United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

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).









Page Last Reviewed/Updated Thursday, March 29, 2012