Page:Operation Crossroads 1946.pdf/30

 theoretically continues to exist forever (albeit in diminishing amounts), a point is reached where it is practically undetectable.

Overall radioactivity of all the fission products formed decays at a rate that is closely approximated by a rule that states that for each sevenfold increase in time the intensity of the radiation will decrease by a factor of ten. Thus, a radiation rate of 1 roentgen per hour (R/hr) at 1 hour after the detonation would be expected to be 0.1 R/hr after 7 hours and 0.01 R/hr after 49 hours. This rule seems to be valid for about 6 months following a nuclear detonation, after which the observed decay rate is somewhat faster than that predicted by this relationship. Activation products, in general, decay at a faster rate than the fission products.

Fission products and the activation products, along with unfissioned uranium or plutonium from the device, are radioactive components of the material in the debris cloud. This cloud and its fallout are the primary sources of potential exposure to residual radiation.

In a nuclear airburst, the central core of intensely hot material, or fireball, does not touch the surface. The bomb residues (including the fission products, the activation products resulting from neutron iteration with device materials, and unfissioned uranium and/or plutonium) are vaporized. These vapors condense as the fireball rises and cools, and the particles formed by the condensation are small and smoke-like. They are carried up with the cloud to the altitude at which its rise stops, usually called the cloud stabilization altitude. Spread of this material then depends on the winds and weather. If the detonation is of relatively low yield, the cloud stabilization altitude will be in the lower atmosphere and the material will act like dust and return to the Earth's surface in a matter of weeks. Essentially all debris from detonations with yields equivalent to kilotons of TNT will be down within 2 months (Reference A.9). Areas in which this fallout material will be deposited will appear on maps as bands following the wind's direction. Thus, airbursts result in less potential for residual radiation exposure to personnel at the testing area from the debris, although there may be some residual radiation fission products from rapid settling of large particles and short-lived radiation coming from activated surface materials under the burst (if the burst altitude is sufficiently low for neutrons to reach the surface).

Underwater nuclear detonations are muffled by the great mass of water that surrounds them. Initial nuclear radiation is absorbed by the water surrounding the device and the intense heat vaporizes the water near the burst. This forms a bubble beneath the surface of the water that expands as the energy released in the explosion works against the mass of water. This expansion continues until the energy is expended, at which point the bubble begins to collapse as it rises toward the surface. Depending upon the depth of the burst and the size of the bubble (which in turn depends on the detonation yield, or total energy released), the bubble may break the surface of the water near its fully expanded size or smaller. Some radioactive products (including activated salt) are vented into the air as the bubble breaks the surface, but most of the device debris and activation products remain trapped in the volume of water that collapses on the bubble. This volume of water is called the radioactive pool.

25