Blast and Fallout Concerns
The blast wind produced by a nuclear bomb will reach 2,000 mph within the first half mile from ground zero, drop to about 1,000 mph at 2 miles, and will still be at hurricane force (200 mph) several miles out. So, unless you know of an impending attack your chances for preparing for the blast will rely upon swift reaction to the super bright flash produced by the bomb.
If you see a bright flash, you should immediately duck and cover. After that, head for the bomb shelter. The fallout that will arrive in as little as a few minutes (distance-dependent) will be extremely hazardous. In fact, the radiation carried with the dust and dirt will very capable of penetrating several inches of steel, your house, your car, and most buildings.
An underground bomb shelter emplaced with its roof at least 36 inches below the surface will provide sufficient protection from radiation. If you are inside your bomb shelter within a few minutes following the attack, you will live. If you are outside, it's curtains. If you stand outside at a point of about 10 miles from the blast, you will likely survive the blast wave from any nuclear strike. But, within 30 minutes the first flakes of radioactive dust will begin to fall on your head. If you are exposed you will die an hour later from internal damage caused by the radiation found in the fall out particles. Those who move inside the underground bomb shelter will simply have to wait about 4 days before emerging unscathed.