklift to stack some skids of massy product into a very effective shelter. Better make it a canned food warehouse...