Twobasic assumptions are made in developing the ship contamination model. The first
is that the mixture of fission products present in the accumulated radioactive material on the hull
and in the piping of a ship decayed radiologically as t-}-3.
This decay rate was verified
experimentally for fission products deposited in seawater and on the decks of target ships at
CROSSROADS. Theuse of t}-3 decay for CASTLE ship contamination calculations is a better
approximation than the land data suggest. The gamma emissions of the actinide radionuclides
contmbuting to the variable decay exponenton land are less energetic than the average. Thus, they
are selectively attenuated in water and through ship hulls, leaving the fission products to dominate
the intensities pertinent to ship contamination calculations.
The second assumption involves the rate of contamination buildup on the hull and
interior piping. The radioactive buildup on a previously uncontaminated ship is assumed to be
initially proportional to the radiation intensity of the water surrounding the ship, but, as buildup
progresses, a limiting or saturation value of contamination is approached asymptotically. The
occurrence of such a saturation effect is indicated by hull intensity readings taken on various ships
after their departure from the lagoon following CROSSROADS operations. Based on these
assumptions, the exterior gammaintensity of the hull I,(t) of a contaminated ship at time t is
given by:
I(t) = St! [1-exp {-CS'! Dy @}] ,
where C and S are constants, and Dw(t), is a parameter proportional to exposure from
contamination-bearing material,
t
D,(t) = J 3 T(t) de .
Here, I(t) is the intensity of the water in whichthe ship is operating at time t. It is evidentthat,
as a ship spendssufficient time in contaminated water, Dy, becomes large and the hull intensity
approachesa saturation value:
Int) > Sri3
The constants S and C were evaluated from CROSSROADS support ship intensity data, as
discussed in Reference 4. The derived values are given below.
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