Document: NUREG-0800
Document ID: 112961d5-bbf7-4ebc-8a51-ee8e2292ddbc
Document Type: srp
Title: LONG-TERM DIFFUSION ESTIMATES
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0523/ML052340585.pdf
Revision Date: 2023-06
Chapter: 2
Section ID: 2.3.5
CFR Part: 
CFR Title: 

Content:
acter- istics is provided by ETSB. RAB provides the locations of receptors of interest. Each release should be characterized as continuous or inter- mittent. Using the criteria presented in Regulatory Guide 1.111, each release is classified as completely elevated, partially elevated, or completely ground level. Turbulent mixing of the effluent into the wake of plant structures is considered where appropriate in accordance with Regulatory Guide 1.111. Topographic characteristics in the vicinity of the site are examined for restrictions of horizontal and/or vertical plume spread, channeling or other changes in airflow trajectories, and other unusual conditions affecting atmospheric transport and diffusion between the source and receptors of interest. Examples of conditions where modifications to standard approaches may be necessary are narrow, deep valleys, land-sea (lake) breeze regimes, and low-level subsidence inversions of temperature. "Fumigation" may be a concern for infrequent releases of short duration from elevated sources. The standard diffusion model used by the staff is described in NUREG-0324. This model is a straight-line Gaussian model with a specific calculational procedure for estimating X/Q values for intermittent releases. Modifications to the straight-line model to consider the effects of variations in space and time in airflow are also described in NUREG-0324. For unusual topographic and meteorological conditions, a variable trajectory model may be used on a case-by-case basis. 2. Atmospheric Diffusion Parameters The vertical plume spread parameter, uz, as a function of distance and atmospheric stability is reviewed. Atmospheric stability should be defined by measurement of vertical temperature gradient, particularly during stable conditions. Other classification schemes (e.g., Refs. 9 and 10) may be used to estimate atmospheric stability class or to determine the plume spread parameter directly for unstable and neutral conditions. These