CANDU Steam Generator Fitness-for-Service Guidelines

Abstract

Steam generator tubes function as part of the CANDU' reactor primary heat transport system coolant pressure boundary. The occurrence of degradation mechanisms in nuclear steam generators has led to the need for industry-standard documents that contain uniform evaluation methods for the fitness-for-service assessment of degraded steam generator tubes. Proposed Fitness-for-Service Guidelines (FFSG) for in-service evaluation of steam generator tubes and preheater tubes in CANDU nuclear power plants are under development. The FFSG are intended to provide Canadian nuclear industry-standard acceptance criteria and evaluation procedures for assessing individual flaws detected during inspection. In addition, they provide a consistent approach for assessing the adequacy of the condition of the entire population of steam generator tubes in a reactor unit. The FFSG are based on safety-related Performance Criteria that require tube structural integrity be maintained during the evaluation period, that operational leak rate is monitored and does not exceed the allowable limit, and consequential leakage during postulated upset or abnormal events is acceptable. The assessments would typically be used to justify continued operation for a planned interval and/or to justify the level of in-service inspection. This paper provides an introduction to the proposed FFSG for CANDU steam generator tubes.