A 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. With high relative humidity, moisture in the cloud could form rain, ice, or snow that could scavenge the fission debris. Scavenging could also occur if the nuclear cloud encountered an ambient rain cloud. This height of burst increases with the yield of the weapon but is roughly 55 m/kt0.4 (~870 meters for a 1 megaton burst). Even above this height of burst, there can still be fallout arising from a variety of mechanisms. The potential for long-term fallout on a global basis is well recognized from atmospheric tests, although dose-rate levels would be much lower than those from the ionized fallout from surface or subsurface bursts, with a potential impact on latent cancer rates.

A subsurface burst introduces additional processes for mixing of fission debris and dust. A main “mushroom” cloud like that from a surface burst is formed if the subsurface burst is not too deep (Figure 5.1); a low-level base surge is formed as the shock wave breaks through the surface (Figure 5.2). Thus, the subsurface burst may create two radioactive clouds that do not merge. As the depth of burst increases, the amount of activity vented to the atmosphere is reduced until the burst is completely contained, if the emplacement hole through which the device has been lowered has been carefully stemmed (70 meters for a 1 kiloton burst), which of course would not be the case for a weapon used to attack a target.

The primary short-term hazard from fallout is the external dose received from gamma rays. In addition, an internal dose can also be received through inhalation or ingestion of debris emitting beta and alpha radiation. Under most conditions, the external dose is the more dangerous, but the internal dose could have latent effects, specifically, cancer.

The total gamma ray activity in a measured fallout pattern is usually stated in terms of exposure rate in roentgens per hour (R/h) at 1 hour after the burst as if that activity were uniformly spread over an area of 1 square kilometer. For a 1 kiloton surface burst, this value is on the order of 9,000 R/h per square kilometer. Of course, the area of the fallout pattern is much larger than 1 square kilometer. The area covered by the associated dose that would cause at least a 50 percent probability of fatality is roughly 2.6 square kilometers per kiloton, assuming that people are in the open and exposed for just the first day after the burst. Over time the radioactivity decays, and eventually the fallout hazard decreases. Some radionuclides, such as cesium-137, have long half-lives, but most of the hazard is due to short-lived radionuclides. Within 1 to 24 hours after the burst, for example, the total gamma ray activity decreases by a factor of about 60.

The following sections focus on the problem of fallout associated with surface and subsurface bursts.

Nuclear Clouds

Nuclear clouds contain three classes of materials—radioactive materials, dust and pebbles, and water and ice. The first is by far the smallest contributor to the mass of the cloud, but it represents the greatest residual hazard and therefore is of prime importance. The radioactive materials consist of fission products from the weapon, as well as activation products produced by the interaction of the weapon’s energetic radiation with the surrounding and/or nearby materials. When this radioactive material becomes attached to larger particles, it can result in fallout.

The dynamics process of the nuclear cloud starts with the ejection and then lofting of surface material. The cloud rises, entraining ambient air and moisture, cooling in the process. This cooling causes the materials to condense on nonvaporized residual material and causes these molten particles to coagulate into larger sizes. As the rising fireball and nuclear cloud continue to rise and entrain ambient

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