For a sizable nuclear exchange you would be wasting your time without filtered air and a very large supply of food and water. Even then your prospects are not good after supplies are depleated. Same can be said for a large asteroid impact. You need air filtering even with a dirty bomb or chemical spill that's upwind of your location. That requires a power source to force air through a filtering system. You will need fuel for that power source. Don't count on solar cells and the associated converters / batteries because their electronics may be taken out by EMP.

More realistically is the need for shorter term shelter in case of tornado, huricane, or other natural disaster.

If B-movies have taught me anything, your okay until the zombies come calling. Then you need those Russian Guns? and lots of ammo.

me and my dad were planing on building a underground cold room for our steers out if 2ftx2ftx3ft cement blocks for $35 a picece. we figgered we get a cold room on the cheap and then have a tax right off as a storm/bomb shelter

Sorry! Left part of it off!
http://www.deepearthbunker.com/survival-pods.html

I don't know if it's only because of all the zombie movies nowadays but when I'm sitting in the blind I often find myself thinking about what I would do if there was a doomsday.