2.6.20 Navajo Surveys. Shot Navajo was also a water shot and was fired on a barge moored
off Site Dog at 0556 on 11 July 1956.
tC.
-
al it was feared that the residual oceanic background
“trom previous shots§might be sufficient to cause large errors in results of the fallout surveys.
To evaluate this, a special survey was made to determine the oceanic radioactivity background
in the anticipated Navajo fallout area. This was accomplished between the first and the eighth
of July, being completed just 3 days before Navajo.
“apparent” mr/hr# as of shot time for Navajo.
The results are shown in Figure 2.40 in
To compute the values in terms of Navajo H-hour, all readings were corrected for decay,
assuming that the background activity resulted from the Dakota device and using a decay constant
of 1.2. No current correction has been applied for either the 7 days during which the survey
took place or for the 3 days between the end of the survey and Navajo shot time.
To give some meaning to the contour values shownin Figure 2.40, it may be stated that a
value of .01 mr/hr#, if measured at H+50 hours during the Navajo survey, would appear as 1.4
r/he at H+1 hour after all correction factors had been applied.
The background shown in Figure 2.40 was not subtracted from the Navajo survey for the fol-
lowing reasons: (1) no current pattern could be deduced by which the current corrections could
pe applied, although it seems likely from the Navajo pattern, that the “warm” area northeast
of Bikini would be carried out of the Navajo fallout area and the “hot” region being fed by Bikini
Lagoon would probably remain; (2) the area being fed by the lagoon wasthe first surveyed during
Navajo, so the background has no effect on the H+1 contours; (3) the largest error that could
result from the background is about 30 percent of the H+1 contour in the northwest region of the
Navajo area.
Aside from this one region, it is doubtful if the background error is as much as
10 percent.
The H+1 hour, 3-foot iso-dose-rate contours for Navajo are shown in Figure 2.41.
The
cross-hatched area at the westernmost part of the fallout region is thought to result from Shot
Apache, which was fired at Eniwetok two days previously. It had already been pointed out that
the area north of latitude 12 degrees 30 minutes north and lying between longitude 163 degrees
and 164 degrees, 20 minutes east may be in error as much as 30 percent, owing to the residual
oceanic radioactivity background.
In determining the areas listed in Table 2.11, the effect of Apache has been taken into consideration, aS shown by the dotted lines enclosing the 3-r/hr and 5-r/hr areas.
The same conSiderations apply to the 2-day accumulated total dose, shown in Figure 2.42,
and to the areas for accumulated dose, which are listed in Table 2.11.
2.6.21 Tewa Fallout Surveys. Shot Tewa was detonated at 0546 M on 21 July 1956. The
total yield was 5 Mt
~ Although it was detonated from a barge, Tewa
has been considered a land shot, because of the shallowness of the water and the yield of the
device. The survey results are therefore subject to somewhat the same considerations as
Zuni. That this is not entirely so, may be seen in the relatively slow penetration rate for Tewa
(Table 2.5).
The 3-foot iso-dose-rate contours for H+1 hour are shown in Figure 2.43.
The geographic
extent of the fallout from this detonation was so large that the survey ships were unable to locate
the western boundary of the 10-r/hr contour in the time allotted for the survey. This is reflected in Table 2.11, which can only indicate the area enclosed by the 10-r/hr contour line as
greater than 29,000 mi’.
The two-day accumulated total dose for Tewa is shown contoured in Figure 2.44. The area
within these contours is listed in Table 2.11.
2.6.22 Comparison of Shots. Table 2.11 summarizes the extent of fallout resulting from each
nuclear detonation and may be used to compare the results derived from the surveys.
To make this comparison, the total yield of each device is normalized te a 5-Mt shot having
a fission yield of 100 percent.
To correct to 100-percent fission, the dose rates are divided by
30