The first thing we'll look at is what happens during a nuclear explosion and the reasons why someone might build a bomb shelter or fallout shelter.
When a nuclear bomb is detonated, most of the immediate fatalities will be a result of the blast and the intense heat created from the explosion. The damage a bomb can cause varies greatly depending on its size, but a one-megaton hydrogen bomb, for example, would destroy every structure within about two miles of ground zero, and a person five miles away would receive third-degree burns.
Only a structure built to withstand about 50 pounds per square inch (psi) could survive close to ground zero, and the majority of it would most likely be underground. The material of such a shelter would have to very heavy and dense, like lead or concrete.
The lingering danger of a nuclear explosion, however, is the effect of nuclear radiation. This is something people outside of the immediate blast area would have to worry about -- radiation sickness can kill as many or more people than a blast would, but it would happen over a much longer period of time.
When nuclear fission or fusion occurs, many types of radiation are created, including alpha particles, beta particles, gamma rays and neutrons. Alpha and beta particles would mostly be harmless. Although they're fast-moving particles, they're too big to pass through much matter -- alpha particles (helium atoms) can be stopped by a few inches of air or a piece of paper, and beta particles (electrons) can be stopped by plastic or light metal. They only pose a serious danger when they're inhaled or fall onto the food we eat.
Gamma rays and neutrons are much more dangerous following a nuclear explosion. Neutrons are heavier than electrons, and when they break off of atoms from nuclear fuel, such as uranium or plutonium, act like extremely small "missiles" and can easily penetrate matter. Gamma rays are photons, very much like light, except they have more energy and can easily pass through several inches of a heavy element like lead.
When a nuclear bomb hits the ground, a crater is formed, and the earth that used to be there gets pounded into trillions of particles. These particles receive the radiation from the explosion and carry it up into the sky in a huge mushroom cloud. The cloud doesn't stay there or come back down to the ground -- wind pushes it along like any other cloud, and the particles drift down along the way. The dangerous material is actually visible, looking like sand or flakes, and coming into contact with large doses of it is life-threatening.
Using fallout shelters is the best way to protect people from falling radiation. See the next page to learn how they work.