Don't quite understand how a nuclear bunker is useful. MAD means you would have maybe 30 - 60 minutes to get into them? Plus, if I was attacking the country where the bunkers are located, I would make sure to blow them sky high, since I can't verify what is in them. Maybe in a pandemic they are useful, although using SARS-CoV-2 as an example, you can't be sure you aren't locking yourself up with someone carrying the virus.