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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).