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Health Physics
Estimation of acute intake by adults of ‘I at
Rongelap, Ailinginae, and Rongerik following the
Bravo test. The estimation of '*'I intake by the highly
exposed populationsin this work, as well as previously in
Lessard et al. (1985) and NCI (2004), was based on
bioassay measurements of urine samples collected within
19 d of the Bravo test originally reported by Harris
(1954) and described more fully in Harris et al. (2010).
The bioassay data provided direct empirical evidence of
the internal contamination following the event to a subset
of the Marshall Islands population. Becauseof the lack of
detailed information on the pathways of the acute intakes, the bioassay data were used as the basis for
estimating intakes to adults at all atolls.
The basic calculation to estimate the average intake
of '*'T amongthe adults from whom a 24-h urine sample
wascollected, is shown in eqn (1):
O —
CRXKXV
EF) X60"
()
where
August 2010, Volume 99, Number 2
considered, and (3) a relationship between intake by
adults and ground deposition density for any radionuclide following the Bravotest.
(1) Time-of-intake (TOI). It is assumed in this
work that the acute intake at Rongelap following the
Bravo test took place during the period of time that the
fallout was being deposited. As a general rule of thumb,
based on Nevada Test Site (NTS) fallout data (Quinn
1990), the duration of fallout is approximately equal to
the TOA (h). While intake might occur at various times
within that period, we made the simplifying assumption
that the entire acute intake occurred slightly before
midway in the period of deposition, i.e., TO] = TOA +
(0.4 X TOA) = 1.4 & TOA. Selection of a point in time
less than halfway during the period of fallout is appropriate as a central estimate since the rate of fallout
deposition generally decreases with time. The estimated
TOA at Rongelap for Bravo was 6 h post-detonation
(Beck et al. 2010); the corresponding TOI, rounded to
one significant figure, is estimated to be 8 h.
Q =acute intake of '*'] intake (Bq, group average);
CR =background adjusted count rate of '*'I per
mL of urine (c s ' mL’);
K =correction factor corresponding to the ra-
dioactive decay of ''I between time of
sampling and time of counting;
V =24-h urine volume (mL) averaged over
sampled population;
EF(t) = urinary excretion fraction for '*'I on day of
sampling; and
Ec = gammadetector counting efficiency (count
per decay).
The calculation of radionuclide intakes for this study
via eqn (1) depends on having relevant data for the
Marshallese population. The data usedin our calculations
to determine the values of the acute intakes of '"'I are
described in the Appendix.
Estimation of acute intakes by adults of radionu-
clides other than "I at Rongelap, Ailinginae, and
Rongerik following the Bravo test. Our estimates of the
acute intake of radionuclides other than '*'I by adults are
based on: (1) an estimate of the time-of-intake (TOD,
which is important for short-lived radionuclides due to
the rapid change of their activity with time after the
detonation, where TOIis derived from the corresponding
value at the fallout time-of-arrival (TOA in h, provided in
Beck et al. 2010), (2) the calculation of the ground
deposition density (Bq m°) at TOI of the radionuclides
(2) Ground deposition density at TOI. In this
work, as in Beck et al. (2010), the model and data
reported by Hicks (1982, 1984) to describe the variation
of the relative ground deposition densities of all radio-
nuclides deposited in the fallout with time, ¢, after the
detonation, were used to estimate the ground deposition
densities at Rongelap, Ailinginae, and Rongerik at the
TOIs following the Bravo test. The data of Hicks, termed
here as normalized deposition factors or ND factors,
relate the ground deposition density of each radionuclide
at time ¢ to the activity of a reference radionuclide at
somereference time. In this work, we have chosen to use
'"Cs activity at 12 h post-detonation as the reference
radionuclide and reference time to be consistent with the
deposition results discussed in Beck et al. (2010) where
it is shown that using '’Cs as the reference allows
comparisons of estimated deposition with contemporary
soul analyses to validate the fallout estimates. Since the
intakesof all radionuclides are based onthe intake of '*'1
at Rongelap, this requires use of the normalized deposi-
tion of '*'I relative to '’Cs as indicated below in eqn(2).
Hicks (1984) developed the nuclide-specific ND
factors only at specific times post-detonation and for a
limited set of fractionation ratios. For the purposes ofthis
work,it was necessary to estimate the ND factors at times
intermediate to the values Hicks provided (i.e., ~8 h for
Rongelap, ~6 h for Ailinginae, and ~11 h for Rongerik).
Using '°’Csas the reference radionuclide for ND simplifies
the interpolation over ¢ since '*’Cs activity varieslittle with
TOA,due to the long half-life of the radionuclide.