Regulatory perspective on CANDU feeder pipe degradation due to flow accelerated corrosion (FAC) and intergranular stress corrosion cracking (IGSCC)

Abstract of the technical presentation presented at:
IAEA workshop on erosion-corrosion including flow accelerated corrosion (FAC) and environmentally assisted cracking (EAC) issues in nuclear power plants
Moscow, Russia
April 21–23, 2009

Prepared by:
John C. Jin and Raoul Awad
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada

Abstract

Feeder pipes at Canadian CANDU stations, which form primary pressure boundary, are experiencing ageing related degradation mechanisms such as flow accelerated corrosion (FAC), intergranular stress corrosion cracking (IGSCC) and potentially low temperature creep cracking (LTCC). Allowance for the continued operation of feeder piping has been based primarily on augmented inspection practices and conservative fitness for service assessments. Although failure of single feeder pipe falls within the envelope of design basis accidents considered in safety analysis, the regulatory staff remains concerned about consequential effects resulting in multiple feeder failures or stagnation flows. Key factors contributing to this concern are current limitations in both understanding of degradation mechanisms and capability of in-service inspection technique. It is the regulator's view that reliable assessment of fitness for service of flawed components requires the integration of different aspects from several disciplines; for example: a mechanistic understanding of degradation, material behavior, principles of engineering structural evaluation, NDE technology and so on.  The limited knowledge regarding the causes of the degradation may lead to susceptible areas that are not inspected. Accordingly, regulatory staff has insisted that inspection planning and structural integrity assessments should take into account of these limitations in a conservative way.  In practical terms, this means that regulatory staff allows a utility to continue operating degraded feeder pipes only when they provide a conservative engineering evaluation of the observed degradation, and commit to an expanded inspection scope to identify other feeders with similar or potentially more severe degradation.

Given that currently available industry codes and standards do not provide sufficient guidelines/criteria for assessing the degradation of feeder pipes, the Canadian Nuclear Safety Commission (CNSC) has asked the utilities to establish feeder pipe specific procedures to provide reasonable assurance that the risk associated with the feeder degradation is maintained at an acceptably low level.  

In response to this requirement, the Canadian CANDU industry has developed and continued to update a feeder fitness for service guidelines to provide evaluation procedures and industry standard acceptance criteria for assessing the structural integrity of the feeder pipes.  The scope and frequency of inspections are determined based on the results of a fitness for service assessments and taking into account of the relative susceptibility of feeder pipes to each specific degradation mechanism.  

This paper presents the regulator's view of the current situation with respect to degradation of feeder piping, its implications for nuclear safety and the regulatory expectations on industry's management of the critical ageing phenomena.

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