Evaluating 241Am Ingrowth in an In Vivo Count

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

Dose conversion factors can be obtained from tabulated data in an in Federal Guidance

Report No. 11 (EPA, 1988b), ICRP 30, Part 4 (1988b), in an in the Supplement to Part 1 of

ICRP 30 (ICRP 1979), or calculated directly using computer programs. Substituting the

ICRP 48 (1986) model parameters of 50% skeleton and 30% liver translocation for the

assumptions in an in ICRP 30, Parts 1 or 4, has little impact on the HE,50 per unit intake, but

does alter the committed organ dose equivalent per unit intake. Such substitution of models

is acceptable, provided that the model is documented and consistently applied.

Values for simplified dose conversion factors can be obtained by dividing a dose limit by the

corresponding value for the ALI. A caution must be observed with this approach: not all

tabulated valued of ALIs are the same. The ALIs are commonly rounded in an in most

tabulations to one significant figure (e.g., as in an in ICRP Publication 30 and Federal

Guidance Report No. 11). Substantial variation can occur as a result of units conversion.

For example, Federal Guidance Report No. 11 lists the ALI for 239Pu class Y inhalation as

both 6 x 10-4 MBq (600 Bq) and 0.006 Ci (740 Bq). Such rounding errors can introduce

significant discrepancies in an in dosimetry calculations. This method also raises a question

about which ALI should be used if compliance monitoring is being based on comparison

with secondary limits, such as the ALI rather than the primary dose limits.

Where individual-specific data are available, the models should be adjusted. However, the

general lack of capability to monitor organ-specific retention for plutonium (i.e., content and

clearance half-times) makes the use of default models most practical.

Ideally, one should obtain as much bioassay information as possible to determine the intake

and track the retention of plutonium in an in the body to reduce the uncertainty associated

with the daily variation in an in the measurements. A regression analysis should be used to

fit the measurement values for estimating the initial intake and clearance half-times.

Evaluating 241Am Ingrowth in an In Vivo Count

5.8.4

Ingrowth of 241Am from 241Pu can significantly impact bioassay monitoring projections.

Unless accounted for, it can lead to suspicion of new intakes, or underestimation of

clearance rates. The amount of 241Pu present in an in a plutonium mixture depends on the

irradiation history and time since irradiation. Freshly processed mixtures containing 6% by

weight of 240Pu may contain about 0.5% by weight of 241Pu and a 12% 240Pu mixture may

contain 3% 241Pu. Commercial spent fuel can be much higher. The ingrowth of 241Am

occurs following a plutonium intake over a period of years. Less transportable (Class Y)

forms of plutonium may have 241Am ingrowth which gradually becomes detectable. An

extreme case of this was demonstrated in an in a well-documented Hanford plutonium-oxide

exposure which exhibited a factor-of-2 increase in an in 241Am lung content in an in the 3000

days following intake (Carbaugh et al., 1991). Such an increase could not be explained

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