Even above this height of burst, there can still be fallout arising from a variety of mechanisms. This height of burst increases with the yield of the weapon but is roughly 55 m/kt 0.4 (~870 meters for a 1 megaton burst). Scavenging could also occur if the nuclear cloud encountered an ambient rain cloud. With high relative humidity, moisture in the cloud could form rain, ice, or snow that could scavenge the fission debris. The amount of dust contaminated and lofted falls off rapidly as the height of burst is increased, primarily because above about 7 m/kt 1/3 there is no crater.Ī burst at or above the fallout-free height of burst, as its name implies, produces aerosolized fission debris but no large particles, because the surface material does not mix with the remnants of the nuclear detonation. A 500 ton surface burst would loft about 500 tons of dust that would be contaminated by the fission debris, whereas a 1 megaton burst would loft 300,000 tons.
The cloud from a 500 ton surface burst could rise to a few kilometers, whereas that from a 1 megaton burst would stabilize in the stratosphere with the top around 20 kilometers. There are four heights of burst (HOBs)/depths of burst (DOBs) of interest for this study of nuclear weapons: (1) a surface, or contact, burst (2) a low-altitude (below the fallout-free limit) airburst (3) an airburst at the fallout-free height of burst and (4) a subsurface burst. The water and ice loading depend critically on the amount of ambient moisture in the atmosphere. There are several critical parameters in determining the fallout pattern: the yield of the weapon, the fraction of the yield derived from fission (versus fusion), the height or depth of burst, the size distribution of the entrained dust, the base surge or material lofted by ground motion (for explosions below the surface), and profiles of atmospheric conditions. The size and intensity of the fallout pattern are heavily influenced by the weapon yield and height or depth of burst and by the meteorological conditions both at the surface and at altitude. After being lofted by the rising fireball, the dust begins to fall out of the cloud, resulting in a fallout pattern on the ground.
Some of this dust will scour the fission debris and become radioactive. But a burst near or under the ground surface will also entrain dust ejected from the associated crater or kicked up by the blast wave. All nuclear bursts produce radioactive fission debris in the form of gases that eventually condense into aerosols.
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