Health Physics Questions and Answers - Question 453

Question 453: Note 2 of Appendix B provides criteria for determining the appropriate derived air concentration (DAC) for a mixture of radionuclides where "the identity of each radionuclide in the mixture is not known, but it is known that certain radionuclides are not present in the mixture." In §20.1204, Determination of Internal Exposure, provisions are made to disregard the concentration of any radionuclide that is less than 10% of its DAC so long as the sum of the percentages for all of the radionuclides disregarded in the mixture does not exceed 30%. Can this approach of disregarding certain radionuclides be applied to the determination of the appropriate DAC, as outlined in note 2 to Appendix B; in other words, can radionuclides that are not present in the mixture in concentrations greater than or equal to 10% of its DAC be disregarded so long as the sum of the percentages for all of the radionuclides disregarded in the mixture does not exceed 30%? This question is intended to affirm a practical approach to truncating the analysis of radionuclide mixtures by disregarding radionuclides that are not present or may only be present in insignificant concentrations relative to other radionuclides in a mixture.

Answer: No. This would be a misapplication of the provisions of 10 CFR 20.1204 (g), which applies to the determination of internal exposure under specified circumstances, not to the choice of the appropriate DAC for a mixture. See the following related questions and answers: Question #121 and answer in section 10 CFR 20.1204 which clarifies the meaning of "total activity" in 10 CFR 20.1204 (g) and provides an example of the proper use of this provision of Part 20; Question 403 and answer in section 10 CFR 20.2106 which concerns the relevance of 10 CFR 20.1202 (b) (3) and 20.1204 (g) to a cutoff levels for radionuclides contributing to the CEDE; and Question 146 and answer in section 10 CFR 20 Appendix B which indicates that the definition of the term "not present" in old Part 20 does not apply to the revised Part 20.

(References: 10 CFR 20 Appendix B, 10 CFR 20.1202, 10 CFR 20.1204)