Document: NUREG-0800
Document ID: 7bbfa5be-36ba-4273-8f24-038130581ae5
Document Type: srp
Title: SHORT-TERM DISPERSION ESTIMATES FOR ACCIDENTAL ATMOSPHERIC RELEASES
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0523/ML052340583.pdf
Revision Date: 2023-06
Chapter: 2
Section ID: 2.3.4
CFR Part: 
CFR Title: 

Content:
of hazardous airborne gases are reviewed for appropriateness of atmospheric dispersion model assumptions and input data, and adequate documentation of this information. The staff makes an independent evaluation of atmospheric dispersion for pertinent distances, usually the exclusion area boundary and the low population zone outer boundary, using the appropriate meteorological data and dispersion model. Two probabilistic approaches are available for. evaluating atmospheric transport and diffusion characteristics. 2.3.4-4 Rev. 1 - July 198' a. A direction-dependent probabilistic approach using the X/Q values which are exceeded 0.5% of the time in each of 16 directions from the plant. This methodology is described in Regulatory Guide 1.145. b. A direction-independent probabilistic approach using the X/Q value which is exceeded 5 percent of the time. This methodology is described in Reference 13. These values are assumed to represent conditions for a two-hour period. X/Q values for time periods greater than two hours are estimated for the LPZ distance by assuming a logarithmic relationship between the "two-hour" value and the annual average value. These values of X/Q based on appropriate models for appropriate time intervals and distances are used in the analyses presented in Chapter 15 for dose assessment of design basis accidents. X/Q values based on site-specific meteorological data are calculated, as needed, for control room dose calculations and onsite and offsite releases of hazardous airborne materials. These estimates are made on a case-by-case basis since the mode of release and, therefore, the dispersion models vary. IV. EVALUATION FINDINGS The reviewer verifies that adequately conservative atmospheric dispersion models, with adequate onsite meteorological data as input to the models, have been used to calculate relative concentrations at appropriate distances and directions from postulated release points during accidental airborne releases of potentially