United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

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).









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