Weld overlay repairs from conception to long-term qualification

Abstract

The weld overlay was initially developed in 1982 as an interim repair for intergranular stress corrosion cracking (IGSCC) flawed welds in boiling water reactor stainless steel piping. Since that time extensive work has been performed to qualify the weld overlay repair technique for longer-term service. Analytical studies of the weld overlay process, experimental programs on laboratory and field specimens, tests to demonstrate the beneficial effects of the weld overlay over its service life, and programs to develop nondestructive examination (NDE) techniques for use in inspecting weld overlays have all yielded positive results. These programs have been sponsored by the utility industry, either individually, through EPRI sponsorship or as part of the BWR Owners' Group (BWROG) effort, and by the US Nuclear Regulatory Commission (USNRC). Weld overlay repairs serve several design functions. These include: structurally reinforcing the flawed location to restore code margins to failure, providing an IGSCC-resistant barrier to crack growth, and imposing a favorable residual stress distribution in the inner portion of the flawed component to inhibit further flaw initiation and growth. The qualification programs confirm that these design functions are maintained with long-term service. A regulatory barrier to long-term operation with weld overlays was the need periodically to inspect overlays and thereby demonstrate continued integrity. Although IGSCC flaws have generally been detected by ultrasonic methods, the repair of these flaws by weld overlay made continued monitoring of the repaired flaw by conventional ultrasonic techniques difficult. Recent developments in ultrasonic examination equipment and techniques, including surface preparation criteria, have enhanced the inspectability of weld overlay repairs and the underlying base material. The original acceptance of weld overlay repairs by the USNRC was for limited service (one or two fuel cycles of operation). However, more recent regulatory guidance, including the recently issued NUREG-0313, Revision 2, recognizes the above technical advances, and provides criteria for acceptance of weld overlays as long-term repairs. Several US utilities have been successful to date in obtaining USNRC approval for extended operation with weld overlays. Some overlays at these plants are currently entering their fourth successful fuel cycle of operation.