Effects of Weld Geometry on Residual Stress and Crack Driving Force for Centerhole Control Rod Drive Mechanism Nozzles: Part II — Circumferential Cracked K-Solutions

The U.S.NRC has undertaken a program to assess the integrity of CRDM nozzles in existing plants that are not immediately replacing their RPV heads. This two-part paper summarizes some of the efforts undertaken on the behalf of the U.S.NRC for the development of detailed residual stress and circumferential crack-driving force solutions to be used in probabilistic determinations of the time from detectable leakage to failure. In this second paper, the weld residual stresses from the first paper were mapped onto detailed fracture mechanics finite element models that contained different length circumferential cracks in the CRDM centerhole nozzle. In each case, the cracks were unpinned after application of the operating pressure and temperatures and the K-solutions extracted. The results from these analyses suggest that the crack-driving force for a circumferential crack at the root of the J-weld slightly increases with increasing weld height but appears to be insensitive to bevel angle.