badge data from a nearby military outpost (S055).
Our external dose value at
Sifo Island, 1.1. gray (110 rad) was greater than the 0.69 gray (69 rad)
originally estimated by Sondhaus from post~-evacuation surveys of exposure
rate.
The difference was due to the presence of very short-lived activation
and transuranic nuclides which, according to the nuclide composition, must
have been present during exposure at Sifo Island.
Medical observations concerning thyroid abnormalities have been tabulated
along with the new thyroid dose estimated for each person.
From these
results, the mean cancer risk rate in the exposed population of 251 people was
150 thyroid cancers per million person-gray~years at risk (1.5 + 2.5 thyroid
cancers per million person-rad-years at risk).
The mean time at risk for
thyroid cancer was 19 years.
The uncertainity derived for the estimate of
risk was based on the standard deviation in adult mean urine activity
concentration, the standard deviation in thyroid absorbed dose per unit
intake, and the standard deviation in the spontaneous frequency of thyroid
lesions in the unexposed comparison group.
In order to avoid unwarrented external and internal dose from the
deposited radioactivity, the inhabitants of these atolls were relocated out of
the affected area. They returned to Utirik in June 1954 and to Rongelap in
June 1957. Environmental and personnel radiological monitoring programs were
initiated in the mid 1950's by Brookhaven National Laboratory.
The objective
was to maintain a comprehensive radiological safety program.
Post-return
body-burden histories and activity~ingestion rate patterns were determined as
were estimates of internal committed effective dose equivalent.
External
exposure rate and living pattern data were also collected.
Relationships
'
between body burden or urine activity concentration and a declining continuous
intake scenario were developed in order to model retrospective and prospective
dose equivalent.
The dosimetric conclusions for the protracted exposure are
summarized in Table 1 (Le84).
Table
1.
Dosimetric conclusions for the protracted exposure of Rongelap and
Utirik Adults from day of return to 50 years.
Rongelap
Nuclide
:
Fe-55
4.8x107* + 2.5x1074
zn-65
$r-90
Cs-137
External
1.9x107>
5.31079
2.2x1072
1.7x1072
Co-60
Uctirik
Committed Effective
Committed Effective
Dose Equivalent, Sv+S.E. Dose Equivalent, SvaS.E.
3.4x107% + 1.3x1074
+
+
+
+
1.0x1072
8.0x107¢
1elxl072
3.4x107°
3.6x1074 + 2.0x107¢
4.4x107* + 3.3x107¢
3.0x1072
1.0x107*
1.3x1072
4.lxl072
+
+
+
+
4.4x107
5.0x107>
1.0x1072
8.2x107>
A decline in the daily activity ingestion rate greater than §bat due
solely togpadioactive decay was ggtimated to be 9% per yeagfor
Cs, 8% per
year for
~“Sr, 80% per year for
Zn and 60% per year for
value of 3% per year for Pu was estimated from sparse data.
are aimed at determining the dosimetric impact of Pu.
~~Co.
A tentative
Current studies
These values for the %
per year decline in activity ingestion rate were observed at both atolls and
do not account for the additional decline due to radioactive decay.