Some Issues in Fitness for Service Assessment of Wall Thinned CANDU Feeder Pipes

Canadian CANDU® feeder pipes experiencing pipe wall thinning due to flow accelerated corrosion (FAC) are accepted for continued service after an engineering evaluation. This evaluation is based on the assumption that FAC degradation is manageable through a comprehensive inspection program and conservative engineering evaluations. The practice of the Canadian nuclear industry is to: establish a minimum acceptable wall thickness, compare the measured thickness to predictions from the previous outage to confirm the conservatism of the predictions in a condition assessment, and predict the thickness at the next inspection and compare against the minimum acceptable value in an operational assessment. If the thickness measured during outage does not meet the pre-established thickness criteria, the feeder should be replaced, unless it is demonstrated to be fit for service through a detailed analysis. The detailed analysis usually involves more complex methodologies which are subjected to regulatory reviews. Several issues have been raised in the fitness-for-service assessments of feeder pipes relating to the definition of primary membrane stress, interpretation of minimum thickness requirements, plasticity analysis, limit load analysis and the applicability of procedures given in Code Case N-597 to Class 1 feeder pipes. This paper presents the Canadian regulatory expectations on these issues.© 2008 ASME