Natural Plutonium Balance in Man
Although plutonium can be found in members of the general public as a result of
worldwide fallout from atomic weapons detonations, the levels are quite small. A
summary of the literature can be found in ICRP Publication 48 (1986). Data from
McInroy et al. (1979, 1981) suggests that median body burdens of plutonium in the
U.S. population peaked at about 12 pCi during the 1960s and declined to about 2 pCi
by 1977. Tissue concentration data from Nelson et al. (1993) can be used to calculate
a median body burden in the early 1970s of 3 to 4 pCi.
These body burdens imply that urinary or fecal excretion associated with worldwide
fallout will not be detectable by routinely available bioassay procedures.
Consequently, it is reasonable to assume that any bioassay detection by a worker-
monitoring problem is likely to be attributable to occupational exposure.
The embryo/fetus is included as part of the 10% of the systemic uptake that is
uniformly distributed in all "other" soft tissues except the liver and gonads. Methods
for evaluating embryo/fetal uptake and dose have been described by Sikov et al. in
NUREG/CR-5631 (1992) and its 1993 addendum (Sikov and Hui, 1993). For uptakes
occurring during the first 2 months of pregnancy, the activity in the embryo/fetus is
assumed to have the same concentration as in the mother's "other soft tissue." For
later uptakes, the embryo/fetal concentration gradually increases relative to the
maternal concentration, but is assumed to remain uniformly distributed in the
embryo/fetus. At 3 months, the embryo/fetal concentration is 1-1/2 times the mother's
"other" soft tissues concentration. At 6 months, it is twice the mother's, and at 8
months it is thrice the maternal "other" concentration. Following transfer to the
embryo/fetus, activity is assumed to remain, without clearance, until birth.
The Nuclear Regulatory Commission has developed simplified methods for assessing
the gestation period dose to an embryo/fetus in Regulatory Guide 8.36 (NRC, 1992).
Application of these methods shows that very large maternal intakes of plutonium or
americium are required to produce uptakes that would deliver 500 mrem, or even 50
mrem to the embryo/fetus. The NUREG/CR-5631 Addendum (Sikov and Hui, 1993)
notes that maternal inhalation intakes of nominally 100 times the annual limit on
intake (ALI) are required to give a 50-mrem embryo/fetal dose. For ingestion intakes,
a 1,000 ALI maternal intake of plutonium is required to give a 50-mrem dose to the
embryo/fetus. Thus, providing adequate radiation protection to limit maternal intake
of plutonium and americium to the occupational limits will adequately provide for
the protection of the embryo/fetus.