Management of Component Fatigue in CANDU Stations for Life Extension

CANDU^® nuclear generating stations and their components were designed for 30 effective full power years (EFPY) of operation. Many CANDU plants are now approaching their design end-of-life and are being considered for extended operation beyond their design life. The Canadian regulator, the Canadian Nuclear Safety Commission (CNSC), has asked utilities to consider component fatigue issues in plant life extension (PLEX) applications. In particular, environmental effects on fatigue is identified as an issue that needs to be addressed, similar to that being addressed for license renewal for U.S. nuclear power plants. To address CNSC concerns, CANDU stations have initiated a program to develop component fatigue management programs for PLEX operation. A pilot study conducted in a typical CANDU plant showed that: • Only 10 to 15% of the numbers of design transients have been used after 25 EFPY of operation. Hence, a significant amount of original design fatigue usage margin remains available for PLEX operation. • Environmental fatigue considerations in heavy water (D₂O) were included in the assessment. Only warm-up transients are assessed to have dissolved oxygen concentrations that can result in a significant environmental effect for the ferritic steels used in the CANDU primary and secondary systems. • Due to the low accumulation of transients, and the relative absence of thermal stratification mechanisms, thermal fatigue is not as significant an issue in CANDU plants as in pressurized water reactor (PWR) and boiling water reactor (BWR) plants. This paper summarizes the results of the pilot study conducted for the Canadian CANDU plants.