Shroud head in a steam separator of ABWR is connected to more than 300 pipes, which are attached by fillet welds. Although the welding causes welding deformation, it is impossible to correct this deformation to the shroud head because the shroud head material is very thick; more than 50 mm. Thus, it is necessary to clarify the mechanism of welding deformation so that it can be quantitatively predicted and controlled. SCC prevention is also essential by residual stress control in reactor internals. The objective of this paper is to predict welding deformation and residual stress for a shroud head, and to investigate the influence of factors such as the inclination angle of the base plate, to which the pipes are connected, on welding deformation and residual stress. Million-finite-element-order large-scale computational method of residual stress and weld distortion has been developed in order to apply directy to complicated weld structures. Details of algorithm and some applications are introduced.

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