Modelling of reactor pressure vessel subjected to pressurized thermal shock using 3D-XFEM

We describe the model developed for integrity assessment of a reactor pressure vessel (RPV) subjected to pressurized thermal shock (PTS). The assessment is based on a multi-step simulation scheme, which includes the thermo-hydraulic, thermo-mechanical and fracture mechanics analyses. The thermo-mechanical model uses the three-dimensional finite element method (FEM) to calculate the stress fields of the reactor pressure vessel (RPV) subjected to the thermal load and pressure during large break loss of coolant accident (LBLOCA). In this contribution, the multistep simulation scheme is extended to the analysis of two-phase steam-water flow occurring in the LBLOCA situation and the fracture analysis of postulated cracks under this accident event. It is demonstrated that the principle of the combined one-way coupled system code, CFD and structural mechanics codes can be used for the fracture analysis in case of a transient accident, which involves two-phase flow. The prediction of the temperature field is achieved by using two-phase computational fluid dynamics (CFD) simulation. To perform the stress analysis, an appropriate finite element discretization of the RPV wall is used, which considered the cladding and the ferritic low allow steel. The calculation of the stress intensity factor (SIF) in mode I for hypothetical cracks located in critical positions of the RPV is based on the linear elastic fracture mechanics (LEFM). The fracture mechanics model is based on the node-based submodeling technique in combination with XFEM in Abaqus. This is used to refine the mesh required by the fracture analysis in the region of interest, especially those regions under the cooling plume formed during the LBLOCA. The submodel may contain three types of cracks: axial, circumferential and inclined. The performed integrity assessment compares the stress intensity factor calculated in the deepest point of a surface crack with the material's fracture toughness. Two approaches to extract the stress intensity factor were applied in the present paper, namely, the classical FEM and eXtended FEM or XFEM. The classical FEM and XFEM solutions are compared. Both techniques give similar results. However, XFEM is more convenient as it is mesh independent.