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

Abstract

Aging steam generator tubes have been experiencing a variety of degradations such as pitting, fretting wear, erosion-corrosion, thinning, cracking, and denting. To assist with steam generator life cycle management, some defect-specific flaw models have been developed from burst pressure testing results. In this work, an alternative approach; heterogeneous finite element model (HFEM), is explored. The HFEM is first validated by comparing the predicted failure modes and failure pressure with experimental measurements of several tubes. Several issues related to the finite element analyses such as temporal convergence, mesh size effect, and the determination of critical failure parameters are detailed. The HFEM is then applied to predict the failure pressure for use in a fitness-for-service condition monitoring assessment of one removed steam generator tube. HFEM not only calculates the correct failure pressure for a variety of defects, but also predicts the correct change of failure mode. The Taguchi experimental design method is also applied to prioritize the flaw dimensions that affect the integrity of degraded steam generator tubes such as the defect length, depth, and width. It has been shown that the defect depth is the dominant parameter controlling the failure pressure. The failure pressure varies almost linearly with defect depth when the defect length is greater than two times the tube diameter. An axial slot specific flaw model is finally developed.

Alternative methodology for assessing part-through-wall cracks in carbon steel bends removed from Point Lepreau Generating Station

“…Almost the same failure pressure of 53.0 MPa is given by the two simulations (different time step), which agree very well with the actual failure pressure. This agrees with conclusions from Reference [5], i.e., that a relatively small number of time steps can be used to obtain an accurate estimate of the failure load. This observation is useful in that it can be used to obtain significant reductions in the computer time required to perform the simulations.…”

“…This HFEM has been extensively validated against experimental measurements for both steels and aluminum alloys under all deformation states: plane stress, plane strain, and 3-D [4]. This HFEM has also been successfully applied to assess the structural integrity of degraded steam generator tubes with a variety of defects, such as pitting, uniform thinning, and axial cracks [5].…”