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

Abstract

To ensure the structure integrity of the RPV, the main challenge is the embrittlement of beltline material. However, the stress of inlet or outlet nozzles of the RPV which are in general reinforced in comparison with the beltline, is more complex especially under the thermal loads. In recently studies, a lot of works have been done 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 discussed using the software ABAQUS 6.12 and Zen Crack 7.9-3. It includes: SIF calculation based on 3D finite element method; structural integrity assessment under a typical LOCA transient; and the fatigue crack growth evaluation under cyclic loading situations. The results show that the SIF along the crack front is obviously asymmetric, and only to assess the safety of the deepest point along the crack front in the ASME and RCC-MR codes may be reconsider. If the KIa 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 KIC criteria, the nozzle is shown to be safe in the case study. The shape of the surface elongated crack (which is often easily produced in the nozzle area) tends to be circle under the cyclic pressure loading situation which shows the crack shape assumed in the ASME and RCC-MR codes is reasonable.