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:
fuel should be assumed to mix instantaneously and homogeneously throughout the free air volume of the primary containment in PWRs or the drywell in BWRs as it is released. This distribution should be adjusted if there are internal compartments that have limited ventilation exchange. The suppression pool free air volume may be included, provided there is a mechanism to ensure mixing between the drywell to the wetwell. The release into the containment or drywell should be assumed to terminate at the end of the early in-vessel release phase. A-2.2 Reduction in airborne radioactivity in the containment by natural deposition within the containment may be credited. Section 6.5.2, “Containment Spray as a Fission Product Cleanup System,” of NUREG-0800, “Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition” (the SRP) (Ref. A-2), and NUREG/CR-6189, “A Simplified Model of Aerosol Removal by Natural Processes in Reactor Containments,” issued July 1996 (Ref. A-3), describe acceptable models for removal of iodine and aerosols (DBA analyses should use the 10th percentile values). The analysis code RADTRAD (Ref. A-4) incorporates the latter model. The NRC staff no longer accepts the prior practice of deterministically assuming that a 50-percent plateout of iodine is released from the fuel because this value is inconsistent with the characteristics of the revised source terms. Some licensees may consider specific containment design features to evaluate aerosol fission product removal. The amount of removal will be evaluated on an individual case basis. Reduction in airborne aerosol radioactivity in the containment by both sprays and gravitational settling should be evaluated on an individual case basis. A-2.3 Reduction in airborne radioactivity in the containment by containment spray systems that have been designed and are maintained in accordance with Section 6.5.2 of the SRP (Ref. A-2) may be credited. Section 6.5.2 of