Analysis of Residual Stresses On the Expanding Transition Zone of Steam Generator Tubes of Apr1400

Abstract

The Advanced Power Reactor 1400 (APR1400) Steam Generator (SG) uses alloy 690 as tube material and SA-508 Grade 3 Class 1 as tubesheet material to form tube-to-tubesheet joint through the hydraulic expansion process. Failures in SGs have been identified to occur due to poorly formed tube-to-tubesheet expansion joint or due to Stress Corrosion Cracking (SCC) of the tube resulting from high residual tensile stresses in the transition zone. This paper investigates this residual stresses. Both theoretical and Finite Element Methodology (FEM) was used to assess and compare the contact pressure in the tube-to-tubesheet joint. In addition, the expansion joint axial strength was assessed by comparing the pull-out force against the joint contact force and an iterative method in FEM used to determine the limit autofrettage pressure. A single U-tube hole and tubesheet with ligament thickness were taken as a single cylinder and plane strain condition assumed. The results obtained shows that the maximum residual longitudinal tensile stress occurs at the inner surface of the tube with a value of 230.32MPa and the maximum residual hoop stress also occurs at the inner surface with a value of 151.76MPa. The joint contact force was estimated to be 20 times more than the pull-out force and the limit autofrettage pressure was estimated to be 141.85MPa.

Influence of expansion rate on circumferential residual stress distribution of expandable casing

“…S.Al-Hiddab et al [1][2] studied the effect of equivalent stress and residual stress on the expansion process, the distribution and condition of residual stress after casing expansion. Kiptisia W et al [3][4][5] used theoretical analysis and finite element method to analyze the residual stress of tube transition zone and joint. Xu Binggui et al 6 proposed the application of stress, strain and residual stress of solid expansion pipe in the process of change to repair casing.…”