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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


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