Document: NRC Regulatory Guide
Document ID: 46b2c829-ce4c-4a6a-8a01-908725558ffe
Document Type: regulatory_guide
Title: Volcanic Hazards Assessment for Proposed Nuclear Power Reactor Sites + HISTORY - HISTORY 03/2020 – DG-4028-Proposed New Guide
Source: NRC Regulatory Guide Division 4
Source URL: https://www.nrc.gov/docs/ML2000/ML20007D621.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-4.26
CFR Part: 
CFR Title: 

Content:
n would necessarily include the suite of considerations used to make risk-informed regulatory decisions (e.g., SECY-98-144, NRC 2019b, and RG 1.174). If these risk insights determine that volcanically induced failure of SSCs results in acceptable performance at (PE x PH), then no further volcanic hazards assessment is warranted. If this conclusion cannot be reached, then Step 6 of the volcanic hazards assessment should be conducted. Step 6: Evaluate Design Bases This step is optional if all previous steps have been completed and the volcanic hazards assessment analysis could proceed directly to Step 7 to evaluate potential mitigating strategies. Nevertheless, the NRC staff concludes that this step could provide additional performance insights from a focused analysis of SSC design bases that considers the unusual demands produced by hazardous volcanic phenomena. The risk-insights steps above make the conservative assumption that SSCs would have unacceptable performance, or fail, from the effects of a volcanic hazard. NPP SSCs have existing design bases that can accommodate large physical demands from other natural hazards, such as seismic ground motions. In addition to the SSC design basis, most SSCs also include additional safety factors in their design margins that provide additional capacity to resist failure during beyond-design-basis events (e.g., Kennedy et al. 1988). Consequently, a direct evaluation of SSC capacity to withstand demands from a volcanic hazard might determine that the likelihood of unacceptable performance could be lower than 100 percent, which was assumed in the preceding risk-insight steps of the volcanic hazards assessment. This evaluation also might determine that modest modifications to existing design bases could provide the additional capacity needed for acceptable performance from potential volcanic hazards. For example, SSCs for air filtration systems typically consider the demands from windblown sands. Volcanic ash falls, however,