Document: NRC Regulatory Guide
Document ID: 4e88bc9c-73b4-419d-a312-fe7e31653337
Document Type: regulatory_guide
Title: Use of ARCON Methodology For Calculation Of Accident-Related Offsite Atmospheric Dispersion Factors
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2116/ML21165A005.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.249
CFR Part: 
CFR Title: 

Content:
sectors is selected for each time interval. The maximum χ/Q value from the 16 sectors becomes the 0–2 hour “maximum sector χ/Q value.” Since ARCON calculates a relative concentration for only one specified direction per code execution, each of the 16 direction sectors needs to be calculated individually. ARCON allows the user to specify the width of the wind direction window in degrees. As such the wind direction window width for each sector should be 45 degrees centered on the compass direction of interest. The following steps can be used as an acceptable method to calculate the 99.5th-percentile χ/Q for any given sector. Each ARCON run produces a “Frequency File” output file with the extension “Cumulative Frequency Distribution (CFD” (cumulative frequency distribution).)”. The ARCON 2.0 User’s Guide (Ref. 9) states that the “Frequency File (.CFD)” name and directory path are the same as for “Input File (.RSF).” The “Frequency File (.CFD)” contains the cumulative frequency distributions of the concentrations calculated for 10 averaging intervals and is designed to be imported into a spreadsheet for further data analysis and display. The distributions may be used to determine concentrations at percentiles other than the 95th percentile. Using the number of hours for a given averaging period from the .LOG file, along with the data provided in the .CFD file, a user can calculate the frequency that any given χ/Q is exceeded in that ARCON run. Appendix A to this RG provides an example of how a user may determine the 99.5th-percentile χ/Q value from the standard ARCON output. DG-4030, Page 17 D. IMPLEMENTATION The NRC staff may use this RG as a reference in its regulatory processes, such as licensing, inspection, or enforcement. However, the NRC staff does not intend to use the guidance in this RG to support NRC staff actions in a manner that would constitute backfitting as that term is defined in 10 CFR 50.109, “Backfitting,” and