[ Index of Health Physics Questions and Answers | NRC Home Page ]
Question 403: NRC Form 5 (Item No. 10) provides for
listing of specific radionuclides in intakes that
contribute to the recorded committed effective dose
equivalent (CEDE). When determining the CEDE only
significantly irradiated organs need be included in the
calculations [10 CFR 20.1202 (b) (3)]. An organ is
considered to be significantly irradiated if the product of
the weighting factor and committed dose equivalent, per
unit intake, is greater than 10% of the maximum weighted
value of committed dose equivalent (CDE), per unit intake
[Footnote 1 to 20 CFR 20.1202 (b) (3)]. Similarly, an
isotope need not be considered to be in an airborne mixture
if the concentration of any radionuclide is less than 10%
of its DAC, and the sum of the percentages of the
disregarded radionuclides is less than 30% [10 CFR 20.1204
(g)]. Given these two examples of cut off levels, can a
licensee disregard the contribution a radionuclide makes to
the CEDE, with regard to recording and reporting specific
radionuclides, if it is less than 10% of the total CEDE
resulting from uptake of a mixture, and the sum of the
disregarded radionuclides is less than 30%? In this way
insignificant doses, the addition of which imply greater
precision than exists for internal dosimetry, can be
disregarded without compromising sound radiological
practices.
Answer: No. The statements concerning significantly
irradiated organs [in 10 CFR 20.1202 (b) (3) and Footnote
1] apply to compliance with requirements for summation of
external and internal dose and are not relevant to cutoffs
of radionuclides used in intake calculations. The
statement in the question concerning the provisions of 10
CFR 20.1204 (g) is incorrect; it does not include the
necessary condition of 10 CFR 20.1204 (g) (1) concerning
use of the "total activity" in demonstrating compliance.
[See the answer to Question 121 for clarification of 10 CFR
20.1204 (g).] There is no basis in the regulations for the
proposed cutoff in the calculation of the CEDE.
(References: 10 CFR 20.2106, 10 CFR 20.2206, Regulatory
Guide 8.7).