Canadian Nuclear Regulatory Perspective on Degradation of CANDU Feeder Piping

Abstract

As some CANDU plants in Canada are approaching the end of their design lives, various degradation mechanisms which were not anticipated during the design phase have been identified in the CANDU feeder piping and resulted in either actual structural failures or the early replacement of components. In particular, inter-granular stress corrosion cracking (IGSCC) and pipe wall thinning due to flow accelerated corrosion (FAC) are the most prominent degradation mechanisms in the CANDU feeder piping. The Canadian CANDU industry has developed and implemented programs to monitor and manage those unanticipated service-related degradations. Fitness for service guidelines are also developed to justify the structural integrity of the components until the next inspection. These programs include augmented periodic inspections that are targeted to specific components, thereby ensuring the early detection of cracks or excessive wall thinning. The inspection scope and frequency adopted in the degradation management programs exceed the requirements of ASME Section XI, “Inservice Inspection of Nuclear Reactor Coolant Systems” and CSA N285.4, “Periodic Inspection of CANDU Nuclear Power Plants Components”. However, those currently effective codes and standards do not specify requirements which are developed based on the consideration of the specific degradation mechanisms such as IGSCC and FAC wall thinning. Accordingly, it has been an issue for the nuclear regulator as well as in the industry to develop criteria for inspection and replacement/repair, which are based on the current level of understanding of degradation mechanisms and the inspection capability. Presented in this paper are the Canadian regulator’s perspective on the assurance of the safe operation of the CANDU feeder piping which endures degradations of IGSCC and FAC.