The detonation of nuclear weapons above the earth sends radioactive materials up to 50 miles into the atmosphere. Large particles fall to the ground near the explosion site, but lighter particles and gases move to the upper atmosphere. The isotopic signature of a bomb's rain is very different from that of a serious accident in a power reactor (such as the one in Chernobyl or Fukushima). The consequences can also relate to nuclear accidents, even though a nuclear reactor does not explode like a nuclear weapon.
The ACS then had to choose between active and static systems to protect the public from nuclear rain. A limited form of nuclear warfare would be like conventional conflict on the battlefield, but using low-performance tactical nuclear weapons. Sensors can fail and the results of a lack of preventive measures would cause local nuclear fallout. A nuclear weapon that explodes at a high altitude does not produce any of the explosion or local rain effects just described.
A nuclear war would produce enormous quantities of ozone-consuming chemicals, and studies suggest that even modest nuclear exchange would lead to unprecedented increases in ultraviolet exposure. Nuclear fallout is the residual radioactive material propelled into the upper atmosphere after a nuclear explosion, so called because it falls from the sky after the explosion and the shock wave has passed. Nuclear explosions also produce clouds of dust and sand-like radioactive particles that disperse into the atmosphere, known as nuclear rain. The exact distribution of precipitation depends crucially on the speed and direction of the wind; in some conditions, lethal rain can extend several hundred miles downwind of an explosion.
The most immediate effect of a nuclear explosion is an intense burst of nuclear radiation, mainly gamma rays and neutrons. In the 1950s and 60s, the United States Atomic Energy Commission (AEC) began developing nuclear fallout safety standards for civil nuclear reactors.