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:
= h5 - ht, hs is the initial height of the plume (usually the stack height) above plant grade, in m, and ht is the maximum terrain height above plant grade between the release point and the point for which the calculation is made, in m. If ht is greater than h5 , then he = 0. For those cases in which the applicant can demon- strate that the vertical velocity of effluent plumes from the plant (because of either buoyancy or mechanical jet effects) will be maintained during the course of the accident, this additional velocity may be considered in the determination of the effective stack height (he) using the same procedures described in regulatory position 2.a of Regulatory Guide 1.1 11, "Methods for Estimating Atmospheric Transport and Dispersion of Gaseous Effluents in Routine Releases from Light-Water-Cooled Reactors." b. For fumigation conditions, a "fumigation X/Q" should be calculated for each sector as follows. The equa- tion for ground-level relative concentration at the plume centerline for stack releases during fumigation conditions is: 1.145-3 X/Q= 'he >0 (27r) Uh, ayhe where Uhe is windspeed representative of the fumigation layer of depth he, in m/sec; in lieu of informa- tion to the contrary, the NRC staff considers a value of 2 meters per second as a reasonably conservative assumption for he of about 100 meters, and Gr is the lateral plume spread, in m, that is repre- sentative of the layer at a given distance; a moderately stable (F) atmospheric stability condition is usually assumed. Equation 5 cannot be applied indiscriminately because the X/Q values calculated, using this equation, become unrealistically large as he becomes small (on the order of 10 meters). The x/Q values calculated using Equation 5 must therefore be limited by certain physical restrictions. The highest ground-level xJQ values from elevated releases are expected to occur during stable conditions with low windspeeds when the effluent plume impacts on a terrain obstruction (i.e., he =