Impact of Welding Sequence on the CRDM Nozzle-to-Vessel Weld Stress Analysis

A three-dimensional finite element model is presented to simulate the welding process of the side-hill control-rod-drive-mechanism (CRDM) nozzle to the vessel head. Emphasis is given to how the weld is laid out in the analysis so that accurate residual stress results can be obtained while the required computing time is viable. In the order of complexity, three approaches are examined in this study: a) the simultaneous approach, i.e., the weld bead (therefore the heat associated with it) is put in the model in a uniform fashion; b) the piece-by-piece approach, i.e., the weld is laid out segment by segment; c) the moving-source approach where the analysis is done by simulating the moving heat source. It is found that there is a significant difference between the stress results by the uniform approach and the piece-by-piece approach. While the moving source method gives the closet representation of the welding process, the computing time for such a multi-pass, three-dimensional model is still prohibitive. The natural choice is therefore the piece-by-piece approach, with the number of segments for the weld dependent on the weld parameters and the geometries of the nozzle and vessel head.