United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

U.S. Nuclear Regulatory Commission

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 Thursday, March 29, 2012