Nuclear fallout

Fallout is the residual radiation hazard from a nuclear explosion and is named from the fact that it "falls out" of the atmosphere in to which it is spread during the explosion. It commonly refers to the radioactive dust created when a nuclear weapon explodes. This radio-active dust, consisting of hot particles, is a kind of radioactive contamination.

Source of fallout

A nuclear explosion vaporizes any material within the fireball, including the ground if it is nearby and this is combined with residual ionizing radiation to produce fallout. The sources of this residual ionizing radiation are:

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• Fission Products. These are intermediate weight isotopes which are formed when a heavy uranium or plutonium nucleus is split in a fission reaction. There are over 300 different fission products that may result from a fission reaction. Many of these are radioactive with widely differing half-lives. Some are very short, that is, fractions of a second, while a few are long enough that the materials can be a hazard for months or years. Their principal mode of decay is by the emission of beta radiation, usually accompanied with gamma radiation. Approximately 60 g of fission products are formed per kiloton of yield. The estimated activity of this quantity of fission products 1 minute after detonation is 1.1 ZBq, equal to that of 30 Gg of radium, in equilibrium with its decay products.
• Unfissioned Nuclear Material. Nuclear weapons are relatively inefficient in their use of fissionable material, and much of the uranium and plutonium is dispersed by the explosion without undergoing fission. Such unfissioned nuclear material decays by the emission of alpha particles and is of relatively minor importance.
• Neutron-Induced Activity. If atomic nuclei capture neutrons when exposed to a flux of neutron radiation, they will, as a rule, become radioactive (neutron-induced activity) and then decay by emission of beta and gamma radiation over an extended period of time. Neutrons emitted as part of the initial nuclear radiation will cause activation of the weapon residues. In addition, atoms of environmental material, such as soil, air, and water, may be activated, depending on their composition and distance from the burst. For example, a small area around ground zero may become hazardous as a result of exposure of the minerals in the soil to initial neutron radiation. This is due principally to neutron capture by sodium (Na), manganese, aluminum, and silicon in the soil. This is a negligible hazard because of the limited area involved.