146
Health Physics
from '*’Cs deposition density estimates. However, as
described in Becket al. (2010), the assumed degree of
fractionation was very important for estimating '’’Cs
deposition density from exposure-rate measurements at
some atolls heavily impacted by Bravo.
August 2010, Volume 99, Number 2
Table 1. Fitted parameter values of a, and A, for use in eqn (2) to
describe the variation of the exposure rate with time after detonation according to Hicks’ (1981, 1984) data for fractionated debris
(R/V = 0.5) for Bravo (thermonuclear tests) and Tesla (nonthermonuclear tests). The values of a, are normalized to an
exposure rate of 1 mR h™' at H+12.
Thermonuclear tests
Estimation of the total exposure from fallout
In order to estimate the total exposure from fallout
from an estimate of exposurerate at any specific time, we
used the temporal variations of exposure rate given by
Componentof
exponential
1
2
3
4
5
6
7
8
9
10
Hicks (1981, 1984) in a manner described below.
First, we developed analytic expressions of the
temporal variation of the normalized exposure rate for
both Bravo and for a non-thermonuclear test (Tesla) that
was conducted at the NTS for the purposes of deriving
the exposure over any interval of time (post-detonation)
from the data provided by Hicks (1981, 1984). The Hicks
dy
(mR h7')
9.30
3.35 xX
1.65 X
5.00 X
1.85 x
3.50 X
9.58 X
1.38 x
1.40 x
7.37 X
10'
10'
10°
10°
10°
107!
10°?
10°?
1077
107°
Non-thermonucleartests
A,
(hy
ay
(mR hy
2.25 10°
830x107!
830x107!
3.88 x 107!
9.67 X 10°?
2.28 x 10°?
5.83 x 1077
1.43 x 1073)
3.05 x 107
2.66 X 107°
1.02107
3.2610!
1.00 x 107°
1.68 x 10°
9.57 x 107!
3.04 x 107!
8.08 x 10°
8.75 x 107°
9.28 x 107°
2.38 x 107°
Ay
(hy
1.86
6.44
6.44
1.34
8.99
2.03
4.35
7.58
4.05
1.00
x
x
x
x
x
x
x
x
x
x
10°
107!
107!
107!
107°
107?
1073
107"
107°
107°
of | mR h! at H+12, but do not take weathering effects
into account, were fit to 10-component exponential
functions such that a mathematical integration could be
easily accomplished. The form for the fitted functions of
the exposure rate was:
10
EQVE12 = dae,
(2)
n=1
Exposure Rate
(normalized to H+12)
exposure-rate data, which are relative to an exposure rate
where
t=the time elapsed since the time of the
detonation of the device (h);
E()/E12 = the ratio of the exposure rate at time ¢ to the
exposure rate 12 h after detonation, ex-
pressed in mR h|;
Time post-detonation (h)
a, =the coefficient to the n” exponential term;
and
X, =the decay constant for the n™ exponential
Fig. 1. Variation with time of the normalized exposurerates for six
thermonuclear tests and a non-thermonuclear test (Tesla) for a
fractionation level, R/V, of 0.5 (Hicks 1981, 1984).
The fitted regression values for a, and A, for Bravo
and Tesla are given in Table | for k/V = 0.5. As shown
in Fig. 1, exposure-rate data for six thermonucleartests
(Hicks 1984) are highly similar. For that reason, we
concluded that the single set of regression parameters,
R/V = 0.5 curves is small. We used the R/V = 1.0 decay
rate regression fit to calculate total exposure and E12
values for close-in distances and short TOAsof fallout
where we assumed R/V to be greater than 0.5. In the
absence of similar data for any non-thermonuclear tests
term (h').
shown in Table 1, would be suitable for all 16 thermonuclear tests listed in Simonetal. (2010a, Table 1). The
regression parameters shown in Table | correspond to a
degree of fractionation (R/V) of 0.5, typical of fallout at
relatively large distances from the site of detonation
where most of the deposited activity was associated with
relatively small particles (<50 um diameter). Wealsofit
Hicks’ data for Bravo for R/V = 1.0 and used those
values for the higher fractionation ratios. As shownlater,
the difference in decay rates between the R/V = 1.0 and
at Bikini or Enewetak, we concluded that the data
derived by Hicks (1981) for the Tesla test conducted at
the NTS would adequately reflect the decay rate and
nuclide composition of the four non-thermonucleartests
(Simon et al. 2010a, Table 1) that deposited relatively
low levels of fallout in the Marshall Islands. As shown in
Fig. 1, the decay rate for Tesla is very similar to that for
the six thermonucleartests.
We subsequently took into account the influence of
weathering on the temporal variation of the exposure