Transfer to the Fetus - doe-std-1128-98

DOE-STD-1128-98

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 plutonium crosses the placenta (Okabayashi and Watanabe,

1973). However, placental and fetal membranes appear to effectively trap a

portion of the plutonium that might otherwise reach the fetus.

The behavior of plutonium in the embryo/fetus changes with the

development of the embryo/fetus (Sikov, 1987; Sikov et al., 1992). Liver

and bone surfaces are the principal sites of plutonium deposition in the

embryo/fetus, accounting for approximately 80% of the deposited

plutonium (ICRP 48, 1986). Plutonium that deposits on bone surfaces

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