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.

Factors affecting fallout

Location

There are two main considerations for the location of an explosion: height and surface composition. A nuclear weapon detonated in the air (air burst) will produce less fallout than a comparable explosion near the ground. This is due to the fact that less particulate matter will be contaminated and kicked up by the explosion. Detonations on or below the surface will tend to produce more fallout material.

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In cases of water surface (and shallow underwater) bursts, the particles tend to be rather lighter and smaller and so produce less local fallout but will extend over a greater area. The particles contain mostly sea salts with some water; these can have a cloud seeding effect causing local rainout and areas of high local fallout. Fallout from seawater is unusually dangerous because it is difficult to remove by washing.

Related Topics:
Cloud seeding - Rainout - Seawater

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For subsurface bursts, there is an additional phenomenon present called "base surge." The base surge is a cloud that rolls outward from the bottom of the column produced by a subsurface explosion. For underwater bursts the visible surge is, in effect, a cloud of liquid (water) droplets with the property of flowing almost as if it were a homogeneous fluid. After the water evaporates, an invisible base surge of small radioactive particles may persist.

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For subsurface land bursts, the surge is made up of small solid particles, but it still behaves like a fluid. A soil earth medium favors base surge formation in an underground burst. Fallout can spread for long distances with the help of wind currents.

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Meteorological

Meteorological conditions will greatly influence fallout, particularly local fallout. Atmospheric winds are able to distribute fallout over large areas. For example, as a result of a surface burst of a 15 Mt thermonuclear device at Bikini Atoll on March 1, 1954, a roughly cigar-shaped area of the Pacific extending over 500 km downwind and varying in width to a maximum of 100 km was severely contaminated.

Related Topics:
Meteorological - Mt - Bikini Atoll - March 1 - 1954

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Snow and rain, especially if they come from considerable heights, will accelerate local fallout. Under special meteorological conditions, such as a local rain shower that originates above the radioactive cloud, limited areas of heavy contamination just downwind of a nuclear blast may be formed.

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