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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
Specifications).