Health Physics

The method of estimating external doses can be
based on either historical data of ground-level exposure
rates, or, alternatively, on data on the deposition density
of particular radionuclides contained in the fallout, such
as °’Cs, combined with information on the ratio of the
nuclide activity at the time of fallout to the exposure rate
at that time. Crude exposure estimates can also be made

from retrospective estimates of '’Cs or *’Sr inventories

measured in soil samples, provided one can estimate the
relative contributions from each of the tests to the total
measured inventory. The reliability of dose estimates is
dependent, however, on having reliable estimates of the

time of transport of the fallout from the detonation point
to the receptor point. Those data, called the fallout
“time-of-arrival” (TOA, measured in h), can consider-

ably affect the dose estimates for locations relatively
close to the detonation point (i.e., within a few hours

transit time for the fallout). In a separate paper, Beck et
al. (2010) describe available post-test data on measured

exposure rates and provide estimates of both TOA and

ground deposition densities (Bq m*) of '*’Cs based on
those and other types of data. '*’Cs deposition density

estimates were developed for each of 32 atolls and
separate reef islands of the Marshall Islands from each of
the 20 tests that took place at Bikini or Enewetak that
resulted in measurablefallout on the atolls.* Estimates of
fallout TOA were also developed for those tests and
atolls so that estimates of dose from external irradiation
could be reconstructed using either method. The names,

dates, and yields of the 20 tests that deposited fallout on
any of the inhabited atolls or separate reef islands, other

than the test site atolls themselves, are provided in Simon
et al. (2010a, Table 1).

In a companion paper (Simon et al. 2010b), the
doses from internal irradiation also are estimated for all
the tests and atoll populations that are considered in this
paper. The risks of cancer resulting from the doses
arising from exposure to radioactive fallout from regional nuclear testing in the Marshall Islands, taking into
consideration age andatoll of residenceat the time of the
tests, are assessed in another companion paper (Land
et al. 2010).

The doses from external irradiation were estimated

in three basic steps:

* The reader will note that this work does notattempt to quantify
the deposition on the test site atolls (Bikini and Enewetak). Not only
was the contamination on the islands of those atolls very heterogeneous, but they were monitored extensively for many years and those

data are reported elsewhere. Moreover, those atolls were never

inhabited during the testing years.

August 2010, Volume 99, Number 2

1. estimation of the outdoor exposure rates at 12 h at
each atoll after each test and of the temporal variation
of the exposure rate after eachtest;
2. estimation of the total external exposures from fallout
from TOA to infinity, obtained by integrating the
estimated exposure rates over time assuming continuous residence on the atoll (with corrections for

relocated populations); and

3. estimation of whole-body and organ doses by applying conversion factors from outdoor exposure to
tissue dose.
Estimation of the outdoor exposure rates

The outdoor exposures at each atoll following each
test have been assessed in one of two ways depending on
whether measured exposure rates were available for the
times and locations of interest. If historical data on
exposure rate were available, the data were assessed and

a best estimate of the island- or atoll-average exposure

rate at 12 h post detonation (termed £12) was made.

Because the quality of the exposure-rate measurements
varied by test and location, expert judgments were often
used to determine the appropriate weighting of measurements of varying quality. As discussed in Beck etal.
(2010), many of the reported measurements were made

before all the fallout from a test was deposited, while
other measurements were obtained many weeksafter the
test when the exposure rate had been attenuated due to
weathering of the fallout by rainfall or humanactivities.
Of course, neither would have been as preferable as
high-quality ground-level exposure-rate measurements
made soon after deposition was complete.
If no reliable exposure-rate data were available to
estimate F12 directly, then the dose estimation method

used was that developed by the Off-Site Radiation
Exposure Review Project (ORERP) for estimating external whole-body and organ doses from fallout originating

at the Nevada Test Site (NTS) (Hicks 1982). That

method relates the '*’Cs deposition densities and fallout
TOA values to £12 using ratios of '’Cs to E12 for a

range of times developed specifically for some of the
tests considered in this paper (Hicks 1984). The types of
data provided by Hicks (1981, 1982, 1984) are: (1)

calculated exposure rates from all radionuclides in the
fallout debris relative to a reference exposure rate of |
mRh'atH+12(12h post detonation), at 31 times after

detonation, ranging from 1 h to 50 y, and (ii) related
radionuclide ground deposition densities, expressed in
pCi m*, for more than 60 of the most important fission

and activation products (the numbervaries from onetest
to another). Activities of fission products per unit of

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