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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.
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