Document: NRC Regulatory Guide
Document ID: 4d46a966-d280-43da-9b03-8b0abe7b29ce
Document Type: regulatory_guide
Title: Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2120/ML21204A065.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.183
CFR Part: 
CFR Title: 

Content:
hat the entire source term is instantaneously released into the containment atmosphere. Therefore, the maximum release rate coincides with the most conservative 0–2 hour χ/Q value. In contrast, the AST is assumed to develop over specified time intervals with the maximum release rate occurring sometime after accident initiation. To ensure a conservative dose analysis, the period of the most adverse release of radioactive materials to the environment should be assumed to occur coincident with the period of most unfavorable atmospheric dispersion. One acceptable methodology for calculating the control room and LPZ χ/Q values is as follows. If the 0–2 hour χ/Q value is calculated, this value should be used coincident with the maximum 2-hour release to the environment. If the maximum 2-hour release occurs at the beginning of the period of releases to the environment, the 2–8 hour χ/Q value should be used for the remaining 6 hours of the first 8-hour time period. If the maximum 2-hour release occurs sometime after the beginning of the releases, the 2–8 hour χ/Q value should be used before and after the maximum 2-hour release for a combined total of 6 hours. The 8–24, 24–96, and 96–720 hour χ/Q values should similarly be used for the remainder of the release duration. Figure 2 provides examples of aligning χ/Q values with the maximum 2-hour release. DG-1389, Page 32 Figure 2. Example Alignments of χ/Q Values with the Maximum 2-Hour Release Period 0 8 16 24 χ/Q Time Since Start of Release (hours) a. Maximum 2-hour release occurs during hours 0-2. χ/Q (0-2 hr) χ/Q (8-24 hr) χ/Q (2-8 hr) 0 8 16 24 χ/Q Time Since Start of Release (hours) b. Maximum 2-hour release occurs during hours 6-8. χ/Q (0-2 hr) χ/Q (2-8 hr) χ/Q (2-8 hr) χ/Q (8-24 hr) χ/Q (8-24 hr) DG-1389, Page 33 D. IMPLEMENTATION The NRC staff may use this RG as a reference in its regulatory processes, such as licensing, inspection, or enforcement. However, the NRC staff does not intend