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:
st to the release point would enter 270 degrees; an analyst facing north would enter 360 degrees. 2.3 Release Characteristics A 95th-percentile overall site χ/Q value, or a 99.5th-percentile relative concentration for each sector, should be determined for each analysis. In determining the bounding combinations, it will be 2 The site meteorological tower wind direction sensors are generally calibrated with reference to true north (360 degrees). Analysts should use caution in measuring directions on site engineering drawings since these drawings typically incorporate a plant grid and a plant “north” that may not align with true north. The source-to-receptor direction input to ARCON must use the same north reference as the wind direction observations. DG-4030, Page 13 necessary to consider the distance, direction, release mode, and height of the release location(s) to the various EAB and LPZ receptor locations. The ARCON code provides options that allow an analyst to model ground-level, elevated stack, and vent-point source releases. In addition, the analyst can model diffuse area sources as a sub-mode of the ground-level release type. The positions that follow discuss these modes and limitations on their use. 2.3.1 Ground-Level Releases The ground-level release mode is recommended for the majority of the estimates from the powerblock area to the EAB or LPZ. If the release type is ground-level, then ARCON ignores all user inputs related to release velocity and radius. Due the limited applicable distance assumed in this RG, ground-level releases are highly recommended. Use of elevated (stack) and vent releases may be appropriate, depending on the design of the plant and with appropriate justification. 2.3.2 Elevated (Stack) Releases The stack release mode is appropriate for releases from a freestanding, vertical, uncapped stack that is outside the directionally dependent zone of influence of adjacent structures. Such a stack should be more than 2.5 times the height