Underground Bunker Design Considerations

Above ground bomb shelters and bunkers are possible. You need an air filtration and a positive pressure system. Nuclear fallout typically consists of particles of a pretty large size that can be filtered out via pretty conventional means of HEPA filters and other off the shelf industrial air filtration devices. And it is not uncommon even for the military to build bomb/fallout shelters more or less above ground. Even in the worst case scenario you are unlikely to need to spend more than a couple weeks in a shelter before it becomes relatively safe outside. Radiation from a nuclear bomb tends to go away rather quickly as the more dangerous shorter lived radioisotopes expire.