2.3
On-Site rall-out
Fall-out on Entwetok and Parry islands was observed only once from
devices fired at Eniwetok Atoll.
Approximately two hours after detonation of
thefMohawk}device on Eberiru Island, an increase in background was noted
on Parry fsiand. The fall-out, which was light, continued for approximately
one hour,
Penk Intensity, reached at the end of the fall-out, was 22 mr/hr.
Ruin showers later tn the day effectively removed most of the contamination.
The only significant fall-out observed on Eniwetok and Parry islands
during the operation resulted from the C. TEWA
Atoll.
~ device fired on Bikini
The fall-out on Eniwetok commenced approxfimately nine hours after
the device was fired.
Two peak intensities in alr concentration were ob-
served, as shown in Fig. 2.11. The Increase in the background was quite
rapid, with a peak of 100 to 120 mr/hr throughout Parry Island. During the
fall-out period, which lasted approximately 17 hr, several rain showers occurred; rain samples assayed showed ccunts in excess of 5,000,000 disintegrations per minute per liter.
The gamma background level fell off quite rapidly once the fall-out
ceased. It is to be noted in Fig. 2.11 that the background decay was much
more rapid than that expected from the normal exponential decay unti] H+48
hr and lIater.
Clean-up operations after the fall-out were conducted around living and
recreation areas. As a result of the rain showers, hot spote were found
around each building where rain runoff collected. These hot spots, along
with other contamination on the surface, were most effectively reduced by
using bulldozers, road scrapers, and hand-grading operations to turn the sand.
The decontamination efforta were successful in reducing the level to such
that no one exceeded the 300 mr/weck tolerance level.
Most serious effect of the fall-out was delay of the roll-up operations.
Initially, personnel from H&N whose dosage records indicated that they were
near the limits of 3.9 > were flown out to Bikin{ to remove them fror. the
contaminated area. Generally, people in this category were urgently required
for roll-up. Accordingly, the max{mum permissible exposure for the operation was raised to 7 r. This allowed roll-up to procecd without further delay.
On the average, the dosage received by any one individual on Eniwetok
or Parry isiands as a result of the fall-out varied from 2 to 3.5 r, depending on the length of stay of the individual and the type of work in which he
was engaged. iIn general, those working in jobs requiring them to remain
outdoors during working hours were at the upper end of the dosage spread.
During the actual fall-out period, personnel were requested to remain
indoors as much as possible and to take full advantage of shower facilities.
Of Interest {s the fact that no evidence of gross personnel contamination was
observed as a result of this fall-out.
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