Document: NRC Regulatory Guide
Document ID: d1045e85-64b0-4a83-8450-067a4fcd130f
Document Type: regulatory_guide
Title: Atmospheric Dispersion Models for Potential Accident Consequence Assessments at Nuclear Power Plants + HISTORY –HISTORY 04/2014 – Periodic Review of Revision 1 – Reviewed with issues for future consideration 02/1983 – Reissued 02/1983 to correct page 1.145-7 (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003740205.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.145
CFR Part: 
CFR Title: 

Content:
ent system, will be necessary. Wind directions during calm conditions should be assigned in proportion to the directional distribution of noncalm winds with speeds less than 1.5 meters persecond.3 1.2 Determination of Distances for X/Q Calculations For each wind direction sector, XJQ values for each significant release point should be calculated at an appro- priate exclusion area boundary distance and outer low population zone (LPZ) boundary distance. The following procedure should be used to determine these distances. The procedure takes into consideration the possibility of curved airflow trajectories, plume segmentation (particularly in low wind, stable conditions), and the potential for wind- speed and wind direction frequency shifts from year to year. For each of the 16 sectors, the distance for exclusion area boundary or outer LPZ boundary X/Q calculation should be the minimum distance from the stack or, in the case of releases through vents or building penetrations, the nearest point on the building to the exclusion area bound- ary or outer LPZ boundary within a 45-degree sector centered on the compass direction of interest. For stack releases, the maximum ground-level concentra- tion in a sector may occur beyond the exclusion area boundary distance or outer LPZ boundary distance. There- fore, for stack releases, X/Q calculations should be made in each sector at each minimum boundary distance and at various distances beyond the exclusion area boundary distance to determine the maximum relative concentration for consideration in subsequent calculations. 1.3 Calculation of X/Q Values at Exclusion Area Bound- ary Distances Relative concentrations that can be assumed to apply at the exclusion area boundary for 2 hours immediately following an accident should be determined.4 Calculations based on meteorological data representing a 1-hour average should be assumed to apply for the entire 2-hour period. This assumption is reasonably conservative considering the