2.6.7
Rate of Penetratien.
The probe installations were mate ca Project 2.63 ships, the
yAG39 andYAG 40. Sincethese ships contained shielded control rooms, they were maneuvered
to be in position to measure the fallout as it arrived. The probes were lowered and raised by
remote control from the shielded control room, and the depth of penetration was measured from
the time fallout started until the YAG 39 station was taken over by the M/V Horizon.
It was
desired that all the measurements during a Single shot be made in the same spot of water, even
though that spot would be moving with the curcent. To accomplish this, a drogue was launched
just prior to fallout, and the ship maintained station around the drogue as it drifted with the
current.
The results of the probe measurements taken aboard the YAG 39 are shown in Figures 2.17
through 2.20 tor each detoration. The downward progress of the fallout clearly can be seen
during the first few hours after fallout arrival. The slnpe of the curve changes during the downward progress of penetration. Because of this, the rate of penetration is here defined as 90
percent of final mixing depth divided by the number of hours required to reach that depth.
rates are listed for each shot in Table 2.5.
These
In the case of Zuni, it is clear that the high rate of penetration is due to particle fall. It is
also evident that some portion of the fallout penetrates below the “depth of penetration.” For
Shot Navajo, the penetration rate is about the same as found during Castle (Reference 2) and
undoubtedly is due to physical mixing.
The penetration rates for Shots Flathead and Tewa are
probably due to a combination of mixing and particle fall.
The factors affecting the rate of penetration are the same as those discussed in the penetration
depth. Certainly it is evident, that in the case of particle fallout, the penetration rate will decrease aS distance from ground zero increases. If the rate is due to mixing alone, it will prob-
ably remain the same throughout the fallout pattern. For purposes of calculating the final 3-foot
dose rate from penetration depth, it will be assumed that penetration rates measured by the YAG
39 and shown in Figures 2.17 and through 2.20 are valid over the entire fallout area for each
shot.
:
2.6.8 Fallout Time of Arrival. In correcting the ships’ track for current drift and in determining the penetration depth for each probe reading, the time of arrival of the fallout must be
taken into consideration.
During Operation Redwing, few measurements of fallout arrival time were made outside the
boundaries of Bikini Atoll. Those that are available beyond the atoll limits were taken by Project 2.63 on skiff stations and aboard ship (Reference 7). But even those do not cover a large
portion of the fallout area.
The only other available information is the predicted central time of arrival for each shot,
which was presented in the Program 2 Preliminary Report (Reference 8) and will be contained
in the overall final summary of Operation Redwing, WYr-1344.
In order to obtain an estimated time of arrival, the results of the two references were combined.
That is, the estimated central time of arrival was used to obtain a general pattern but
was modified to fit the actual measurements of Reference 7.
Concerning the actual measure-
ments, whenever the data was presented the measured arrival time was taken as the midpoint
between time of first arrival and the time of peak activity.
This was done to ensure that arrival
time would correspond to the time at which sufficient fallout had occurred to give a significant
dose-rate reading in water.
Figures 2.21 through 2.24 show the estimated times of arrival used for final data reduction.
During the fallout after Shot Zuni, the estimated central time-of-arrival pattern in Reference
8, showed the contours foiding back and producing double fallout. In Figure 2.21 this has been
averaged to show a single, smooth contour line for each period of time after detonation.
2.6.9 Deduced Ocean Currents.
Prior to and during Operation Redwing, efforts were made
o establish ocean-current patterns that could be used in correcting fallout observations for
Current drift. The results of these efforts have been presented in Reference 6, wherein it is
Shown that the current directions and velocities fluctuate over periods of a few days to a week.
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