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.

Answer: (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, March 29, 2012