U.S. Nuclear Regulatory Commission
Question 121: 10 CFR 20.1204 (g) provides that when a mixture of radionuclides in air exists, licensees may disregard certain radionuclides in the mixture if the licensee uses the total activity of the mixture in demonstrating compliance with the dose limits in section 20.1201 and if certain other conditions are met. How can a licensee both disregard certain radionuclides and use the total activity?
Answer: The term "total activity" in this section refers to "gross activity" measurements that are correlated with other measurements of individual radionuclides. For example, "gross beta" measurements of air samples might be used for determining intakes of a mixture of beta-emitting radionuclides when (a) gamma-ray spectrometry of representative air samples has identified radionuclides that account for more than 70% of the activity in the air samples (i.e., the percentage of radionuclides disregarded does not exceed 30%) and (b) the concentration of any radionuclide disregarded is less than 10% of its DAC.
(Reference: 10 CFR 20.1204)