Development of the Technical Basis for the New Code Case “Performance and Qualification Criteria for Mitigation of Stress Corrosion Cracking by Surface Stress Improvement”

Abstract Stress corrosion cracking (SCC) of nuclear power plant components is perhaps the most significant issue affecting nuclear power plant availability and plant lifetime management. Actions were taken within ASME Section III Task Group Weld Residual Stress to ensure mitigative actions, such as surface stress improvement (SSI), are taken when a construction repair is made on a surface that will be wetted and susceptible to SCC. The intent is to minimize the potential for SCC in operation by mitigating potentially susceptible areas during construction. However, no performance criteria specific to such construction repairs exist for pre-service SSI methods that mitigate SCC. Thus, TG-ASSIT is developing an alternate set of performance criteria that will be captured in a Section III Code Case. Additional testing was undertaken to support the development of this Code Case. A number of organizations including DOOSAN and the Welding and Repair Technology Center (WRTC) and Advanced Nuclear Technology (ANT) programs of the Electric Power Research Institute (EPRI) jointly executed a three-year test program focused on evaluating the viability of novel-to-nuclear surface stress improvement techniques such as Ultrasonic Nanocrystal Surface Modification (UNSM) and Low Plasticity Burnishing (LPB) techniques. The program also included further demonstration of laser peening (LP) processes as a complement to the LP test data already captured by prior EPRI Materials Reliability Program (MRP) work (i.e., MRP-267 R2). This paper summarizes the results from the DOOSAN-WRTC-ANT test program and follow-on inspectability testing for UNSM and LPB that supports the SSI performance criteria for mitigation of SCC of ASME Section III components.