United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

Question 379: Should radioactive noble gas concentrations be excluded

(a) with regard to evaluating and posting Airborne Radioactivity Areas and

(b) in determining Derived Air Concentration-hours (DAC-hours)? The definition of Airborne Radioactivity Area refers to areas where airborne radioactivity concentrations exceed the DAC values or where an individual could exceed 12 DAC-hours in a week. DAC is defined as the "concentration of a specific radionuclide in air which, if breathed...results in an intake of one ALI [Annual Limit on Intake]." The values listed for radioactive noble gases in the DAC column in 10 CFR 20 Appendix B are identified as "submersion" values that apply to external, rather than internal, exposure. Also, there are no ALI values listed in 10 CFR 20 Appendix B for radioactive noble gases. From this, it appears that radioactive noble gas concentrations do not apply to evaluating and posting Airborne Radioactivity Areas or to DAC-hour determinations.


(a) Radioactive noble gas concentrations should not be excluded with regard to evaluating and posting airborne radioactivity areas. See the discussion below.

(b) Radioactive noble gases of the "submersion" class (which have no inhalation ALI) should be excluded in determining DAC hours for use in determining the committed effective dose equivalent (CEDE). In other words, the DACs for noble gases are based on external dose and should not be used to assess internal dose.

Discussion: The following discussion relates to the posting question. Although the definition of DAC in 10 CFR 20.1003 does not include concentrations calculated on the basis of the external dose resulting from "submersion," Appendix B clearly states that the DAC values listed in Table 1 of Appendix B "relate to one of two modes of exposure: either external submersion or the internal committed dose equivalents resulting from inhalation of radioactive materials." The definition of "airborne radioactivity area" refers to "...concentrations - (1) In excess of the derived air concentrations (DACs) specified in Appendix B...." Thus, the definition of "airborne radioactivity area" includes the DACs in Appendix B that are noble gases and that are based on "submersion." The preamble to revised Part 20 (56 FR 23379, second and third columns) also indicates that areas that meet the definition of "airborne radioactivity area" because of the presence of noble gases are required to be posted.)

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

Page Last Reviewed/Updated Thursday, September 03, 2015