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:
d efficiency than PE. In such situations, initially evaluating the risk significance of either PE or PH provides a risk-informed basis to decide whether additional analyses are warranted. In addition, this approach allows sites with only a potential hazard from volcanic ash fall to directly analyze the conditional hazard of ash fall exceeding certain design bases or limits without having to first evaluate the probability of an ash-fall eruption occurring. This approach allows the volcanic hazards assessment to evaluate a range of future eruption conditions (e.g., eruption volume, duration, grain-size characteristics) without having to determine the likelihood of an ash-fall eruption occurring in the future. The conditional ash-fall hazard, typically expressed as an exceedance probability, could provide an appropriate technical basis to develop a proposed NPP’s design basis or determine if an existing design basis was resilient to the conditional ash-fall hazard. Typically, PE is based on past patterns of eruption in the history of the volcanic system. This eruptive history generally will be incomplete, due to erosion and burial of older units. The PE evaluation will need to develop a suitable technical basis to determine how much of the volcanic system’s record is appropriate to use in the PE calculations. A common concern arises when the most recent eruptions are the best documented, whereas older eruptions have increasingly larger uncertainties in their timing and character. The selection of a subset of a volcanic system’s history should be supported by a technical basis that provides confidence that the PE calculation was based on an appropriate record of the system’s past activity. Insights from the tectono-magmatic model often provide a technical rationale for determining what part of a volcano’s history is representative of expected future conditions. The evaluation of the uncertainties in the timing and character of past events represents a