Evaluation of the structural integrity of a BWR core shroud with non-regular locations of circumferential cracks. Part I fracture analysis

Abstract

ABSTRACT Boiling water reactors have internal components manufactured with stainless-steel type 304. Some relevant cracks have been detected through the periodic inspections of their internal components. Regarding the core shroud, cracks have been observed along the circumferential welds. The causes of such cracks are: radiation, vibration, stress corrosion cracking (SCC) and residual stresses, acting alone or simultaneously. Initially, the material was ductile. However, it becomes brittle as hours of operation are accumulated. In the case of this paper, three asymmetric cracks, at the circumferential horizontal weld H7, have been postulated after 25 years of operation. The loading case considered was the combination of an earthquake with a loss of coolant accident (LOCA). The structural integrity was evaluated after 30 and 60 years of operation. In the last case, such evaluation was carried out during an extended period of operation of 30 years. In all cases, a fracture mechanics approach was followed. The results showed that one of such cracks was below the safety factor. In order to get an accurate evaluation, an elastoplastic and limit load analysis is required. This has been discussed in a companion paper.