Examination of Service Cracks in CRDM Nozzles and Relevance to Flaw Evaluation Procedures

In Section XI of the ASME B&PV Code, a flaw evaluation procedure was recently developed for axial surface cracks in control-rod-drive mechanism (CRDM) nozzles in PWR reactor pressure vessel heads, i.e., Code Case N694-1 and IWB-3660. Some of the aspects of the analysis procedures are not rigorously defined due to numerous unknown aspects of this complicated problem. The U.S. operators of PWR plants were required by the NRC to make inspections of their CRDM nozzles and report the inspection results. The data was recently analyzed to assess how the flaw sizes develop and how that might provide guidance to the ASME flaw evaluation criterion. The results from numerous plants showed that the surface cracks tend to develop with a relatively constant aspect ratio for a certain plant, but that aspect ratio can significantly change from plant to plant. Using weld residual stresses and operating stresses with rigorous K-solutions for the depth and length of the surface cracks and the PWSCC crack growth rates, however, predicts surface flaws that would be much shorter in length than occur in service. It is suggested that rather than calculating the length of the surface flaws using the K-solutions, that the growth in the depth direction be calculated using the K-solutions (with some additional guidance) and the length be based on the aspect ratios found in the particular plants.