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:
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 small variation of xIQ values with averaging time (Ref. 8). If releases associated with a postulated event are estimated to occur in a period of less than 20 minutes, the applicabil- ity of these models should be evaluated on a case-by-case basis. Procedures for calculating "2-hour" X/Q values depend on the mode of release. The procedures are described below. 3Staff experience has shown that noncalm windspeeds below 1.5 meters per second provide a reasonable range for defining the distribution of wind direction during light winds. 4See § 100.11 of 10 CFR Part 100. 1.145-2 1.3.1 Releases Through Vents or Other Building Penetrations This class of release modes includes all release points or areas that are effectively lower than two and one-half times the height of adjacent solid structures (Ref. 9). Within this class, two sets of meteorological conditions are treated differently, as follows: a. During neutral (D) or stable (E, F, or G) atmos- pheric stability conditions when the windspeed at the 10-meter level is less than 6 meters per second, horizontal plume meander may be considered. xIQ values may be determined through selective use of the following set of equations for ground-level relative concentrations at the plume centerline: XIQ = I Ul 0 (irayaz + A/2) ( 1) U1 0(37r=a ) (2) X/ Q= U. l_,a 3 Ui17TI yCz (3) where X/Q is relative concentration, in sec/m3, X/Q values should be calculated using Equations 1, 2, and 3. The values from Equations I and 2 should be com- pared and the higher value selected. This value should be compared with the value from Equation 3, and the lower value of these two should then be selected as the appro- priate xJQ value. Examples and a detailed explanation of the rationale for determining the controlling conditions are given in Appendix