A fallout shelter is an enclosed space specially designed to protect occupants from radioactive debris or fallout resulting from a nuclear explosion. Many such shelters were constructed as civil defense measures during the Cold War.
After a nuclear explosion, matter vaporized in the resulting fireball is exposed to neutrons from the explosion, absorbs them, and becomes radioactive. When this material condenses in the cloud, it forms dust and light sandy material that resembles ground pumice. The fallout emits both beta particles and gamma rays. Much of this highly radioactive material then falls to earth, subjecting anything within the line of sight to radiation, a significant hazard. A fallout shelter is designed to allow its occupants to minimize exposure to harmful fallout until radioactivity has decayed to a safer level