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DOE-STD-1128-98
Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities
the bone volume by resorption and burial. Because of the extremely slow nature of this
redistribution, plutonium is considered to be uniformly distributed over bone surfaces at
all times following skeleton deposition. A small fraction of the translocated plutonium
reaches the gonads. Although the gonadal fraction is different for males and females, the
calculated gonadal doses are the same regardless of gender because the plutonium
concentration in the tissues is assumed to be the same. The ICRP assumes that the
remainder goes directly to excretion. Although the ICRP did not specifically state the
fraction of systemic excretion occurring by urine as opposed to feces, a 0.5 fraction for
each is often assumed.
Metabolic distribution and retention parameters for the three ICRP models are shown in
Table 2.7. The table also includes the absorption factors from the GI tract to the
bloodstream, as well as the inhalation class of common forms of plutonium.
Americium, as an ingrown impurity from the decay of 241Pu, can behave the same way as
the plutonium host matrix in which it is contained. This implies that the 241Am associated
with a class Y inhalation of plutonium might exhibit class Y behavior, rather than the
class W behavior assigned by the ICRP. This observation has been made in ICRP 48
(1996) and by Eidson (1980).
Experience has shown that the biokinetic models in Table 2.7 are subject to some
significant variations. A Hanford plutonium-oxide-exposure case described by Carbaugh
et al. (1991) has demonstrated lung retention far greater than that expected for a class Y
material, leading to the suggestion of a tenaciously retained "super class Y" form. This
phenomenon has been informally verified by dosimetry personnel at the Rocky Flats,
Savannah River, and Los Alamos sites, and is supported in the literature by Foster
(1991). At the other extreme, La Bone et al. (1992) have identified a circumstance in
which a 238Pu oxide inhalation class appeared to exhibit biokinetic behavior more
characteristic of an inhalation class D material. These extremes emphasize the
importance of addressing the uniqueness of individual workers and exposure
circumstances when dealing with known intakes, rather than relying on the assumed
standard models.
2.4.3
Transfer to the Fetus
In its most recent review of the metabolism of plutonium and related actinides, it was
noted in ICRP 48 (1986) that there is no strong evidence for preferential deposition of
plutonium in the fetus and that the concentration of plutonium in the bone of the embryo
or fetus is rapidly diluted by growth. However, experimental animal studies have shown
that plutonium crosses the placenta after injection in pregnant animals (Green et al.,
1979). For fallout plutonium, it has been qualitatively confirmed in humans that
2-18


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