U.S. Nuclear Regulatory Commission
Question 427: The word "external" in 10 CFR 20.1302 (b) (2) (ii) refers to any radiation source which could irradiate an individual from outside the body. Since sources include both airborne radioactive materials and contained sources, the dose from airborne radioactive materials could be double-counted -- as a concentration pursuant to 10 CFR 20.1302 (b) (2) (i) and as direct radiation pursuant to 10 CFR 20.1302 (b) (2) (ii). In a situation where the licensee was approaching the 50 mrem/yr limit from direct radiation from contained sources, the additional direct radiation component from airborne releases may cause this limit to be exceeded. Clearly, this situation could be addressed through use of 10 CFR 20.1302 (b) (1); however, the intent of the revised Part 20 appears to provide viable alternatives to complying with the regulations whenever feasible. Must a licensee who elects to use the method of 10 CFR 20.1302 (b) (2) for demonstrating compliance with the public dose limits "double-count" the dose from airborne radioactive materials?
Answer: No. External sources ordinarily include all radiation sources outside of the body, such as direct radiation from contained sources and direct radiation from airborne radioactive materials. To the extent that doses from airborne radioactive materials (e.g., noble gases) are accounted for as concentration values pursuant to 10 CFR 20.1302 (b) (2) (i), they need not be accounted for as external sources under 10 CFR 20.1302 (b) (2) (ii) in determining compliance with the 50 mrem/yr limit. (However, airborne radioactive material does need to be accounted for in determining compliance with the limit of 2 mrem in any one hour).
(References: 10 CFR 20.1302, 10 CFR 20.1301).