A challenge for owners operating nuclear power plants and spent fuel canisters is to provide sound technical basis for the safety and security of long-term operation and storage, respectively. Two code cases N-729-6 and N-770-5 provide performance requirements for mitigation of Primary Water Stress Corrosion Cracking (PWSCC) in ASME Section XI and Code Case N-860 “Examination Requirements and Acceptance Standards for Spent Nuclear Fuel Storage and Transportation Containment Systems” is under development in ASME Section XI. The primary degradation concern being addressed in N-860 is Chloride Induced Stress Corrosion Cracking (CISCC). ASME Section III has formed two Task Groups with the intent to reduce PWSCC and CISCC in operation by mitigating residual stresses during new construction of components.
ASME Section III formed Task Group Weld Residual Stress (TG-WRS) to incorporate lessons learned into the Code from operating plants regarding their experience with stress corrosion cracking. An example of expected guidance from TG-WRS is to consider residual stresses after weld repair of a susceptible material when the repair surface is exposed to the wetted environment. To address this concern, one form of mitigation could be the use of SSI which has been used in operating plants. However, there is a gap because no acceptance or performance criteria exists for mitigation of SCC by surface stress improvement for ASME Section III components.
The executive committee of ASME requested that a new Task Group named “Advanced Surface Stress Improvement Technology” (ASSIT) be formed by the Korean International Working Group (KIWG) and Materials Fabrication and Examination (MF&E) to address this gap by developing a code case for mitigation of PWSCC and CISCC for new ASME Section III components by SSI techniques. This paper will discuss the background and current approach being taken by Task Group ASSIT in developing SSI acceptance or performance criteria for mitigation of SCC. The necessary technical reports supporting this code case are also under development as part of joint projects between Doosan Heavy Industries and Construction (DOOSAN), Electric Power Research Institute (EPRI), and Sun Moon University (SMU). Interim reports summarizing the status of these joint projects will be presented as separate papers in the same session.