A Finite Element Study on the Integrity Evaluation Method of Subclad Cracks Under Pressurized Thermal Shock Transients

The reactor pressure vessel is usually clad with stainless steel to prevent corrosion and radiation embrittlement, and thus, the population of subclad cracks is higher than that of surface cracks. In fact, a number of subclad cracks have been found during in-service-inspections. These subclad cracks should be assumed for a safe operation under normal conditions and faulted conditions such as pressurized thermal shock (PTS). There are several elastic stress intensity factor (SIF) solutions for subclad cracks which are used for ASME fracture assessment procedures. However, these solutions are known to be overly conservative due to intrinsic assumptions. In this paper, full 3-D elastic-plastic finite element analyses were performed to investigate the validity of the present ASME code in evaluating the integrity of vessel under PTS conditions. The solution in ASME Section XI code gives accurate SIF values when the crack tip is close to cladding. However, the difference between the code solution and elastic plastic finite element analysis (EPFEA) results become significant with increasing crack depth. This difference is caused by the stress discontinuity at the interface between cladding and parent material. Elastic finite element analysis (EFEA) results, which do not consider plastic deformation at the clad material, also showed a big difference compared to those from EPFEA. Therefore, it is recommended to perform EPFEA for the accurate assessment of subclad cracks under PTS which causes plastic deformation at the cladding.