Chapter 4
CONCLUSIONS AND REC OMMENDATIONS
Shot Fig provided sufficient data to determine the general characteristics of fallout of military importance for small yields.
The scaling techniques in Reference 1 lead to gross overestimates of downwind extent
for H + 1 hour dose rates of 100 r/hr and lower for the particular conditions of Shot Fig
.
5-knot winds). Apparent reasons for the failure are: (1) Cloud
height for Shot Fig was considerably greater than that implicitly assumed by the scaling
technique. (2) No accounting is made for the drift of the cloud during its rise. (3) Activity
distributions with height and fall rate are not similar over the largeyield range involved.
Scaled estimates are more nearly correct for intensities greater than 100 r/hr.
A dynamic fallout model is proposed (Section 3.11) to estimate close-in fallout from fission weapons of 1- to 100-ton yield. This model accounts for cloud drift and growth and
is consistent with Fig measurements of the amountof activity deposited as a function of
downwind distance, intensity as a function of downwind distance, and cloud shine.
Calculations made on the basis of this model
/
show that intensi-
ties greater than 100 r/hr increase in downwind extent as windspeed decreases, whereas
intensities less than 100 r/hr reverse this behavior. This is not consistent with present
techniques of accounting for variation in windspeeds where the downwind extent of any intensity is increased with the increase of windspeed. It is recommended that the dynamic
model be used to estimate effects of windspeed variation.
Winds that existed during Shot Fig exhibited a very small directional shear, and mean
windspeeds ranged from 11 to 16 knots when averaged from pertinent altitudes to the surface. The visible cloud had not reached its ultimate height of 6,000 feet by the time it had
drifted beyond the instrumentation array, some 10,500 feet from ground zero.
Only 4 percent of the fission products formed was accounted for within the array. From
cloud shine measurements, it was deduced that most of the activity was carried beyond the
array in the lower portion of the visible cloud between 0.3 and 0.7 of its maximum height.
The earliest measurement obtained within the crater itself was at H+ 3.5 hours, at
which time a monitor descended inside the crater lip and recorded about 140 r/hr on the
Jordan survey meter. This reading implies an intensity less than 1,700 r/hr at H + 26
minutes, which is a much lower value than that reported in Reference 7.
Cave-in of the
crater walls could have been responsible for this apparent inconsistency.
The use of t~'*? decay law will generally lead to errors no greater than those involved
with dose~rate measurementin the field and is recommended for field use.
Although only a few plutonium contamination measurements were made during Fig, the
levels were low enough to indicate that no serious long-term problem would result from
this source.
As expected, it was difficult to make completely definitive fall-out radiation measure-
ments without the ability to make full-field dose-rate measurements at any point within the
instrumentation array. Use of sticky-pan fallout collectors gave results reasonably consistent with survey meter readings for the high-intensity regions that resulted on the shot
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