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Questions about RegGuide 8.36 Fetal Dose Assessment

Greetings RADSAFERs,

I have two questions about the NRC's RegGuide 8.36:

1. In Appendix A, Table A-1 there are eight radionuclides that are
listed twice.  (Nb-89, In-110, Sb-120, Sb-128, Tb-156m, Eu-150, Re-182
and Np-236)
I am fairly certain that these are listed twice because they have two
different half-lives (as listed in the 10CFR20 DAC and ALI tables).
How do I distinguish which half-life is which Dose Equivalent Factor??
I have tried assuming that they are listed in the same order in both the
10CFR20 ALI table and in the RegGuide, and tried to see if the higher
Dose Equivalent Factor (rem/uCi) corresponds to a lower ALI.  But there
is no correlation.

2. In Appendix C of the RegGuide, there are some radionuclides that have
negligible doses for an intake occuring in the first 30 days.
The RegGuide states:
"For these nuclides, an approximation of the ... dose for an intake ...
during the first 30 days ... based on a time-weighted average of the
31-day intake data."  
As an example it states:
"...assuming a maternal intake of C-14 resulting in a 1-uCi blood uptake
on the 20th day of the pregnancy, the gestation dose should be
determined by multiplying the cumulated dose from an intake at day 31 by
the ratio of 20 days to 30 days (i.e., 20 divided by 30)."

Shouldn't this be the ratio of the number of days during the first 30
days that the fetus would be exposed instead of the days-to-date?

In the example, shouldn't you use 10 days (the number of days of
exposure from day 20 to day 30) divided by 30 days?

If any one can clarify this for me it would be greatly appreciated.
You can respond directly to me if you do not think this is of general
interest to the RADSAFE list.


Chris Hogan