Using Spiers’ model, we calculate the dose rate D,, to a small, tissue-
filled cavity in bone from the “Sr concentration in mineral bone.
Then from geometrical considerations, the dose rates to the bone
marrow D,, and endosteal cells D, are calculated using conversion
factors D,,/D,, = 0.32 and D,/D,= 0.43, respectively. These factors are
quoted by the United Nations Scientific Committee on the Effects of
Atomic Radiation (UNSCEAR)[16,17] and are equivalent to a bonemarrow doserate of 1.4 mrad/y per pCi *'Sr/g calcium and an endosteal
cell dose rate of 1.9 mrad/y per pCi “’Sr/g calcium.
The '’Cs and Co Methodology
Ingestion
For '’Cs and “Co, the methods of the International Commission
on Radiological Protection (ICRP) [18-20] and the National Council
on Radiation Protection and Measurements (NCRP) [21] as developed
by Killough and Rohwer in their INDOS code [22] are used for the
dose calculations. This code is used as published; however, the output
is modified to show the body burdens for each year. For Cs, which
is of major importance in the Marshall Islands, the model for adults
consists of two compartments with removal half-times of 2 and 110 d,
with 10% of the intake going to the 2-d compartment and 90% to the
110-d compartment. These data are consistent with preliminary data
obtained by Brookhaven National Laboratory (BNL) on the half-time
of the long-term compartment in the Marshallese [23]. The gut trans-
fer coefficient for '’Cs is 1.
The half-time of '’Cs in children is determined in two stages. The
equation usedto determine the half-timeof '"’Cs, developed by Snyder
at Oak Ridge National Laboratory, is T., = 1.63 M, where M is the
body mass in kilograms [24]. The constant of 1.63 is adjusted from
the original 1.43 to account for the now-accepted, 110-d long-term
compartment. The M as a function of age is determined using equations given by Spiers [13]. When the Snyder and Spiers equations are
combined, the half-time as a function of age can be determined. The
average half-time using the above approach for ages 5 through 10 is
c
children in this age bracket is 43 d. For ages 11 to 15, the SnyderSpiers method gives an average half-time of about 70 d, while the
BNL data for nine adolescents in this age bracket is 69 d [25].
46
fxternal Carmina
The primary external gamma exposure is
from '' (Cs, with a very
small contribution from "Co. To convert exter
nal gamma measurements in ur/h to an absorbed dose in
tissue, we chose to use the
conversion factor from exposure dose in air
to absorbed dosein tissue
given in the UNSCEAR report [17] that is
(0.87) (0.82) = 0.71 where
0.87 is the conversion from exposure to absor
bed dose in air and 0.82
is the conversion from absorbed dose in
air to absorbed dose in the
body.
In ICRP Publication 21, the conversion facto
r for '"Cs gamma
rays (0.66 MeV) is 0.65 andit is 0.7 for Co (1.17
MeV) [26].
The value for total body given by O’Brien
and Sanna for 0.5-MeV
gamma rays is 0.52; for 1 MeV the valuei
s 0.56 {27}. For the skeleton,
the conversion factors are 0.49 and 0.54
for 0.5 and 1.0 MeV, respectively.
Transuranic Radionuclides Methodology
Inhalation
The inhalation model used for the vario
us isotopes of plutonium
and for **'Am is that of the ICRP Task Grou
p [28,29]. Parameters for
the lung model are also those of the ICRP
—the gut-to-blood transfer
for
plutonium isotopes is 107' and for “Am it
is 5 x 10°* (30}. Both
‘"'Am and plutonium are assumed to be clas
s-W compounds.
Ingestion
For the ingestion pat hway, the gut trans
fer coefficients are, as stated
above, 10°* for plutonium and 5 x 10°*
for *4'Am. Thecritical organs
are bone and liver with a biological halflife of 100 y in bone and 40 y
in liver.
Of the plutonium and 2*Am transferred
to
blood, 45% is
assumedto reach the bone and 45%
is assumed to reach the liver. The
rema
ining 10% is distributed a
The"
u
Ds
dose to bone marrow and endostea
! cells is calculated
by Spiers’ method in a manner anal
agous to "Sr (7,31,32]. First, a
dose to bone mass Dx is determined
based on the concentration in
pCi/g. Second, the ratios D,,/Dy and
D,/Dy are applied to find the
47