Evaluating 241Am Ingrowth in an In Vivo Count

DOE-STD-1128-98

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

Report No. 11 (EPA, 1988b), ICRP 30, Part 4 (1988b), 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 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 Alias are the same. The ALIs are commonly

rounded in most tabulations to one significant figure (e.g., as 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 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 the body to reduce the uncertainty

associated with the daily variation in the measurements. A regression analysis should

be used to fit the measurement values for estimating the initial intake and clearance

half-times.

5.8.4

Evaluating 241Am Ingrowth in an In Vivo Count

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 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 a well-documented Hanford plutonium-oxide exposure which

exhibited a factor-of-2 increase in 241Am lung content in the 3000 days following

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

standard 500-day class Y lung clearance half-time; finally, a 17-year biological

clearance half-time was estimated. The subsequent committed effective dose

equivalent was estimated to be a factor of 3 higher than if the standard 500-day half-

time had been used. Similar difficulties have occurred with initial detection of 241Am

5-33