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We are trying to apply surface stress improvement process (SSIP) to the repair and maintenance of nuclear power components, mainly in the United States and Japan. A representative example is applying water jet peening (WJP) and laser peening (LP) technology to parts to extend their lifespan. The biggest problem with the lifespan of nuclear power equipment is the occurrence of stress corrosion cracking (SCC), which is directly related to safety. The SSIP is one of the good ways to mitigate SCC. The Code Case N-931 “Performance and Qualification Criteria for Mitigation of Stress Corrosion Cracking by Surface Stress Improvement: Section III, Division 1 and 3’ was developed since 2017 by the task group “Advanced Surface Stress Improvement Technology” in Korea International Working Group (KIWG) of the American Society of Mechanical Engineers (ASME). This case provides performance and qualification criteria for the pre-service application of a SSIP to items potentially susceptible to SCC, as a means to prevent crack initiation. Mitigation of items to the requirements of this case does not modify or change the in-service inspection (ISI) requirements for those items. This Code Case is to be used for SSIP such as ultrasonic nanocrystal surface modification (UNSM), WJP, LP that satisfy the performance criteria and qualification criteria defined in this case. Two major technical bases for this Code Case are “PVP2022-85151” and “EPRI Report 3002018458”. Brief summary of UNSM technology and two technical bases is introduced.

Following the inquiry “What requirements may be used for performance and qualification criteria of surface stress improvement processes (SSIP) to mitigate the potential for stress corrosion cracking (SCC) for Section III, Division 1 and 3 items?” The code case N-931 “Performance and Qualification Criteria for Mitigation of Stress Corrosion Cracking by Surface Stress Improvement: Section III, Division 1 and 3’ was developed since 2017 by the task group “Advanced Surface Stress Improvement Technology” in Korea International Working Group of ASME (American Society of Mechanical Engineers). This case provides performance and qualification criteria for the pre-service application of a surface stress improvement process (SSIP) to Section III Division 1 and 3 items potentially susceptible to SCC, as a means to prevent crack initiation. Mitigation of items to the requirements of this case does not modify or change the in-service inspection (ISI) requirements for those items. Items with base material potentially susceptible to SCC due to environmental conditions that have been identified in the design specification, as required by NCA-3211.19(b) (Division 1) or by WA-3351.2 (Division 3). Items with weld material potentially susceptible to SCC due to tensile residual stresses caused by weld repairs at the wetted surface. Paragraphs NB- 4451, NCD-4451, and NG-4451 prohibit weld repairs on the wetted surface of items susceptible to SCC unless a mitigation action is applied. This Code Case is to be used for surface stress improvement processes such as ultrasonic nanocrystal surface modification (UNSM), water jet peening, or laser peening that satisfy the performance criteria and qualification criteria defined in this case. Two major technical bases for this code case are PVP2022-85151 and EPRI Report 3002018458. Brief summary of UNSM technology and two technical bases is introduced.

Fitness-for-Service is a useful technology to determine replacement timing, next inspection timing or in-service when nuclear power plant's buried pipes are damaged. If is possible for buried pipes to be aged by material loss, cracks and occlusion as operating time goes by. Therefore Fitness-for-Service technology for buried pipe is useful for plant industry to perform replacement and repair. Fitness-for-Service for buried pipe is studied in terms of existing code and standard for Fitness-for-Service and a current developing code case. Fitness-for-Service for buried pipe was performed according to Code Case N-806 developed by ASME (American Society of Mechanical Engineers).