Stay inside for 24 hours, unless other instructions are given by local authorities. The u.s. department of health and human services recommends staying home for at least 24 hours in the event of a nuclear explosion. After 48 hours, the exposure rate to a 10-kiloton explosion (the kind that could damage but not destroy a city) drops to just 1%.
An accident at a nuclear power plant, a nuclear explosion, or a dirty bomb are examples of radiation emergencies. If something like this happens, you may be asked to enter a building and take shelter for a while instead of leaving. The walls of your home can block much of the harmful radiation. Because radioactive materials weaken over time, staying indoors for at least 24 hours can protect you and your family until it's safe to leave the area.
Entering a building and staying there is called sheltering in place. Nuclear explosions can produce clouds of dust and radioactive sand-like particles that disperse into the atmosphere, known as nuclear fallout. Everyone should know how far they live and work from major nuclear power plants and potential nuclear attack sites. Nuclear explosions produce a powerful phenomenon called a nuclear electromagnetic pulse (PEM), an invisible burst of energy that can cut off power lines, telephone and Internet.
In a nuclear attack, a nuclear bomb is detonated in the air or on the ground, causing a devastating explosion.