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).