CNSC proposed guideline for the examination of steam generator tubes Removed for periodic surveillance

An abstract of the technical paper presented:
6th International Canadian Nuclear Society (CNS) Steam Generator Conference
Toronto, Ontario, Canada
November 8–11 2009

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

Abstract

This paper discusses one aspect of the approach used by the Canadian Nuclear Safety Commission (CNSC) towards ensuring that licensees operate and maintain their steam generators in a safe operational condition, namely the requirements for steam generator tube extraction and examination. The vast majority of data available to a licensee concerning the current condition of steam generator tubes is inferred from nondestructive examination (NDE), which typically relies on an interpretation of electromagnetic or ultrasonic signals. Furthermore, bulk solution primary and secondary side chemistry control practices are aimed at minimizing degradation of steam generator tubes, however, experience has shown that impurities can locally concentrate to corrosive levels even when bulk solution chemistry is maintained within specified parameters. The knowledge acquired from periodic extraction and metallurgical examination of tube samples can reduce uncertainties associated with the above practices and provide valuable insights for SG life cycle management plans and aging management programs.

The requirements for ensuring the safe operation of ageing nuclear units are described in the Canadian Nuclear Standard CSA N285.4, which necessitates the periodic metallurgical examination of removed steam generator (SG) tubes. However, CSA N285.4 does not provide adequate details concerning specific requirements for tube examinations. With this in mind and based on EPRI guidelines for PWR SG tubing, the CNSC has recently proposed a set of guidelines to improve the consistency in the examinations conducted and in the reporting of results. This paper provides the details and technical basis concerning the guidelines for SG tube examinations with respect to the tube removal method, the critical elements of the examination, and the reporting process.

It should be noted that the CSA N285.4 standard requires relatively few tube extractions over the life of a plant representing considerable cost to the licensee, both monetarily and in terms of personnel dose. The guidelines proposed by the CNSC are intended to maximize the value of each tube extracted for metallurgical examination in accordance with the CSA N285.4 requirements, with consideration given to evaluating inspection tool capabilities, confirming tube material performance and gathering information on the operating environment's potential contribution to observed or plausible tube degradation mechanisms.

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