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:
entrations in support of design-basis radiological habitability assessments for the control room DG-4030, Page 6 Both the ARCON96 and ARCON 2.0 versions of the model calculate χ/Q values using hourly meteorological data. The hourly χ/Q values are then combined to estimate relative concentration averages for periods ranging from 2 hours to 30 days. With the default averaging periods, the χ/Q values for 1, 2, 4, and 8 hours are centerline values. Average χ/Q values for periods longer than 8 hours consist of centerline χ/Q values for the first 8 hours and sector-average χ/Q values for the remainder of the period. For example, each 24-hour average χ/Q is made up of an average of 8 one-hour centerline χ/Q values followed by 16 one-hour sector-average χ/Q values. This is consistent with NRC guidance on the use of centerline and sector-average χ/Q values found in Regulatory Guides 1.111 and 1.145. Because wind direction is considered as the averages are formed, the averages account for persistence in both diffusion conditions and wind direction. Cumulative frequency distributions are prepared from the average relative concentrations, and relative concentrations that are exceeded no more than 5 percent of the time (95th- percentile concentrations) are determined for the cumulative frequency distributions for each averaging period. As described in RG 1.194, the ARCON96 code implements an improved building wake dispersion algorithm; assessment of ground-level, elevated, and diffuse source release modes; use of hour-by-hour meteorological observations; sector averaging; and directional dependence of dispersion conditions. This guidance provides procedures for using the ARCON code to estimate χ/Q values out to distances of 1200-meters (m) from the PBA, which could include the EAB and the outer boundary of the LPZ. Large light-water nuclear power plants typically have EAB and LPZ distances that range from 800 to 6,000 m (2,625 to 19,685 ft), whereas expected small