Health Physics Questions and Answers - Question 459

Question 459: In the answer to Question 379, the NRC addressed the issue of whether noble gases should be included in assessing the requirement to post an area as an airborne radioactivity area. This question is intended to obtain further clarification with regards to the two separate provisions that require posting of airborne radioactivity areas. The first provision requires posting of areas in which concentrations of airborne radioactive materials are "in excess of the derived air concentrations (DACs) specified in Appendix B." As pointed out previously (in the answer to Question 379), Appendix B includes DACs for noble gases, and therefore noble gas concentrations should be included in posting considerations. The second provision requires that posting be established for areas where an individual could "exceed...an intake of 0.6 percent of the annual limit on intake (ALI) or 12 DAC-hours" in a week. The answer to Question 379 states, "radioactive noble gases . . . (which have no inhalation ALI) should be excluded in determining DAC hours for use in determining the committed effective dose equivalent (CEDE)." From this it appears that for the second provision regarding posting of airborne radioactivity areas, which established precautions to limit internal exposures from intakes, one should not take into account noble gas concentrations because they result in external exposures from submersion. However, noble gas radioactive daughters must be included when determining posting requirements under either provision. Is this clarification of the differences between the two provisions and respective applicability of radioactive noble gas concentrations correct?

Answer: Yes, assuming that it is understood that the "two provisions" in the statements preceding the question refer to the two parts of the definition (in 10 CFR 20.1003) of "airborne radioactivity area", which are separated by the word "or". There is only one "provision" that requires posting of airborne radioactivity areas, the "provision" of 10 CFR 20.1902 (d).

(References: 10 CFR 20.1902, 10 CFR 20.1502, 10 CFR 20.1003)