Advice for the Public on Protection in Case of a Nuclear Detonation
https://www.icrp.org/page.asp?id=611
summary 1 - if you survive the blast - radioactive fallout will arrive
in 10 minutes or less - stay away from the fallout for at least 24
hours
summary 2 - if you have warning of an impending blast - take shelter
in a basement or the core of a building