assumptions seem to be contradictory in view of certain facts concerning cloud rise.
For
example, the rise rate of clouds increases with yield. If the activity is associated with
the same particle~size distribution for all yields, it would seem that particles of a given
fall rate would be left behind the rising cloud at relatively lower cloud heights for smaller
yields. Thus, if there is only one fall-rate distribution for all yields, vertical distribution
of activity should differ for different yields.
However, it is known that the speed of turbulent after-winds, which draw in and mix
debris with active material inside the fireball, increases with yield. Thus, for a larger
yield, a particle of given cross section sweeps out a greater volume in a given time and
has a correspondingly higher probability of colliding with other particles to form relatively
large fallout particles. Thus, it seems likely that the usual assumptions regarding vertical activity distributions and activity fall-rate distribution are both wrong for close-in
fallout.
The errors produced are apparently such that close~in fallout intensity is underestimated by scaling from small yields to large yields. Within the available experience for
one type of surface, the larger the yield difference, the larger the scaling error.
An interesting question from a military point of view is whether this apparent trend is
real and continues up the scale of yield into the megaton range. If so, the present estimates of close-in intensities from megaton yields could be significantly low.
3.11 DYNAMIC FALLOUT MODEL
Only about 4 percent of the total activity produced by Shot Fig was accounted for within
the instrumentation array. However, this included all of the activity associated with fall
rates great enough to cause intensity levels of military interest. For these reasons, only
4 percentof the total activity produced is incorporated in the dynamic fallout model proposed for use with any wind structure to estimate close-in fallout from fission weapons of
1- to 100-ton yield. The main features of the model are:
1. -Fall-rate distribution for the activity (3 x 10° r/hr-ft? per ton of fission yield) is log
normal with o = 0.48 and f) = 24 knots.
d
Where:
() =
1
=
Thatis,
- E al
e
20 2
f
°
(f) = fraction of activity associated with the fall-rate range f to f+ df.
2. All activity is associated with that part of the visible cloud that corresponds to the
region between h, and hy shown in Figure 3.2. Scaling of hy and hg with yield should be
made by a '/.-power law using Fig data as the basis. Scaling for diameter should be made
by a 44 -power law. For all yields, equal time is required for h»), hy, and diameter to
reach a given fraction of their maximum values.
3. Activity is assumed to rise until the rise rate of the cloud corresponds to the fall
rate of the activity, at which point free fall commences.
This model is reasonably consistent with the measured amountof activity deposited as
a function of downwind distance, intensity as a function of downwind distance, cloud shine
measurements, and cloud drift and growth for Shot Fig.
Calculations have been made that utilize these assumptions and some approximations ta
allow relatively easy hand computation. The Fig yield
and cloud dimensions were
46
i