described at distances ranging from 25 to 250 miles from ground zero.
An evaluation of
the model for points closer to ground zero has been hamperedby lack of data. Also, the
assumption of a stabilized, fully developed cloud over ground zero at zero time is usually
made for calculations utilizing this model, since this assumption is good for calculating
intensities at points of primary concern around NTS. However, for small yields and fallout intensities high enough to be of military importance, all particle trajectories, including
rise and fall, must be considered. Figures 1.1 and 1.2 show space-and-fall-rate distributions of the model used for fallout prediction during Operation Hardtack. The advantage of
a model of this kind is that it may be used to estimate the influence of different wind conditions on surface contamination intensities.
1.2.4 Model Calculations for 2 |
‘Surface Burst. On the basis of the assumptions
stated above, the fallout pattern of a
surface burst was calculated, using the Hard~tack model, for the wind condition expected for Shot Fig. Results are shown in Figure 1.3.
A hodograph for the expected wind structure is shown in Figure 1.4. (Hodographs are
described in Section 1.3.2.)
Since calculated intensities are sensitive to the cloud geometry chosen, it is important
to make a realistic estimate of cloud dimensions. Cloud dimensions used are summarized
in Table 1.1. The cloud was assumedto be fully developed and stabilized over ground zero
at zero time.
It should be understood that slight variations in burst environment can cause large variations in cloud dimensions, with attendant changes in the fallout intensities. For example,
uncertainty in cloud height is indicated in Figure 1.5 by the broad band necessary to cover
data points of cloud height versus yield. Variations in burst environment apparently can
affect cloud heights to the extent of a tenfold change in yield. The shaded region shown in
Figure 1.5 indicates the range of cloud heights possible from a nuclear detonation within a
yield range of 10 to 100 tons.
1.2.5 Estimates from Geometric Scaling. Fallout contamination for discrete wind situations can also be estimated by a relatively simple geometric scaling method (References 1
and 2). In this method a measured contamination pattern from one yield is scaled to that
expected for another in a manner consistent with the assumptions stated above. The usual
practice in this method is to assumethat linear dimensions of clouds scale as the cube root
of yield. Then, linear dimensions of a given surface contamination contour as well as the
intensity label on that contour are scaled as the cube root of yield.
The advantage of this method is that details of space and fall-rate distributions of activity within the cloud are unnecessary.
Best results are expected when the yield range covered is minimized. The two land
surface shots from which fallout information has been obtained nearest the yield range of
10 to 100 tans were Coulomb C of Operation Plumbbob at 600 tons and another one-point
detonation18 January 1956,
_ Results of scaling data from both
shots by this simple geometric method
are comparedin Figure 1.6.
Also shown for comparison are intensity-distance curves
taken from the
Hardtack model calculation (Figure 1.3) and from an extrapolation of prediction curves
given in Reference 1. Wind velocity differences
may have been sufficient to account for some of the difference between estimates based on
these two events; also, the assumptions made as the basis of the scaling method may not
be valid over the yield range of
to 600 tons. Higher estimated intensities are obtained
when scaling from the higher yield. Experience with nonnuclear one-point detonations ~
suggests that this may be due to a lower meanfall rate (smaller size) for the contamination
12