Chopter 3
LAGOON RADIOACTIVITY SURVEY
3.1 OBJECTIVES
The Bikini Lagoon radioactivity survey was a continuation of the lagoon current survey.
It
was to determine the pattern of fallout and the movement of radioactive water in all sections of
the lagoon. Also, the fallout pattern in the lagoon was to be tied in with the oceanographic fallout survey when possible.
3,2 BACKGROUND
During Operation Castle, radioactive water welled up around the ships at anchor in Bikini
Lagoon. The lagoon survey was to keep track of the movement of radioactivity to prevent this
from happening again. Previous studies indicated that radiation might be used to study the move-
ment of water throughout the lagoon.
The findings on the use of radioactivity for current direc-
tion and velocity measurements was discussed in Reference 6.
3.3 THEORY
The lagoon would tend to hold radiation within its system of circulation. Clean water would
be entering over the windward reefs continuously to dilute the radiation, but the influx of clean
water would be small in comparison to the total volume of contaminated water in the lagoon.
The general circulation of the lagoon would tend to concentrate the radiation in the lower layers.
The uppec layer would lose its radioactive water rather quickly across the leeward reefs, where-
as the radiation in the lower layers would be held there and upwelled at the windward end of the
lagoon. In addition, due to mixing, part of the radioactivity in the lower levels would be brought
into the upper layers.
3.4 OPERATIONS
The procedure for gathering this information was new to the personnel on the LCU. Asa
result, changes in technique were made as the survey progressed. At first the probe was only
inthe water when a vertical cast was made.
There was, at this early time, no arrangement for
towing the probe just under the surface, as was being done in the ocean survey. Later, a method for towing the probe was developed, and it was towed at the surface. Vertical casts were
also made at various intervals along the track of the vessel.
At first, the operation was concentrated near the site of the detonation, rather than over Che
entire fallout pattern on the first few days. Later, the procedure was changed to cover as much
of the fallout boundaries as possible during the first day, leaving the most radioactive water until the following day or later, as seemed practical at the time.
This seemed most advisable,
Since the contamination of the LCU reached a point where several hours had to be spent in deContamination. This time could have better been spent in gathering more data on shot day.
The final method for collecting the data was to tow the probe just under the surface of the
Water. At various times the vessel was stopped and vertical casts taken. To avoid internal
Contamination of the instrument, the low-level head was not taken out of the probe and replaced
by the high-level head when the radioactivity became too high (150 mr/hr). Instead, the readings of radiation were taken on a hand instrumentin the instrument trailer at the time the probe
85