United States Nuclear Regulatory Commission - Protecting People and the Environment

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))

Page Last Reviewed/Updated Thursday, March 29, 2012