Service stresses in expansion transitions of heat-exchanger U-tube joints

In recent years, the service performance of steam generator tubes in many pressurized water reactors has been adversely affected by the integranular attack (on the secondary side) and stress corrosion cracking (on the primary side), especially in the localized area near the tube-to-tubesheet junctions including the expansion transitions. For a reasonably complete assessment of such failures and related issues (e.g., remedial measures) it is often essential to know the stresses and relative importance of various factors causing these stresses. Although the corresponding residual stresses are expected to play a significant role, it is also important to better quantify the expected stresses due to operational loads and geometric effects. The purpose of this paper is to present and discuss the results of stress analyses within the expansion transition zones including the local discontinuity and transition geometry effects mainly from the operating stress point of view. The local stress calculations were simplified by assuming axisymmetric loads and axisymmetric geometry; these stresses were superimposed on the non-axisymmetric bending stresses due to the overall (global) structural deformations resulting in peak stress values. The local stress evaluation consisted of elasto-static, thermal-structural analysis using finite elements; four models were analyzed to examine the effects of (a) proximity of transition zone relative to the top of the tubesheet and (b) the profile of transition zone itself (including thickness variations). Pressure and thermal conditions included (a) hot leg and cold leg, (b) fouled and clean surfaces, and (c) sludge-pile effect. The local stresses are mainly of the wall bending type with comparable magnitudes in both the hoop and the axial directions and with steep gradients in axial as well as through-wall directions. Generally, the secondary side stress magnitudes were found to be higher than on the primary side. The tube-to-tubesheet discontinuity effect was found to dominate the resultant stresses more than either the effect of zone profile or zone location (from the point of view of operating loads). Significance of these factors and the resultant stresses are briefly discussed in relation to the local corrosion failures of the tubes.