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:
e 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 of the adjacent structures or be located as follows: a. more than 5L downwind of the trailing edge of upwind buildings, and b. more than 2L upwind of the leading edge of downwind buildings, and c. more than 0.5L crosswind of the closest edge of crosswind buildings, where L is the lesser of the height or width of the building creating the downwind, upwind, or crosswind wake. Since L will be dependent on wind direction for most building clusters, it will generally be necessary to assess the zone of influence for all directions within the 90 degree wind direction sector centered on the line of sight between the stack and the EAB or LPZ receptor location. As shown in Figure 1 of RG 1.194, if multiple receptors are involved such that upwind, downwind, and crosswind orientations are confounded, 5L could be used for each orientation. ARCON does not calculate the plume rise from buoyancy or mechanical jet effects. The analyst may determine plume rise and add the amount of rise to the physical height of the stack to obtain an effective plume height, as described in Regulatory Position 6 of RG 1.194. Although ARCON does not determine plume rise, it uses the input values of stack flow, radius, and vertical velocity to assess downwash and to estimate a limiting χ/Q value. If the EAB and LPZ receptor locations are close to the base of a tall stack, the elevated release model in ARCON generates negligibly low χ/Q values. Although perhaps numerically correct, these model results may not be sufficiently conservative for a design-basis assessment since the model does not adequately address meteorological conditions that could result in higher χ/Qs. Concentrations greater than those predicted by ARCON could result from diurnal wind direction changes, meander, or stagnation.