CHAPTER 17
fallout from the barge shots settled in the water more slowly than that
from the island shot. Probe measurements” of particle-penetration depth
indicate that the rate of penetration of radioactive particles from Shot
Zuni was about 11.0 meters/hr, whereas rates for Flathead and Navajo
were about 3.5 meters/hr and 2.3 meters/hr, respectively. Shot Tewa was
a 5-MI burst detonated on a barge over very shallow water (about 20-ft
depth), and was considered more nearly a Jand-surface shot than a watersurface shot. However, Ref. 77 states that the thin film of water mst
have had a modifying effect on the fallout particles, as evidenced by the
slow rate of penetration, only about 3.8 meters/hr, for relatively closein fallout. At the same time, however, the region of fallout was extremely widespread, as in the case of a land-surface burst.
Comparisons of plots™ of depth of penetration vs activity for Navajo
and Tewa indicate dose rates of about 2 to 3 mr/hr at about 3 hr after
Navajo, at ocean depths of between 10 and 20 meters (33 to 66 ft) whereas
at the same depths at about 3.8 hr after Tewa, the activity levels were
between 100 and 200 or/hr.
Reference 77 indicates that at about 2.5 hr after
Tewa, saturation prevented the instruments from recording levels higher
than 2.7 r/hr at depths of about 55 ft.
This measurement was obtained
by one of the Geiger-counter units which were moored to skiffs and suspended at various levels in the sea. The one unit that operated was
located approximately 10 mi from surface zero, and was triggered by fall-
out at 18 min after burst.
All other available water-probe contamination
measurements for all the Operation Redwing shots were made from the sur-
vey ships et later times (7 to 10 hr after burst) and indicate very low
activity levels, of the order of a few m/hr.
The nature and behavior of activity from a surface burst at sea over
deep water would probably resemble that from Shot Flathead or Navajo,
particularly if the burst were a hit or near miss, such that the fireball
engulfed a ship.
The mass of a DD or DL may be from 6 to 11 million
pounds, and thet of a CVA may vary from 100 to 200 million pounds, where-
as the total mass of the barges from Navajo and Flathead was only between
840,000 and 900,000 pounds.
Ships would provide more insoluble solids to
agglomerate with the fission products than did the test barges. However,
eome bottom material was probably also involved in the fallout from the
test shots. Thus, it is estimated that following a nuclear burst on or
near a ship at sea, fallout would consist of slurry particles of sizes
and densities similar to those of the barge shots, and would be similarly
distributed in the water.
2.
Underwater Bursts
Operation Crossroads: The first nuclear underwater detonation on
record, a shallow detonation, ia Shot Baker of Operation Crossroads (23
XT at 90 ft in 180 ft of water in July 1946). According to Ref. 79, the
radioactivity in the water was important, and between 10% and 50% of the
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