Fracture mechanics investigation regarding the effects of cracks on the structural integrity of a BWR-core shroud

As a consequence of core shroud intergranular stress corrosion cracking (IG SCC) detected in the course of inservice inspections, a fracture mechanics analysis was carried out to evaluate the effects of postulated cracks on th e structural integrity. In this study, critical crack sizes and crack growt h were calculated. Due to the comparatively low stress acting on the core s hroud during normal operation, the residual stresses in the welds make up t he major proportion of the tensile stresses responsible for IGSCC. In order to consider residual stresses of the lower core support ring welds, a fini te element analysis was performed at MPA Stuttgart using the FE-code ANSYS. The crack growth computed on the basis of USNRC crack growth rates da/dt d emonstrated that crack growth in depth direction increases quickly at first , then retards and finally comes almost to a standstill. The cause of this 'quasi-standstill' is the residual stress pattern across the wall, being ch aracterized by tensile stresses in the outer areas of the wall and compress ive stresses in the middle of the wall. Crack growth in circumferential dir ection remains more or less constant after a slow initial phase. As the cal culation of stress intensity factors K-1 of surface flaws under normal cond itions demonstrated, a 'lower bound' fracture toughness value is only excee ded in the case of very long and deep surface flaws. It can be inferred fro m crack growth calculations that under the assumption of intergranular stre ss corrosion cracking, the occurrence of such deep and at the same time lon g flaws is unlikely, regardless of the initial crack length. Irrespective o f the above, the calculated critical throughwall crack lengths, which were determined using a 'lower bound' fracture toughness value, demonstrated tha t even long throughwall cracks will not affect the component's integrity un der full load. Moreover, it can be concluded from the studies of crack grow th that-assuming intergranular stress corrosion cracking-a sufficiently lon g period will elapse before a crack which has just been initiated reaches a relevant size. Therefore, it can be stated that these cracks will likely b e detected during periodic inservice inspections. (C) 1999 Elsevier Science S.A. All rights reserved.