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Modes of Entry into the Body - doe-std-1128-98_ch10031
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Transfer to the Fetus cont'd - doe-std-1128-98_ch10033


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
2-18


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