Current Regulatory Guidance and Proposed Methodology for External Flooding Assessments

Abstract

In the wake of Fukushima, there is continuing focus on the assessment of external hazards on Nuclear Power Plants. In Canada, CNSC REGDOC 2.4.1 and 2.4.2 compliance requires that safety assessments are completed for external hazards. One hazard not consistently reviewed across Canadian Nuclear Power Plants is external flooding (either from precipitation or body of water threats). This is, at least partially, due to the lack of a single standard or accepted methodology for external flooding assessments.Historically, external flooding challenges were assessed with conservative modelling, in compliance with S-294 and followed the superseded IAEA Safety Series 50-P-7. Since Fukushima, methodologies using more realistic approaches have been applied. One area of particular concern is the definition of an extreme rainfall event which should be considered in these assessments.This paper discusses the various regulatory guidance in place for external flooding assessments and provides a hazard screening methodology. This methodology includes direction in defining bounding rainfall events and could be applied at Canadian nuclear generating stations.