United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

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).

Page Last Reviewed/Updated Thursday, March 29, 2012