Assessment of the Integrity of Degraded Steam Generator Tube by the Use of Heterogeneous Finite Element Method

Abstract

Steam generator tubes at Ontario Power Generation (OPG) have been experiencing a variety of degradations such as pitting, fretting wear, erosion-corrosion, thinning and denting. To assist with steam generator life cycle management, OPG has developed Fitness-For-Service Guidelines (FFSG) for steam generator tubes. The FFSG are intended to provide standard acceptance criteria and evaluation procedures for assessing the condition of steam generator tubes for structural integrity, operational leak rate, and consequential leakage during an upset or abnormal event. Based on inspection results in conjunction with representative, postulated distributions of flaws in the un-inspected tubes, the FFSG provide an acceptable method of satisfying the intent of CSA-N285.4 and justifying the continued operation of degraded steam generator tubes. Some non-mandatory empirical axial and circumferential flaw models are also provided in the FFSG for structural integrity assessments. The test data from the OPG Steam Generator Tube Test Program (SGTTP) showed that the FFSG axial flaw model is conservative for a wide range of defect morphologies. A defect-specific axial flaw model was proposed for lattice-bar fret defects in I800 tubes by utilizing the SGTTP database of extensive test results. A defect-specific flaw model for outer diameter (OD) pitting and inner diameter (ID) intergranular attack in Monel 400 tubes was also developed using the SGTTP test data. More tests have been scheduled to support the development of defect specific models for axial flaws (OD cracks or ID laps) in Monel 400 and to supplement the database for Monel 400 pits. This paper explores the use of simulated testing for use in developing defect specific flaw models to reduce the amount of expensive tests. The Heterogeneous Finite Element Model (HFEM) has been developed and successfully applied to predict the failure behaviour of ductile metals under various deformation modes, i.e. plane stress, plane strain and 3-D. This two-scale (micro-macro) model takes into account the heterogeneous microstructural distribution and the consequential scatter in the mechanical properties. These inhomogeneities are then explicitly incorporated into a large deformation finite element program. In this work, the application of this HFEM to the assessment of integrity of degraded steam generator tubes is illustrated. The HFEM is validated by comparing the predicted failure modes and failure pressure with experimental tests for the tubes with uniformly thinned circumferential defects and various axial defects. The Taguchi experimental design method is then applied to prioritize the influencing parameters that affect the integrity of degraded steam generator tubes such as the defect length, depth, morphology, position and the number of defects, and internal pressure.