A Fracture Mechanics Evaluation of an Indication in an RPV Vertical Shell Weld Produced by the Upjohn Welding Technique

A reactor pressure vessel (RPV) manufacturer used the Upjohn welding technique to fabricate the longitudinal and meridional seam welds in a number of RPVs. The Upjohn welding process allowed fabrication of heavy plate sections without the use of positioning devices. With this welding technique, the major dimension of fabrication flaws extends through the weld thickness rather than parallel to the thickness as with other fabrication welding techniques. One vertical weld seam of a RPV fabricated using the Upjohn weld process was examined by ultrasonic techniques consistent with PDI requirements. Flaws were detected during the ASME Code required in-service inspection. After the application of the flaw proximity rules, one indication was characterized as a planar flaw 4.24 inches long (along the weld thickness direction) and 2-inches wide. One end of the indication was located 0.7 inch from the ID surface, which included a clad thickness of 5/16 inch. The use of Section XI flaw characterization and proximity rules would have required the characterization of this flaw as a large surface flaw. Therefore, a three-dimensional finite element model, including crack tip elements, was developed to determine the increase in the theoretical infinite plate K value due to the edge effect. The flaw evaluation was based on the guidelines of Appendix A of ASME Section XI. The allowable K values considered adjusted reference temperature (ART) based on cumulative fluence from operation to end of design life, which included a 20-year license renewal period. Because the calculated values of K were less than the allowable values of K, it was concluded that the indication is not expected to become a surface indication during future operation. Discussion on recommendations for RPVs with Upjohn welds, suggested changes to ASME Section XI Paragraph IWB-3610 and a comparison with the RPV flaw database is also provided.Copyright © 2004 by ASME