Understanding Radiation: Radiation and Fallout
Fallout is simply the dirt and dust which falls to the ground following a nuclear explosion. It will "charged" with radiation and will eventually "burn" itself out - a process that will take several days.
Radioactive fallout will fall in a manner similar to that following a volcanic eruption. It will be flaky in appearance and its size may reduce to dust particles or smaller. Expect it to be thicker near the detonation site and thinner as it travels down wind.
The bad news about fallout is that it's radioactivity can penetrate thick surfaces (including steel, wood, and earth) even though the actual dust from the fallout can not. In short, if you are exposed outside a shelter to an amount of just 400 R/hr, you will be dead within a few hours. The good news is that fallout's radioactive properties reduce to near normal levels around about 48 hours.
This is where an underground bomb shelter comes into play. Ideally following a nuclear explosion in which you have survived the initial blast, you would simply collect your family inside your "by-the-book" constructed shelter and wait it out. Four days later, you come out and start rebuilding your lives.
Understanding fallout and what you need do to be near 100% free of its dangers is critical. The construction of your underground bomb shelter should satisfy the amount of barrier you need to survive. Basically, you should build you shelter so its roof is at least 48 inches underground (36 for undisturbed soil). Whether you use steel-reinforced concrete or a layer of lead, the 3 to 4 feet of soil will provide effective protection and act as the first barrier for keeping radioactive elements from entering your body.