Effects of Initial Crack Location on Failure Assessment Curves in Dissimilar Metal Weld Joints in Nuclear Power Plants

Based on detailed three-dimensional finite element analyses, accurate, option 3, failure assessment curves (FACs) based on the R6 are constructed for a dissimilar metal weld joint (DMWJ) connecting the safe end to the pipe-nozzle of a reactor pressure vessel. The effects of initial crack location in the DMWJ structure on FACs are investigated. The results show that the plastic collapse of the DMWJ structure and the limit moment M_L is mainly dominated by the plastic yield of the 316L material with lowest yield stress, and the crack locations in the DMWJ structure have less effect on the M_L. When the load ratio L_r is less than 0.8, the crack locations have almost no effect on the FACs; while after the L_r is larger than 0.8, the crack locations have significant effect on the FACs. With the crack location moving from the safe end through the DMWJ toward the thickness transition in the pipe-nozzle, the FACs shift upward, which leads to a enlargement of the safe region in the failure assessment diagrams (FADs). This is mainly caused by the different properties of the materials in the DMWJ structure. To accurately assess the integrity of the DMWJ structure, the R6 option 3 FACs based on three-dimensional finite element analyses should be constructed and used for the cracks with different positions.