PREEMPTIVE WELD OVERLAYS TO MITIGATE PWSCC CONCERNS IN PWR PIPING SYSTEM DISSIMILAR METAL BUTT WELDS

摘要

Primary water stress corrosion cracking (PWSCC) continues to be a concern in nickel-based alloys (Alloy 600 and the associated weld metals, Alloys 82 and 182) in pressurized water reactors (PWRs). It has caused cracking and leakage in a number of components, including steam generator tubes, vessel head penetrations, and most recently, the dissimilar metal butt welds (DMWs) commonly used to connect vessel nozzles to PWR primary system piping. Weld overlays (WOLs) have been used extensively in the past twenty years to repair nuclear plant piping that has been found to be cracked or leaking due to stress corrosion cracking [1]. This paper summarizes the advantages of and technical justification for applying preemptive weld overlays (PWOLs) before cracking or leakage is observed, to mitigate PWSCC in Alloy 82/182 butt welds. PWOL design is governed by a number of considerations. The PWOL must supply sufficient thickness of resistant material (Alloy 52 weld metal) to provide new structural reinforcement of the original pipe weld sufficient to sustain design basis loads within ASME Code margins. Structural reinforcement calculations are presented demonstrating the achievement of this capability in accordance with ASME Section XI rules for evaluation of flaws in austenitic piping. The PWOL must supply sufficient thickness to effectively reverse the highly tensile residual stresses from the original DMW, including the potential detrimental effects of an in-process repair weld. Residual stress evaluations using elastic-plastic finite element models are presented that demonstrate the achievement of this objective for several typical nozzle geometries. Finally, analyses are presented to demonstrate that a dissimilar metal weld, with PWOL applied, meets the Nuclear Regulatory Commission (NRC) criteria for leak-before-break (LBB).