U.S. Nuclear Regulatory Commission
Question 146: The term "not present," which is defined in paragraph 5 of the Note (concerning mixtures of radionuclides) following Appendix B of the old Part 20, is not defined in the corresponding "Note" in the revised Part 20. Does this definition from the old Part 20, which indicates when a radionuclide may be considered as not present in a mixture, continue to apply in the revised Part 20?
Answer: No. Nothing in the old Part 20 (§§20.1-20.601) applies in the revised Part 20 (§§20.1001-20.2401). This definition of "not present" is not included in the revised Part 20; however, 10 CFR 20.1204 (g) stipulates the conditions under which licensees may disregard certain radionuclides in a mixture when determining internal dose. See the answer to the related Question 121 for clarification of 10 CFR 20.1204 (g).
(References: Appendix B, 10 CFR 20.1204)