Does a fallout shelter protect you from the initial nuclear missile or only the nuclear radiation after impact?

Answer 1

As the name says, it protects from one thing: the fallout, which is radioactive dust lifted into the atmosphere by the rising fireball and is carried off by the wind then falls out on distant places. Fallout can look like anything from invisible specks that can only be seen using a microscope to sand to white "ash" flakes a half inch in diameter.

A fallout shelter cannot protect from blast, firestorm, or the initial "prompt" radiation. Therefore you must be lucky enough to already be outside any actual target zone (for a hydrogen bomb this distance can be around 10 miles or more). To provide full fallout protection due to the fact that much of the fallout is microscopic dust, all fallout shelters must be operated at positive internal pressure supplied by forced outside air filtered through HEPA filters. As electrical supplies will likely be out following an attack, these air pumps in many fallout shelters will have to be operated by hand 24 hours a day by the occupants for as long as the shelter is occupied (which may in some cases be a couple of months).

I'm not sure what you mean by "protect you from the initial nuclear missile" as the missile itself never came anywhere within 500 miles of its target (and on true intercontinental missiles it probably didn't come within even 5000 miles of its target). The missile only carries the warhead outside the atmosphere, where it then separates from the warhead and quickly falls back down burning up in the atmosphere, only the warhead (a small package that can weigh under 100 pounds but have an explosive yield of 300 kilotons or more) continues from that point on to the target.