[hga]

More like 2.3 miles for a 1MT airburst, that's just for a 15 PSI blast shelter. If you're building these for real, with concrete and so on in a real Civil Defense program, that's easy. It's pretty easy, if you have the time, to build an expedient blast shelter with wood and earth that'll handle 15 PSI.

And most warheads are less powerful (see my other comments); using my handy "RAND" nuclear effects calculator from the back of my copy of The Effects of Nuclear Weapons, 15 PSI for the following weapon air bursts (targeting cities, not ground bursts against ICBMs etc.):

  500 Kt: 1.8 miles
  300 Kt: 1.5 miles
  100 Kt: a bit over 1 mile
   50 Kt: 0.85 miles

So we're really talking a mile or two, depending on the standards to which the shelters are built. 1 MT 1 mile requires 45 PSI, call it 50 per the above. Don't know how intense a real shelter would have to be, then again that's an uncommon threat level.