Natural PWSCC Crack Growth in Dissimilar Metal Welds With Inlay

The commercial nuclear power industry has proposed several mitigation techniques to address safety concerns due to primary water stress corrosion cracking (PWSCC) in nickel-based dissimilar metal (DM) welds (specifically Alloy 82/182 welds) in pressurized water reactors (PWRs). Since many of these welds reside in primary piping systems that have been approved for Leak-Before-Break (LBB), the mitigation strategies chosen must ensure that these systems still satisfy the LBB criteria. Mechanical Stress Improvement Process (MSIP), Full and Optimized Structural Weld Overlay (FSWOL, OWOL), and Inlay and Onlay cladding are examples of the currently proposed mitigation methods. This paper focuses on an evaluation of the inlay process for the mitigation of PWSCC since it may be the technique of choice for the large-diameter reactor coolant nozzles. Currently the ASME Section XI code is developing Code Case N-766 ‘Nickel Alloy Reactor Coolant Inlay and Cladding for Repair or Mitigation of PWR Full Penetration Circumferential Nickel Alloy Welds in Class 1 Items.’ This code case is documenting the procedures for applying inlay welds. The residual stresses caused by the inlay process were used to model the natural crack growth through the inlay in this paper. The inlay residual stresses and modeling methods are presented in a companion paper. Since the PWSCC crack growth rate is much slower in the inlay material (Alloy 52) compared with the Alloy 82/182 weld metal, the crack growth shape retains a ‘bubble’ appearance. This shape is a challenge to model within the framework of advanced finite element based natural crack growth methods. This paper focuses on the crack growth modeling challenges, the actual growth shapes for different weld repair and inlays processes, and finally compares crack growth rates to those made using a simple crack growth shape.