Health Physics Questions and Answers - Question 403

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)