Question 458: Some Part 50 power reactor licensees have
developed "weighted" or "effective" derived air
concentration (DAC) values for airborne mixtures of
radionuclides, on the basis that the mixtures are well
known and relatively stable, as demonstrated through
periodic analysis of primary sources (e.g., reactor coolant
and other process fluids), airborne and removable
contamination samples, and waste streams (i.e., 10 CFR 61
analysis). These weighted DACs utilize a known ratio of
the readily detectable radionuclides in a mixture to the
more-difficult-to-detect radionuclides, to infer the total
activity and the DAC fraction of a mixture from gross
counting methods (i.e., without having to do isotopic
analysis of each and every sample). Given adequate quality
control measures, is the use of such "weighted" or
"effective" DACs acceptable for posting, survey and
monitoring purposes?
Answer: Yes, in general, the "weighted" or "effective"DACs can be used for inferring the total activity and the
DAC fraction of a mixture from gross counting methods
provided that the method (s) for calculating the "weighted"
or "effective" DACs (which are not described in the
question) are appropriate, have been validated, and that
the uses of these weighted / effective values are not
inconsistent with other regulatory requirements, such as 10
CFR 20.1203, 10 CFR 20.1204, 10 CFR 20.1502, 10 CFR
20.1902, and the Footnotes and Note to Appendix B. The DAC
values used in the calculation of the "weighted" or
"effective" values (and the DAC values used for any other
purpose) must be the values listed in Appendix B to Part 20
unless the licensee has obtained approval, under the
provisions of 10 CFR 20.1204 (c) (2) or 20.2301, to use
other values. (References: 10 CFR 20.1501, 10 CFR
20.1203, 10 CFR 20.1204, 10 CFR 20.1502, 10 CFR 20.1902,
Appendix B).