Regulatory Perspective on Fitness for Service Assessments of CANDU Pressure Boundary Components

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

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

Abstract

Major pressure boundary components, such as pressure tubes, feeder pipes and steam generators, at Canadian CANDU reactors are being operated beyond their initially-assumed design life. The Canadian Nuclear Safety Commission (CNSC) has approved long-term operation of nuclear power plants based on fitness for service assessments and aging management programs developed by Canadian nuclear licensees. The fitness for service assessments are conducted to demonstrate with a high level of confidence that components are and continue to be within their design basis for the proposed licensing period. In the event this cannot be demonstrated, licensees are required to schedule component replacements to maintain safe operation.

The demonstration of fitness for service can be achieved by completing comprehensive condition assessments that consider past operating history, knowledge of the current conditions of the components and use of analytical models to predict future performance. The effects of known and plausible service-induced degradation should be assessed to determine the potential impact on structural integrity. This requires a fundamental understanding of degradation mechanisms and appropriate management plans, including periodic in-service inspections and mitigating strategies. The results of the condition assessments are compared against the design requirements to confirm that safety margins will not be eroded during long-term operation of the plant.

The CNSC has accepted Fitness for Service Guidelines (FFSG) for the major components which provide various engineering methodologies and predictive models along with acceptance criteria, depending on the type of degradation mechanism. The aging management programs provide methods and strategies for timely detection and monitoring of material degradations and structural configuration. CNSC staff continue to monitor the effectiveness of the aging management programs through compliance verification programs.

This paper presents the CNSC's regulatory practice adopted in the course of technical reviews of fitness for service assessments of major pressure boundary components conducted by Canadian nuclear licensees to ensure safe long-term operation.

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