Acute and chronic intakes of fallout radionuclides @ S. L. SIMoNn ET AL.
as a basis to estimate the chronic intakesfor the residents ofall
other atolls, with the exception of Utrik.
The detection of substantial levels of °Zn in the bodies
of the Rongelap and Utrik evacuees poses a dosimetric
estimation problem since normalized deposition factors for
Zn were not reported by Hicks (1984). We assumedthat
©Zn was produced by neutron activation of weapons
materials and of entrained sea water, admittedly in small
amounts, and was, therefore, present in local and regional
fallout. The °Zn was then apparently absorbed by phyto-
plankton and zooplankton and further concentrated by fish
and other aquatic animals feeding on plankton in ocean and
lagoon areas close to each atoll (Donaldson 1963*; Donaldson
et al. 1997). The fact that most of the activity of plankton and
fish in the mid-1950’s was due to activation products (Fe,
’Co, “Co, Zn) seems to indicate the avidity of plankton and
seafood for those elements (Welander 1958). On the other
hand, “Sr and '°’Cs are mainly found in terrestrial foodstuffs
contaminated as a result of root uptake.
Because most of the atolls were not evacuated and
their populations not monitored, it 1s essential to estimate
the variation of the dietary intake rate with time after the
test. We assumedthat the temporal variation of the dietary
165
results of the bioassay measurements conducted in 1957
among the Rongelap Island community were not only
due to Bravo, but also, to some extent, to fallout at
Rongelap from all other tests conducted in 1948, 1951,
1952, 1954, and 1956, in addition, to a small degree, to
fallout at Kwajalein and Majuro from the tests conducted
before or during the periods of residence of the evacuees
at those atolls (Table 1, Simon et al. 2010). The envi-
ronmental inventories of the long-lived radionuclides on
Rongelap Atoll in 1957, the year when the whole-body
and bioassay measurements were made, include contributions from all tests that resulted in measurable fallout on
the atoll before that year. Taking ®Zn as an example, we
estimated that the inventory of that radionuclide at
Rongelap in 1957 was mainly due to Bravo (73%), with
only minor contributions from the other 1954 tests (15%)
and from the 1956 tests (12%). Therefore, the °Zn whole-
body contents measured in 1957 could also have been
obtained if Bravo hadledto a “theoretical” '"’Cs deposition
density at Rongelap 1.4 times greater than what was
estimated (100 kBq m7; Table 7 of Beck et al. 2010) and
if no other test had contributed to the °Zn whole-body
contents measured in 1957 among the Rongelap Island
intake shown in egn (5) also holds for the initial period of
community. In our calculations, we assumed that for each
was not inhabited and, therefore, no measurements were
made. Eqn (5) can therefore be modified as:
was heavily fractionated at Rongelap, the relationship be-
time of approximately three years, during which Rongelap
q(Z, Bravo, Rongelap, t) = g(Z, Bravo, Rongelap, 0)
x eh AZ) + KZ, Rongelap) |< 7}
(6)
test, the “initial” intake rate of °Zn was proportionalto the
deposition density of '°’Cs. Taking into account that ®Zn
tween the initial intake rate of “Zn and the theoretical
deposition density of '*’Cs can be expressed as:
q(®Zn, Bravo, Rongelap, 0) = a(°Zn)
Using eqn (6), the radionuclide intakerates at the time of the
x K(°Zn, Bravo, Rongelap)
Bravo test, g(Z, Bravo, Rongelap, 0), are estimated to be
3,900 Bq d| for *Fe, 1,600 Bg d' for Co, 164,000 Bq
d' for °Zn, 2.8 Bg d' for *’Sr, and 540 Bq d! for '°’Cs.
Those “‘initial” intake rates are theoretical because it would
have taken some time for the chronic intake pathways to
becomeestablished since they involve contamination of the
vegetation by root uptake and the contamination of seafood,
and the populations of Rongelap and Utrik were evacuated
within two days after the Bravo test before any significant
chronic intake could occur.
As will be evidenced later, it is essential to establish
a relationship between the “initial” intake rates (which
are only available for Bravo at Rongelap and Utrik) and
the '"’Cs deposition densities (which are available forall
tests and all atolls). The '*’Cs deposition density for
Bravo at Rongelap, estimated as 100 kBq m*in Becket
al. (2010), cannot be used for that purpose because the
X Depinel'*’Cs°Zn), Bravo, Rongelap],
(7)
where
g(°Zn, Bravo, Rongelap, 0) = 164,000 Bq d!
a(°Zn) = the ratio of the initial
dietary intake of Zn,
in Bq d', andofthe
deposition density of
SCs, in kBq m’, for
a reference level of
fractionation, R/V, of
0.5;
K(°Zn, Bravo, Rongelap) = 4.07 is the degree of
fractionation of ®Zn
relative to '’Cs for
Bravo at Rongelap;**
and
* Donaldson LR. Evaluation of radioactivity in the marine environment of the Pacific Proving Ground. Conference on Nuclear
Detonations and Marine Radioactivity, Kjeller, Norway, 16—21 September 1963.
* This means that the Zn to '°’Cs activity ratio at the time
of fallout from Bravo was 4.07 times greater at Rongelap than at
distant atolls.