Integrity analysis of a reactor pressure vessel subjected to a realistic pressurized thermal shock considering the cooling plume and constraint effects

The fracture mechanic analysis of a reactor pressure vessel subjected to Pressurized Thermal Shock (PTS) loading is one of the most important issues for the assessment of life time extension of a nuclear power plant. The most severe scenario occurs during cold water injection in the cold leg due to a Loss-Of-Coolant Accident (LOCA). Two hypothetical LOCAs are assumed for an adopted reference design of a two-loop pressurized water reactor. Boundary conditions obtained from the RELAP5 code are used as input for three-dimensional computational fluid dynamics which provides the accurate description of the transient including the plume cooling effect. The safety assessment considers the comparison of the stress intensity factor for the deepest point of a surface crack front with the fracture toughness of the material. However, the fracture toughness of the materials is influenced by crack constraints determined by the T-stress. Therefore, safety margin of the reactor pressure vessel should be based on the K-T instead of on the K approach. The existence of a cooling plume affects the stress intensity factor and T-stress. Even if the cooling plume affects the T-stress, it is not translated into an increase of the safety margin. Therefore, in a PTS event with pronounced plume cooling a detailed model of the plume has to be considered in the structural analysis.