The result is the formation of a crater with a considerable amount of
close-in fallout and throwout.
Since in cratering the cavity never stabilizes, a significant amount of
fused rock will be present in the fallout, as well as species that are
volatile, primarily present as a surface coating on many of the
particles formed during the fracturing and spallation of the overburden.
The fraction of activity, including transuranics, released to the fallout
field depends on the yield and the depth of burial (scaled to 1 kiloton).
The optimum emplacement depth of the device for producing
the largest
crater is approximately that depth for which spallation and gas acceleration contribute about equally to crater formation.
This depth is not
necessarily the depth at which the maximum amount of radioactivity
appears in the fallout, however.
When the emplacement of the device is
at shallow depth so that the cavity or even the fireball breaks through
the surface, the characteristics of a surface burst are approached.
However, fallout from a surface burst and from bursts above the surface
is better understood by an approach starting with a consideration of
free-air bursts.
For a more extensive discussion of free-air bursts, see
Glasstone (1962).
A free-air burst is defined as a burst in which the fireball does not
interact with the land or water surface beneath it.
Here, we limit the
definition to include only those bursts occurring at an altitude such
that no surface materials enter the fireball before it has cooled to at
least the solidification temperature of the vaporized species resulting
from the chemical interaction of the device materials with the air.
As the fireball increases in size and cools, the vapors condense and
form a cloud of solid particles of device debris.
Thus, the solid
contents of the cloud consist almost entirely of highly radioactive
device debris in the form of small, generally smooth, round particles
having an approximately lognormal size distribution with a geometric
mean (median) diameter of about 0.14 pm and a geometric standard deviation of about 2.1 (Nathans, 1976a).
The radionuclide concentrations
more or less follow the radial distribution theory of Freiling in the
particle size range above several micrometers; that_is, the concentrations have a particle size dependence as (diameter)
, where m lies
between 0 and -1, depending upon the volatility of the radionuclide
species condensing in the fireball.
However, in the particle size range
below a few micrometers, the radionuclide concentrations increase quite
sharply with decreasing particle size (Nathans, 1971).
Because of the
small size of the particles that are formed, a free-air burst leaves
virtually no local fallout.
Usually, as the cloud rises, some of the particles are left behind to
form a "stem.'' There is some evidence that the mean size of the parti-~
cles in this "stem" is a little larger than that of the particles in the
main cloud (Nathans, 1971).
In addition,
the cloud rise causes the
appearance of a strong updraft in its wake with inflowing winds ("afterwinds").
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