United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

Question 455: Part 50 license standard technical

specifications define "Dose Equivalent I-131" as "...that

concentration of I-131 (microcurie/gram) which alone would

produce the same thyroid dose as the quantity and isotopic

mixture of I-131, I-132, I-133, I-134, and I-135 actually

present..." and "the thyroid dose conversion factors used

for this calculation shall be those listed in NRC

Regulatory Guide 1.109." (a) After implementation of the

revised 10 CFR Part 20, should licensees continue to use

the Regulatory Guide 1.109 thyroid dose conversion factors

or should they use the thyroid dose conversion factors in

EPA Federal Guidance Report No. 11? (b) Will this be

addressed in NRC's forthcoming generic letter on changes to

technical specifications related to the revised Part 20?


Answer: (a) Licensees must continue to use the thyroid dose

conversion factors DCFs) that are referenced in their

technical specifications (TS). A TS amendment would be

needed to allow the use of other technically acceptable

values. It should be noted that in the absence of such

regulatory requirements, the NRC has allowed licensees to

use sources of intake-to-dose conversion factors other than

Regulatory Guide 1.109.

(b) The use of Federal Guidance Report No. 11 thyroid DCFs

is not planned to be included in the generic letter on

changes to power reactor technical specifications to

incorporate the revised Part 20 but will be addressed in a

forthcoming health physics position document (which will be

made publicly available). (Reference: Reactor Technical


Page Last Reviewed/Updated Thursday, March 29, 2012