U.S. Nuclear Regulatory Commission
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))