DNA 1240H-2
Water-Surface Bursts. Data indicate that slurry-type radioactive
particles will deposit on weather decks of ships caught in the fallout
resulting from water-surface shots, and the deposited particles generally are not visible.
On the unwashed areas of test ships, a dose
increase of 40% to 50% was recorded during the 24-hr
period following
cessation of fallout from 2 test shots.
The increase indicated that
although it was invisible, deposited activity was present.
The wash-
down countermeasure has been effective in removing slurry deposit, and
has reduced doses on washed weather decks to about 10% of the unwashed
deck doses.
The interaction of a ship's structure with radiations from the de-
posited material serves to attenuate the gamma rays, the amount of
attenuation being dependent on the thickness and density of the
structural shielding. The effectiveness of the shielding is indicated
in terms of the shielding factor, which is the dimensionless ratio of the
below-decks dose or dose-rate to that at 3 ft above the weather deck.
At below-decks locations where deck plating served as part of the shield,
test data from target ships indicated that doses were 10% to 20% of weather
deck values.
Test data showed that the alumimm superstructure also to
some extent attenuated the gamma radiations from deck-deposited activity;
depending on the location of the exposure point, doses were reduced to
10% to 60% of weather deck doses.
Underwater Bursts.
A burst at mid-depth in comparatively shallow
water, such as Shot Baker, Operation Crossroads, may be expected to
produce a large base surge, as well as fallout. Records are not
available to indicate whether the deposited activity from Shot Baker
was visible; but it is expected that for a burst of this type, some
bottom material (which would be visible) would be included in the fali-
out. It was estimated that first-hour doses ranging from 3800 r to
140 r resulted on the weather decks of ships from 500 yd to 2000 yd
from surface zero, respectively. Below-decks dose records, of dubious
reliability, indicate shielding factors from 0.25 to 0.025 for various
locations.
Deeper underwater test shots produced base surges; however, no
visible fallout occurred, and data indicated negligible deposited
activity on the target ships.
However, very small (less than 1 micron)
radioactive particles were found in some of the activity samplers at
Operation Hardtack. Particles, such as those in the samplers, may have
been deposited on the weather decks and rapidly removed by the washdown
systems operating on the target ships, since very little dose was
accumulated on the weather decks after the passage of the base surge
(in the first few minutes).
Shielding Factors. One theoretical method described for calculating
shielding factors is rather cumbersome and has not been proven entirely
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