Health Physics Questions and Answers - Question 458
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)