Document: NRC Regulatory Guide
Document ID: 6828ed7d-219b-4105-acf2-37d8a69c3f72
Document Type: regulatory_guide
Title: Use of ARCON Methodology For Calculation Of Accident-Related Offsite Atmospheric Dispersion Factors + HISTORY - HISTORY 08/2021 – DG-4030 , Proposed Revision 0
Source: NRC Regulatory Guide Division 4
Source URL: https://www.nrc.gov/docs/ML2116/ML21165A005.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-4.28
CFR Part: 
CFR Title: 

Content:
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 derived from the CFD output file. A second method to estimate χ/Qs for these timeframes is to adjust the ARCON Program Defaults. This can be found in Figure 2.8 of the ARCON 2.0 User’s Guide (Ref. A-1). The frequency file is intended to be imported into a spreadsheet and divided into columns. Once the columns are delimited, the χ/Q exceedance frequencies can be identified. A new column can be created that provides the percent of the time that a given χ/Q value is exceeded. This can be done by identifying the total number of hours in the .LOG file for each averaging interval (in Figure A-2, this would be 8,742 for the 2-hour averaging interval). The user would then subtract the number of hours in the averaging interval column for each χ/Q threshold (from Figure A-1, this would be 2 for the χ/Q threshold of 2.291 x 10-03 seconds per cubic meter (s/m3), 14 for the χ/Q threshold of 2.089 x 10-03 s/m3, etc.) from the total number of hours, divide by the total number of hours, and multiply by 100 to calculate the percentage of the time each χ/Q is exceeded in the .CFD file. DG-4030, Appendix A, Page A-2 Figure A-1. Portion of the ARCON 2.0 .CFD File from Example 1 in the ARCON 2.0 User’s Guide As the χ/Q values in the first column get smaller, the more likely it becomes that the χ/Q value is exceeded in the defined wind direction window. Therefore, the number of hours above any given threshold increases as the χ/Q decreases. For χ/Qs associated with an averaging interval value of 0, this indicates that this χ/Q limit is never exceeded, and therefore 100 percent of the hourly χ/Qs are below this threshold. This formula and relationship can be expressed as: % ா௫௖௘௘ௗ௔௡௖௘ୀ൬ ಹ೟೚೟ೌ೗షಹೌೡ೒_೔೙೟ ಹ೟೚೟ೌ೗