Document: NRC Regulatory Guide
Document ID: 687c884d-2807-4ca3-ac21-69b87c59f4ab
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:
dix A, Page A-1 APPENDIX A EXAMPLE PROCESS FOR CALCULATING 99.5TH-PERCENTILE χ/Q The assumptions and methods in this appendix are acceptable to the staff of the U.S. Nuclear Regulatory Commission for deriving the 99.5th-percentile atmospheric relative concentration (χ/Q) values from ARCON standard output files. These assumptions and processes supplement the guidance provided in the main body of this regulatory guide (RG). As discussed in Section 2.4.2 of this RG, ARCON produces standard output files that can be used to analyze the data and produce χ/Q exceedance frequencies not calculated in typical ARCON runs. The standard output files include a “Frequency File” output file with the extension “.CFD” (cumulative frequency distribution). 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. Figure A-1 contains a portion of a frequency file output created as part of Example 1 in the ARCON 2.0 example problems (Ref. A-1). The second column in Figure A-1 represents the number of 1-hour averaging intervals that are exceeded by the χ/Q thresholds, or concentrations, provided in the first column (with a header of “XOQ”). The third column (with a header of “2”) represents the number of 2-hour averaging intervals that are exceeded by the χ/Q thresholds. This is the case for each of the averaging intervals up to 720 hours, or 30 days. The 2-hour averaging interval should be used to derive the 0-2-hour χ/Q values, as would be consistent with Regulatory Positions 1.3 and 1.4 in RG 1.145 (Ref. A-2). It is noted that RG 1.145 (Ref. A-2) recommends that the time periods used to determine the sector χ/Qs for the LPZ represent appropriate meteorological regimes, e.g., 8 and 16 hours and 3 and 26 days. Some of these durations are not represented in the standard ARCON output, but may be