Effects of location, thermal stress and residual stress on corner cracks in nozzles with cladding

Abstract

The stress intensity factors ( K I ) for corner cracks in a boiling water reactor feedwater nozzle with stainless steel cladding are obtained for loading by internal pressure and a fluid quench in the nozzle. Conditions both with and without residual stress in the component are considered. The residual stress is simulated by means of a reference temperature change. The stress distribution for the uncracked structure is obtained from a three-dimensional finite element model. A three-dimensional influence function (IF) method, in conjunction with the boundary-integral equation method for structural analysis, is employed to compute K I values from the uncracked stress distribution. For each type of loading K I values are given for cracks at 15 nozzle locations and for six crack depths. Reasonable agreement is noted between calculated and previously published pressure-induced K I values. Comparisons are made to determine the effect on K I of crack location, thermal stress and residual stress, as compared with pressure stress. For the thermal transient it is shown that K I for small crack depths is maximised early in the transient, while K I for large cracks is maximised later under steady state conditions. Computations should, therefore, be made for several transient time points and the maximum K I for a given crack depth should be used for design analysis. It is concluded that the effects on K I of location, thermal stresses and residual stresses are significant and generally too complex to evaluate without advanced numerical procedures. The utilised combination of finite element analysis of the uncracked structure and three-dimensional influence function analysis of the cracked structure is demonstrated and endorsed.