Fallout deposition in the Marshall Islands @ H. L. Beck er at.
131
In this analysis, it is only of interest to compare the
level of fractionation at each location of interest to the
the calculated '°’Cs to E12 ratios as a function of R/V in
unfractionated radionuclide mix; therefore, the degree of
As was shownin Table 1, the degree of fractionation
fractionation is simply denoted as R/V wherea ratio of
R/V = 1.0 represents unfractionated fallout, while a ratio
of k/V = 2.0 represents fallout where there is twice as
much activity of each refractory nuclide in the cloud
relative to that in an unfractionated cloud, and, conversely, a degree of fractionation R/V = 0.5 represents
fallout where one half of the atoms of each refractory
radionuclide has been removed, typical of fallout at long
distances from the detonation site (Hicks 1982).
Because of the dependence of fractionation on
particle size and because the time of deposition varies for
particles of different sizes, we define a critical time, T..,,
to be the length of time since detonation forall particles
greater than 50 wm diameter to be deposited. At distances where the TOA is greater than T.,, refractory
nuclides are assumed to be depleted relative to volatile
Table 1.
has a substantial impact on the ratio of '*’Cs to F12.
From Table 3, it can be seen that the deposition of '*’Cs,
per unit of '*'I or per unit of ’’Sr deposited, is 30% to
40% less when the TOA is small comparedto T,,, 1.e., at
locations with a greater degree of fractionation.
Based on estimated TOAsand estimated T,,, which
are based on reported debris cloud top heights (DNA
1?
1979), we believe, as shownlater, that only fallout from
Bravo wasenriched in refractory nuclides to any significant degree and only at a few of the northern atolls in the
Marshall Islands. This phenomenon was primarily a
consequence of the relatively high speed at which the
cloud traveled, resulting in a greater than usual downwind transport of both large and small particles and a
greater than usual degree of fractionation. This is con-
nuclides as described in Hicks (1982). Conversely, at
sistent with the estimate of T., for Bravo, which was
about 48 h. At the locations of the southernmostatolls,
volatile nuclides including '’Cs are assumed to be
and, thus, the deposition calculations there are based on
increasingly closer distances to the point of detonation,
increasingly depleted in the deposited particles relative to
refractory nuclides. '*’Cs and ”’Sr, because their precur-
sors ('*’Xe, ”’Kr) are noble gases, are assumed to be even
further depleted compared to other volatile nuclides such
as *'T, ie, °’Cs/V <1. The calculated ratios of '°’Cs to
the exposure rate at H+12 (termed £12) for several
values of the degree of fractionation are given in Table 1.
Hicks (1982) reported the radionuclide composition for
Bravo fallout as a function of time for both R/V = 0.5
and R/V = 1.0. Using Hicks’ unfractionated nuclide
mixture, we extended his calculations to other values of
R/V up to R/V = 3.0. Thecalculatedratios of '°’Cs to '°'1,
7Cs/Sr, and '°’Cs/V as a function of R/V and TOA/T.,
are given in Table 3. The approximate relationship
between A/V and TOA/T.. was inferred from compari-
sons of measuredratios of '°’Cs in soils downwind from
the NTSto corresponding post-test measurements of £12
(McArthur and Miller 1989; Thompsonet al. 1994) with
Table 3. Estimated activity ratios of '’Cs/V,* '’Cs/'*'I, and
B87Cs/°Sr, as a function of R/V.
R/V
TOA/T,,
(7Cs/V)*
0.5
1.0
1.5
2.0
3.0
1
0.5 to <1.0
0.25 to >0.5
0.1 to <0.25
<0.1
1.1
1.0
0.7
0.6
0.5
7Cs/3'1)?
1.32
1.20
0.84
0.72
0.6
x
x
x
xX
X
107°
107
107
107
1077
(°7Cs/Sr)
1.1
1.0
0.85
0.75
0.63
*137Cs/V represents the activity of '*’Cs to any other volatile nuclide
relative to the ratio for unfractionated debris.
> At H+12. Does not include '*'I that will grow in from '!Te and *!"Te
precursors.
almost all the fallout occurred at times greater than T.,
a relative activity ratio of refractory to volatile radionuclides (R/V) of 0.5, implying a deficit of one-half of the
refractory nuclides when compared to a radionuclide mix
without fractionation.
The A/V for Bravo fallout at each atoll was estimated from various ratios of nuclides obtained from
radiological analysis of soil samples, including '*’Cs to
2395240Dy 37Cs to Sr, Sr to 2°'4°Pu, as well as the
ratio of TOAto T,,. As explained above, greater degrees
of fractionation (i.e., larger R/V values) result in smaller
ratios of '’Cs or “Sr to refractory nuclides such as
°FPy in soil samples, as well as smaller ratios of '*’Cs
to Sr.
Our best estimates of R/V for Bravo fallout at a
range of atolls are presented in Table 4. The measured
87Cs to *?'*Pu ratios in soils collected in 1978 (Robi-
son et al. 1981) are showntoillustrate the clear variations
in the ratio of a refractory nuclide activity (°’*~°Pu) to
a volatile nuclide ('*’Cs) activity in the soil sample data
as the level of fractionation changes with distance. The
ratio varies from about 4 at atolls close to the test site to
15-20 at atolls far away. Because the various ratios used
to estimate R/V did not always provide consistent results
(for example, the '’Cs to *’**°Pu ratio measured at
Rongerik Island, as shown in Table 4) and also because
the soil samples also contained various amounts of
unfractionated fallout from tests other than Bravo, expert
judgment was used to correct for the fraction of '’’Cs
from other tests, to evaluate all the soil data activity
ratios, and to make a best estimate of the R/V for Bravo
fallout. These estimated R/V are also consistent with the