United States Nuclear Regulatory Commission - Protecting People and the Environment

Health Physics Questions and Answers - Question 396

Question 396: The Appendix B tables of the revised Part 20 include ALIs and DACs for daughter radionuclides, in contrast to the Appendix B tables in the "old" Part 20, which do not include these daughter radionuclides. For example, ALIs and DACs for rubidium-88 and cesium-138 are listed. Are the listed ALIs and DACs expected to be considered separately with regard to posting and other requirements in the revised Part 20?

Answer: Yes. However, the statements preceding the question itself may reflect misunderstandings; these possible misunderstandings are addressed in the following discussion.

The Appendix B tables of both "old" and "revised" Part 20 do include many "daughter radionuclides." The tables of "revised" Part 20 include more radionuclides, and therefore more daughter radionuclides, than the tables of "old" Part 20. Although rubidium-88 and cesium-138 (daughters of krypton-88 and xenon-138, respectively) are not included in "old" Part 20, they do need to be "considered separately with regard to posting and other requirements..." of "old" Part 20; the relevant concentrations for rubidium-88 and cesium-138 are those listed at the end of Appendix B for "Any single radionuclide not listed above with decay mode other than alpha emission or spontaneous fission and with radioactive half-life less than two hours." The "old" Part 20 does include the rubidium-87 daughter of krypton-87 and the cesium-135 daughter of xenon-135, as well as many other daughters whose parent radionuclides are listed in the tables. Some very short-lived daughters of long-lived radionuclides are not included in either "old" or "revised" Part 20 (e.g., the 2.55-min barium-137m daughter of 30-year cesium-137).

The tables in Appendix B of "old" and "revised" Part 20 are consistent with respect to treatment of daughter radionuclides in the dose modeling used to derive the ALIs and DACs listed in the tables. When a listed radionuclide has a radionuclide daughter, the value in the table for that parent radionuclide takes into account the dose from the daughter radionuclide produced in the body from the decay of the parent following intake of the parent (unless a "submersion" value is listed for the parent). However, the value in the table for a parent radionuclide does not take into account any simultaneous intake of the daughter radionuclide. Thus the ALIs and DACs for daughter radionuclides need "to be considered separately" from their parent radionuclides "with regard to the posting and other requirements of Part 20."

(Reference: 10 CFR 20 Appendix B)

Page Last Reviewed/Updated Wednesday, October 07, 2015