The Gamma Atmospheric Dispersion Factor for Finite-Cloud Radiation Dose Computations

Estimates of radiation doses to receptors in the vicinity of nuclear power plants from gaseous effluents are important to ensure that plant operation is in compliance with the as-low-as-reasonably-achievable dose objectives delineated in 10 CFR 50, Appendix I. The U.S. Nuclear Regulatory Commission (NRC) recently issued an Advance Notice of Proposed Rulemaking (ANPR) on the development of a new regulatory basis in support of aligning the NRC regulations governing dose assessments for radioactive effluents with the most recent methodology published by the International Commission on Radiological Protection (ICRP) contained in ICRP Publication 103 (ICRP-103) (2007). The ANPR specifically recommends that the dose conversion factors (DCFs) in Regulatory Guide 1.109 (RG 1.109) be revised as part of any effort to more closely align the NRC’s regulations with the ICRP-103 recommendations. Section C.2 of RG 1.109 provides a sector-average (SA) finite-cloud model for computation of annual doses at off-site receptors from noble gas releases from freestanding tall stacks. One of the limitations of this model is that embedded in the applicable equation is the DCF, and as such, the model is not suitable for implementation of the ANPR (if approved) and the recommended use of stand-alone updated DCFs. This limitation can be circumvented through use of the “gamma atmospheric dispersion factor” in the finite-cloud dose model, referred to as the gamma (χ/Q). A second limitation of the SA finite-cloud model is the dose overestimation at close-in receptors under very stable conditions and elevated plumes, which can be eliminated by using the corresponding finite-cloud gamma (χ/Q) for the plume-centerline (PC) model. Presented are analytical details on the derivation and use of the SA and PC gamma (χ/Q)’s, which are suitable for incorporation of stand-alone updated sets of DCFs based on ICRP-103.