Regulatory Perspective for the Definition of Probabilistic Acceptance Criteria for the CANDU Pressure Tubes

Abstract of the technical paper/presentation presented at:
ASME 2016 Pressure Vessels and Piping Division Conference
July 17–21, 2016

Prepared by:
Sankar Laxman, Blair Carroll and John Jin
Canadian Nuclear Safety Commission

Abstract:

For the assurance of fitness-for-service of CANDU pressure tubes (PTs), guidelines and acceptance criteria are provided in CSA N285.8-15, Technical requirements for in-service evaluation of zirconium alloy pressure tubes in CANDU reactors. With respect to the assessment of risk of operation associated with degradation mechanisms and aging of the PTs in the entire core of a given reactor unit, clause 7 of CSA N285.8 allows licensees to use either a deterministic or probabilistic method to assess the likelihood of PT failures. When a probabilistic method is used, the licensee is obligated to demonstrate that the combined frequency of PT failure(s) over the evaluation period, due to the various potential degradation mechanisms, is less than the maximum acceptable frequency provided in table C.1 of CSA N285.8-15.

The maximum acceptable frequency provided in table C.1 of CSA N285.8-15 was developed in the early 1990s based on reactor operating experience and knowledge at that time, drawn from Station Siting Guides and Consultative Regulatory Guide C-006 (Revision 1). A task group was established by the CSA N285.8 Technical Steering Committee to re-evaluate the allowable failure frequencies to confirm that they remain relevant given the current state of knowledge and the additional evaluation tools available. This paper provides Canadian Nuclear Safety Commission staff with views on the technical basis for revisions to the allowable frequencies based upon current industry practices in probabilistic risk assessments and risk informed decision-making with inputs from deterministic core assessments.

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