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:
ffuse Area Sources,” of RG 1.194. 2.4 Determining the 95-Percentile and 99.5-Percentile χ/Q Values This RG provides a method for applicants and licensees to use ARCON for offsite (EAB and LPZ) χ/Q estimates. For each of the 16 downwind direction sectors (e.g., N, NNE, NE, ENE), ARCON DG-4030, Page 16 calculates the 95th-percentile χ/Q values for each source-receptor combination for various time-averaged periods ranging from 2 hours to 30 days. ARCON does not calculate a maximum sector 99.5th percentile, as performed by PAVAN and stated in Regulatory Position 2 of RG 1.145. Using the methods described in Sections 2.4.1 and 2.4.2 of this RG, the user can calculate the 99.5th-percentile χ/Q value for each sector and should select the larger of the two χ/Q values, either the 99.5-percent maximum sector value or the 95-percent overall site value to represent the χ/Q value for the 0-2 hour time interval. If the applicant or licensee uses other methodologies to determine the maximum 99.5th-percentile χ/Q value, the methodologies should be well documented and provided with appropriate justification. 2.4.1 Determination of 95th-Percentile Overall Site χ/Q ARCON provides time-averaged directional χ/Q values for the 95th percentile. To determine the overall site χ/Q, analysts should apply a “wind direction window” width of 360 degrees. This will effectively include all the hours in the dataset and will therefore represent each wind direction, regardless of sector. Each of the other ARCON inputs should be entered as appropriate for the design of the plant. 2.4.2 Determination of 99.5-Percentile Maximum Sector χ/Q A 99.5th-percentile χ/Q value should be calculated for each of the 16 downwind sectors, and the maximum 99.5th-percentile χ/Q value out of all the 16 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