Question 28: How are annual average concentrations (AAC)
to be calculated, and is it acceptable for nuclear power
plants to use this AAC in lieu of instantaneous limits (as
currently required by the operating license) which are
derived from NUREG-0133?
Answer: AACs are calculated by multiplying the annual
effluent release of individual radionuclides by the annual
average atmospheric dispersion factor for the most
prevalent downwind sector at the controlled / unrestricted
area boundary. The instantaneous limits, on the other
hand, are based on a whole body dose limit of 500 mrem/y
and a thyroid dose limit of 1500 mrem/y for gaseous
releases and Appendix B concentration values for liquid
releases. In both cases, the dose rate or concentration
values are applied on an instantaneous maximum basis at the
boundary of the unrestricted area. Annual average
dispersion estimates are used to relate the concentration
or dose rate to a release rate, and, ultimately, to an
effluent monitor alarm set point. For purposes of
maintaining effluent releases ALARA pursuant to 10 CFR Part
50, Appendix I, power reactor licensees are restricted by
Technical Specifications to the instantaneous limits. To
permit effluent releases at levels corresponding to the AAC
described above would not enable a licensee to meet the
Appendix I design objectives. (Reference: 10 CFR 20.1302
(b) (2))