Program for the Inspection of Nickel Alloy Components Report from the Program's Steering Committee: Inspection of Bottom-Mounted Instrumentation Nozzles Round Robin

The U.S. Nuclear Regulatory Commission (NRC) executed agreements with organizations in Japan, Sweden, South Korea, Finland, and the United States to establish the Program for the Inspection of Nickel Alloy Components (PINC). The objectives of the PINC program participants are: (1) To join together for cooperative research; and (2) To address the problem of primary water stress corrosion cracking (PWSCC), a form of degradation observed in some pressurized water reactor pressure boundary components. Specifically, the research was designed primarily to understand the morphology of PWSCC cracks, to assess nondestructive evaluation (NDE) techniques for detecting and characterizing cracks with such morphology, and to distinguish them from other types of flaws or other innocuous weld conditions. This program provided data that enabled a quantitative assessment of available NDE techniques to detect and size PWSCC in nickel-based alloys. The PINC participants identified and ranked all PWSCC and component configurations for consideration for study in the PINC framework. The three areas that were ranked highest were bottom-mounted instrumentation (BMIs), dissimilar metal welds (DMWs), and control rod drive mechanisms (CRDMs). The BMIs were identified as top priority because it is not easy to replace a reactor pressure vessel bottom head and repairs are challenging, as was learned at the South Texas Project. The issue of dissimilar metal welds was considered to also be very important based on the cracking that had been experienced at V.C. Summer and Ringhals. The CRDM issue was also assigned a high priority because of the number of plants world wide that have experienced cracking and the Davis-Besse event. However, the low availability of CRDM assemblies and the need to complete the PINC round robin in a timely fashion made it possible to address only the dissimilar metal welds and the bottom-mounted instrumentation nozzles. Dissimilar metal piping weld assemblies were available immediately so this round-robin study was able to start first. The PINC was focused on studying two aspects of PWSCC. One was to document the crack morphology and NDE responses of PWSCC and compare these data with methods to simulate PWSCC for NDE capability studies. The other was to study the capability of various NDE methods to detect and size the through-wall extent of PWSCC. The studies involving NDE capability were carried out as international round robins with PINC participants. The results are, in some cases, discussed with reference to the requirements of ASME Code, Section XI, Appendix VIII. It should be noted that the round robin study was not designed to constitute an Appendix VIII compliant demonstration. This report describes the efforts of the PINC participants to assess the capability of NDE techniques to detect and characterize the through-wall depth and length of PWSCC in bottom-mounted instrumentation penetration tubes and J-groove welds. Based upon the information that was developed from conducting round-robin exercises on BMI nozzles, this study provided the following conclusions. The conclusions and recommendations presented in this report are based on the probability of detection, false call rate, and sizing statistics measured in the round robin studies.