New options for improved steam generator U-bend integrity

Abstract Primary side intergranular stress corrosion cracking (IGSCC) has been noted in the U-bend region of first- and second-row Alloy 600 tubes in PWR recirculating steam generators. The cracks have been found primarily at the apex and near the tangents of the bends on the extradose (outside bend radius) and in the sides near the small radius of curvature portion of the distorted tube. During fabrication the tubes were bent without stress relief. High residual strains are therefore present in these tube sections. Further, laboratory tests have shown Inconel 600 material to be susceptible to IGSCC at nuclear steam supply system operating temperatures. Although the cracking mechanism is generally understood, detection and remedial methods were not well established. The right radius bend, the tube ovality, and the complications of working in a radioactive steam generator make inspection and repair of these regions challenging. Responses to this problem had been to use an extremely conservative plugging criteria or to plug all the row 1 tubes as a preventive measure. However, as the plugging margins are reduced, this approach of taking large numbers of tubes out of service began to threaten the ability of the plant to operate at full power over the design life of the plant. Babcock & Wilcox has initiated three programs to attack the U-bend cracking problems, applying rotating probe technology for better tube examinations, stress relieving the tubes to retard IGSCC, and removing plugs to return previously plugged tubes to service. Rotating eddy current has already proven to be more sensitive than the standard bobbin coil in detecting and characterizing IGSCC in the straight tube region. The application of this technology to the U-bend offers reduced noise due to geometry variations, as well as improved sensitivity to defect conditions. The second program involved developing the capability to stress relieve row 1 and row 2 tubes. As for the second program, testing has demonstrated that stress relief substantially reduces the risk of IGSCC and improves the ductility of the original tubing. The third program had as its objective remote removal of rolled and ribbed plugs from previously plugged tubes. This capability will permit the stress relief and return to service of previously plugged tubes.