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:
fects caused by variations in the duration of daylight and darkness (e.g., limited inversion depths, extended persistence of various conditions); d. seasonal variation of dispersion and meteorological conditions such that separate modeling approaches and models may be necessary at certain times of the year; and e. the possible need for field studies to characterize and model dispersion conditions. The staff considers 5 years of hourly offsite observations to be representative of the conditions at a proposed or existing site, for the purposes of estimating χ/Q values at the EAB and LPZ. However, given this relatively short time scale, added attention should be given to the comparison between the offsite station(s) and the conditions at the site, as discussed above in Sections 2.1.2 (6) and 2.1.2 (7) of this RG. 2.2 Source-Receptor Pair Distances and Directions 2.2.1 Distance from Source to Receptor Figure 2-1 depicts an example of how an applicant may use the building locations within the powerblock area to determine the most limiting distance from the edge of the building to the EAB or LPZ. Analysts only need to consider the buildings from which radioactive material could potentially be released. Other buildings, such as support buildings and cooling towers, do not need to be considered when calculating the distance to the EAB or LPZ. The preferred, and most conservative, method is to use the limiting distance from the nearest building edge and apply that distance over all 16 sectors, thus creating a uniform circle around the powerblock area. Using the limiting distance over all 16 directional sectors would ensure that the most conservative χ/Q value is calculated for each given sector. When used in conjunction with the preferred release characteristics discussed in Section 2.3, this method would ensure that the 95th-percentile overall site χ/Q is conservatively determined. The source to receptor distance is the shortest horizontal distance between the release