PLUTONIUM
103
3. HEALTH EFFECTS
Table 3-8. Parameters of ICRP (1994a) Model
of Plutonium Biokinetics in Humans
a
Age
Parameter
b
3 months 1 year
5 years
10 years
15 years
Adult
Blood to ovaries
8.0x10
-6
1.0x10
-5
2.6x10
-5
4.5x10
-5
7.8x10
-5
7.1x10
-5
Liver (1) to small
1.33x10
-4
1.33x10
-4
1.33x10
-4
1.33x10
-4
1.33x10
-4
1.330x10
-4
intestine
Blood to upper large 1.29x10
-2
1.29x10
-2
1.29x10
-2
1.29x10
-2
1.29x10
-2
1.290x10
-2
intestine contents
Blood
to kidney
6.47x10
-3
6.47x10
-3
6.47x10
-3
6.47x10
-3
6.47x10
-3
6.470x10
-3
(urinary path)
Blood to urinary
1.29x10
-2
1.29x10
-2
1.29x10
-2
1.29x10
-2
1.29x10
-2
1.290x10
-2
bladder contents
Soft tissue (ST1) to
4.75x10
-4
4.75x10
-4
4.75x10
-4
4.75x10
-4
4.75x10
-4
4.750x10
-4
urinary bladder
contents
Kidneys (urinary path) 1.386x10
-2
1.386x10
-2
1.386x10
-2
1.386x10
-2
1.386x10
-2
1.386x10
-2
to bladder
Gastrointestinal tract 5.0x10
-3
5.0x10
-4
5.0x10
-4
5.0x10
-4
5.0x10
-4
5.0x10
-4
to blood
c
a
See Figure 3-6 for schematic representation of model.
b
Units are in days
-1
, except for gastrointestinal tract to blood, which is unitless.
c
Values shown for the absorption fraction are for general public exposures (e.g., diet).
Recommended values for
occupational exposures are as follows: oxides (excluding poly-disperse oxides), 1x10
-5
; nitrates, 1x10
-4
; other
compounds or unknown mixtures, 1x10
-4
.
PLUTONIUM
105
3. HEALTH EFFECTS
Bone is divided into trabecular and cortical components, with each further divided
into bone surface, bone
volume, and bone cavity (marrow compartment). Initial deposition of plutonium is assumed to occur
from blood directly to bone surfaces, where it can be transferred to bone marrow or to bone volume.
Elimination of plutonium in bone surface and bone volume is assumed to occur through bone marrow to
blood. Transfers of plutonium within the cortical or trabecular bone compartments are modeled
based on
assumptions about rates of bone formation and resorption, which are assumed to be vary with age (ICRP
1990; Leggett 1985). Movement of plutonium to the marrow compartment is determined by the bone
resorption rate, whereas movement from the bone surface to the bone volume is assumed to occur by
burial of surface deposits with new bone and is determined by the bone formation rate.
During growth,
bone formation and resorption are assumed to occur at different sites within bone; therefore, the rate of
removal of plutonium from the bone surface is approximated by the sum of the bone resorption rate
(represented in the model by the movement of plutonium to the marrow compartment) and the rate of
bone formation, which results in burial of surface deposits (represented by movement of plutonium from
the bone surface to bone volume). In adults, the possibility of resorption and formation of bone occurring
at the same site is assumed; therefore, only a portion (50%) of the bone formation rate results in
burial of
surface deposits and movement of plutonium from the bone surface to the bone volume. Rates of uptake
of plutonium into bone surface are assumed to be relatively fast (half-life=3–6 days, adults) compared to
rates for distribution within bone and exit from bone (half-life=10
3
–10
4
days, adults; 10
2
–10
3
days,
children); this results in relatively rapid uptake and long retention of plutonium in bone. Rates of
distribution within bone are assumed to be higher in children (by a factor of approximately 10), reflecting
more rapid bone turn-over in children. Rates of uptake of plutonium into liver are assumed to be
relatively fast (half-life=3–11 days) compared to elimination from liver (t
1/2
=10
3
days), which results in
relatively rapid uptake and long retention of plutonium in liver. Predicted kinetics of skeletal and liver
plutonium burdens in
adults and children, following a single dose of plutonium to blood (e.g., intravenous
dose) are shown in Figure 3-7.
Validation of the Model.
ICRP 1994a has been evaluated with data on plutonium excretion and
postmortem tissue levels in plutonium workers (e.g., Carbaugh and La Bone 2003; Filipy and Kathren
1996; Fritsch 2007; Hodgson et al. 2003; James et al. 2003; Singh et al. 2003). Uncertainty analysis of
model predictions has been reported (Suzuki et al. 2002).
Risk Assessment.
The model has been used to establish the radiation dose (Sv) per unit of ingested
or inhaled plutonium (Bq) for intake ages 3 months to 70 years (ICRP 1994a, 2001).
The dose integration