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Considerations for the Use of Probabilistic Assessments in Regulatory Decision Making Related to Pressure Boundary Component Aging

Abstract of a technical presentation presented at:
American Society of Mechanical Engineers
Pressure Vessels and Piping Conference
July 16-20, 2017

Prepared by:
Blair Carroll, M. Eng., P. Eng.
John Jin, Ph.D, P.Eng.
Canadian Nuclear Safety Commission


Within the current Canadian regulatory framework, the structural integrity of pressure boundary components with detected service-induced degradation must be demonstrated using deterministic evaluation techniques. However, CSNC staff has recognized that the inherent conservatism in these deterministic assessment approaches may generate overly conservative conclusions when they are applied to assess the impact of postulated service-induced degradation to establish aging management requirements for nuclear power plant pressure boundary components. This may have the unintended effect of reducing the effectiveness of aging management programs by directing resources toward activities that will have minimal benefits for improving plant safety and could result in unnecessary doses to personnel. With this in mind, CNSC staff has accepted the limited use of probabilistic assessments prepared by licensees to support aging management activities for pressure boundary components. These probabilistic assessments form a part of risk informed decision making strategies intended to reduce excess conservatism that could arise if decisions are based only on the results of deterministic assessments. Examples include assessments of the risk of CANDU pressure tube rupture and the risk of failure of CANDU feeder piping dissimilar metal welds due to stress corrosion cracking.

This paper provides an overview of CNSC staff’s experiences with the review and acceptance of licensee submissions incorporating probabilistic assessments of pressure boundary component aging for risk informed decision making.

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