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)