Canadian regulatory approach to steam generator life cycle management

An abstract of the technical document presented:
The Eighth International Seminar on Horizontal Steam Generators OKB “GIDROPRESS”
Podolsk, Russia
May 19-21, 2010

Prepared by:
Raoul Awad, Blair Carroll, and Jovica Riznic
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada

Abstract

Effective ageing management programs of key safety-related structures, systems, and components (SSCs) are an efficient means for ensuring the long-term safe and reliable operation of nuclear power plants. In the early days of design of nuclear power plants, it was assumed that the operational life-cycle of steam generators would be the same or similar to that of other key components in the reactor primary-heat-transport-system. However, widespread degradation of the steam generator tubing that has occurred at a number of plants has shown that this original assumption was incorrect or at least too optimistic. Observed degradations can be attributed to a number of factors ranging from shortcomings in the design codes,  manufacturing processes or water chemistry, and unanticipated mechanisms of material and component degradation resulting from high temperature, high fluid flow, cycling loads and presence of corrosive species. The licensees (utilities) have responded to this challenge with extensive inspection and maintenance programs supported by research and development in the areas of corrosion and mechanical degradation of tubes and internals, chemistry, thermal hydraulics, fouling, inspection and cleaning, and specialized inspection tools.

This paper discusses the Canadian Nuclear Safety Commission's (CNSC's) approach towards ensuring that licensees operate and maintain their plants in a safe operational condition. It briefly describes elements of the CNSC requirements and the overall regulatory oversight process to achieve these goals. The paper also discusses the known degradation mechanisms in Canadian steam generators and describes the requirements in place to ensure licensees sufficiently monitor the condition of their SSCs and appropriately disposition the results of inspections.  Particularly, the tube degradation is a major driving force for development of CANDU-specific fitness-for-service guidelines, and for specialized inspection and monitoring technology. The paper introduces the periodic inspection program required by CSA N285.4 standard and provides a discussion of the technical basis behind the expectations for periodic inspection and material surveillance of tubes in steam generators and separate preheater vessels. The regulatory approach ensures that the utilities demonstrate that no tubes degrade to the point where their integrity could be impaired and that any leakage due to degradations would not exceed the regulatory limits.   

CNSC staff recognizes the importance of cooperation and information exchange and is actively involved with a number of ageing-management and structural integrity initiatives with industry and other national regulatory agencies both, within Canada and at international level. To strengthen the role of proactive ageing management at Canadian nuclear power plants, the CNSC is continuously maintaining and improving as well as developing new regulatory documents, standards and compliance program activities, and is encouraging further research on ageing degradation of SSC's important to safety. On a final note, the paper describes the current and planned initiatives to improve the Canadian regulatory requirements for ageing management programs, as well as the oversight for the surveillance of critical SSCs, including the use of probabilistic methods for condition monitoring and operational assessment, and risk-informed in-service inspections.

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