fore selected for the depth of surface mixing. The radiation intensity at a point above an infinite
slab of uniformly distributed activity is calculated from the following simplified express:on,
which assumes that the buildup factors for the two media are the same:
-
Jy(t) h
lw
=
{ix e HA% ~ ByX [- Et (- ad}
Ww
2 Uy
In order to estimate attenuation due to the coracle itself, the value for 1, ,
iS separated into two
components, viz, that due to water beyond the intersection of a tangent to the coracle edge and
the water surface, and that transmitted through portions of the coracle itself. The effective Z
numbers calculated for various coracle materials by the method in Reference 41 are used to
compute an average linear attenuation coefficient (Section A.1). The expression for Iy using
these coefficients is:
Iw = 0.019 Ja(t) Vp (t— te)
It was therefore apparent that, with the exception of upwelling radioactive water, the principal factors affecting the gross intensity I, are the radiation from deposited material Ip and
radiation from the cloud I, - The GITR, however, does not have a 47 response (Figure 1.4).
The corrected expression for I, determined by averaging instrument response over the solid
angle subtended by the deck is:
Ig = 1.02 LN + 0.98 Ip + 0.98 ly
The general expression for Ig in which the relative contribution for all deposited sources is
represented by the sum of Ip and Iy is:
Ig = 1.8% 10 Salt) + 1.54 Ja(t) Vp (t — ty)
To estimate the relative contribution from the radiating cloud, some expression must be
assumed for Ja(t). The mathematical complications of moving fields are avoided by assuming
an infinite stationary cloud, and motion is simulated by allowing the concentration of radioactive
material to change as a function of time. This approximation, of course, overestimates the
relative importance of the deposit dose rate during the simulated approach and underestimates
it during the latter phases of the simulated departure. Since the matter of primaryinterest is
the approximate maximum contribution due to deposited activity prior to and during the peak
dose rate, both these inaccuracies in the model can be tolerated. An analysis of previous gamma dose rate histories (Reference 42) has indicated that the time to reach peak activity is approximately twice the time of arrival; thus, by assuming further that cessation occurs at eight
times the time of arrival, cloud movement is simulated by varying the concentration factor
Ja(t) as follows:
Ta(t) = kt7h? (ng(t) + n4(t)]
where
na(t) = re for t, St = 3t,
and
ni(t) = [ ~ tas] for 3t; <t < Bt
ty
33