On welding residual stresses in structural integrity analyses of NPP components

It is typically assumed that weld residual stresses (WRSs) in the pipings of nuclear power plants (NPPs) are confined to the weld region and its immediate vicinity and have locally relatively high values, of the order of tensile yield stress in and near the inner surface. In many cases the welds have not been post weld heat treated, and thermal relaxation of the WRS levels is small due to the relatively modest operational temperatures. Therefore, the WRSs in NPPs can potentially affect the structural integrity through the risk of stress corrosion cracking (SCC). In this study the WRS assumptions of commonly used fitness-for-service procedures are compared for a set of representative NPP pipe sizes. Also, the behaviour of WRSs as a function of typical load transients, corresponding to several years of plant operation, is simulated with finite element analysis (FEA) for a nozzle connected to a safe-end and pipe. Finally, from the results of the computational analyses, conclusions and recommendations are given on the application of WRS assumptions.