fired at or near the surface of the ground, a large fraction of the fissionproduct activity is deposited within the first day in the general area of the
test site, due to condensation and scavenging by the large particles sucked
into the rising fireball.
Those particles which are too small to have
appreciable fall rates are carried away from the burst site by the winds.
For detonations with yields of less than 1 megaton, all of these small
particles are left in the troposphere, where they are eventually removed from
the air by precipitation scavenging and impaction at the ground.
There is
general agreement (Lippy, 63) Stewart et ai,(*) National Academy of Seiences‘>)
and many others) that the mean lifetime of a particle in the troposphere, be-
fore it is removed,is about a month.
Here, however, we are concerned primarily with the effects of very large
yield bursts, those powerful enough to inject large quantities of debris into
the stratosphere, i.e., ylelds greater than one megaton, and in particular
with the fate of the Sr-90 resulting from these bursts.
Strontium-90 is singled out, of course, because it is thought to repre~
sent the principal long-term potential biological hazard from nuclear testing.
However, this isotope is also of particular interest in studying stratospheric
motions because of apparent very small particle size.
It is produced as a
daughter product of Krypton-90 as follows:
Kr-90 33.85 ppago?LMM sy9028 YF y-goHFPF zpgol stable)
This means that the Sr-90 will tend to condense later than the bulk of the
fission products, resulting in the formation of sub-micron particles which
are easily carried into the stratosphere by the ascending cloud, and which
may heave negligible fall rates.
.
BAS
Stratospheric injections of Sr-90, as estimated by Libby!) are shown
in Table 1.
These estimates are based on a series of assumptions and specu-
lations concerning the nature of the fallout.
The stratospheric content of
Sr-90 is obtained by the subtraction of local and tropospheric fallout from
the total Sr-90 radioactivity produced by nuclear explosions whose clouds
enter the stratosphere.
Local fallout is assumed to be 80 percent for land
surface shots, 20 percent for surface water shots and 10 percent for air shots.
Shots of more than a megaton yield are assumed to put 1 percent of the debris
which did not fall out locally into the troposphere and the remainder into
the stratosphere.
Shots in the kiloton range are assigned to the troposphere.
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