RADIATION STANDARDS, INCLUDING FALLOUT
35
The local fallout is material that falls out close to the point of
detonation, tropospheric fallout travels around the world and the
stratospheric fallout which behaves quite differently from the local and
tropospheric fallout.
he relative abundance of each is determined by the nature of the
weapon, its yield, and the conditions of detonation, particularly the
altitude.
Local or near-in fallout occurs when the fireball of the bomb
touches or comes sufficiently close to the ground to draw up into the
vaporized cloud matter from the surface of the earth. The radionuclides produced by the explosion may be deposited on or incorporated into this material depending upon whetheror not the latter
is more or less completely fused or totally vaporized. After the fireball cools, the larger particles deposit on the earth over a period of
from a few minutes up to many hours after the detonation. This
constitutes the local or near-in or early fallout which for small
weapons may extend out from the point of burst a few miles and cover
tens to a few hundreds of square miles while for megaton weapons
it may extend out to several hundred or more miles and cover
thousands of square miles.
Tropospheric or latitudinal fallout is a more delayed fallout of
the debris which has not penetrated the tropopause to the stratosphere
and the particle size of which is such that it does not fall rapidly.
It occurs over a period of 2 weeks to a month or so after a detonation
andconsists of relatively fine material (a few micra to small fractions of a micron) suspended in the lower part of the atmosphere,
the troposphere, where rain and other weather phenomena occur.
It is carried around the world in the same general band of latitude
as that of its origin although excursions of tropospheric debris as far
as 20° to 30° from the test latitude are not uncommon. Tropospheric
fallout from tests at temperate or polar latitudes does not in any
significant amount cross the equator. It is deposited on the earth’s
surface by weather events, principally rain or snow and possibly to
some extent by dew, dry deposition being important only in relatively
arid areas. Thus, the distribution of tropospheric fallout is determined generally by the paths of air masses passing over thesite of
formation, with factors such as local weather conditions and distance
traveled (in thousands of miles) determiningregions of greater (“hot
spots”) or less concentration.
Tropospheric fallout has contributed significantly to the radioactive debris in the Northern Hemisphere as a result of weapons
testing in the Pacific, in Nevada, and by the U.S.S.R.
The principles stated above for near-in and tropospheric fallout
were apparent at the time of the 1959 hearings. Since then there has
been some refinement in our knowledge of the tendency for local or
near-in fallout to contain greater amountsof certain fission products
as compared to others, and the tendency for the closer in fallout
particles to have certain nuclidesin a less soluble form than the smaller
particles which predominate farther out. This affects the availability
of these nuclides for early incorporation into the food chain via the
soil. Additional information on these points has been developed from
analysis of data collected prior to 1959. Local fallout consists in large
part of larger particles which tend to fall out first. These particles are
somewhat poorer in Sr © as comparedto the finer particles which fall
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