Fracture Assessment for Pressurized Water Reactor Vessel Nozzles Subjected to Pressure and Thermal Loading

To ensure the structure integrity of the reactor pressure vessel (RPV) in pressure water reactor plants, the main challenge is the embrittlement of beltline materials. However, the stress at inlet or outlet nozzles of the RPV which are reinforced in comparison with the beltline is more complex, especially under the thermal loads. In recent studies, significant research efforts have been conducted to show that the nozzle region may be more challenging under some conditions. In this paper, a fracture assessment for the RPV nozzles subjected to pressure and thermal loading is studied using the software abaqus 6.12 and Zen Crack 7.9-3. It includes: stress intensity factor calculation based on three-dimensional finite element method; structural integrity assessment under a typical loss of coolant accident (LOCA) transient; and the fatigue crack growth evaluation under cyclic loading situations. The results show that the stress intensity factor along the crack front is evidently asymmetric, and only to assess the safety of the deepest point along the crack front in the ASME and RCC-MR codes may be unreasonable. If the K_Ia criteria is applied, under a typical LOCA transient, it is difficult to obtain an effective fracture safety margin for a 1/4 thickness crack, while based on the K_IC criteria, the nozzle is shown to be safe in the case study. The long shallow surface crack (which is often easily produced in the nozzle area) tends to be circle type under the cyclic pressure loading situation which shows that the crack shape assumed in the ASME and RCC-MR codes is reasonable.