Fracture mechanics analyses of a reactor pressure vessel under non-uniform cooling with a combined TRACE-XFEM approach

Abstract

This paper presents the integrity analyses of a model reactor pressure vessel (RPV) subjected to pressurized thermal shocks (PTS). The analyses are performed with a one-way multi-step strategy that includes the thermo-hydraulics, thermo-mechanical and fracture mechanics analyses to simulate three hypothetical loss of coolant accidents (LOCA). The thermo-hydraulics analyses are performed with the system code TRACE and a three-dimensional (3D) model of the RPV, providing the input for the structural analyses with the finite element code ABAQUS. These employ a sequential use of global model (entire RPV) and submodels (a portion of the RPV containing the crack), where the eXtended Finite Element (XFEM) approach is employed to compute the stress intensity factor (SIF) of a postulated crack in the RPV wall.The results first present a verification of the multi-step strategy with the FAVOR code for uniform temperature distribution in the RPV wall. Under uniform temperature and pressure load, a significant effect of the nozzle geometry on the asymmetric stress distribution is demonstrated. The second part of the results shows that the stresses and the SIFs are also sensitive to the non-uniform temperatures due to the presence of the cooling plumes. It is also confirmed that the analyses with ABAQUS and FAVOR provide very similar results for the medium and large break LOCA transients. For the small break LOCA, the FAVOR code underestimates the SIFs due to the missing nozzle geometry in combination with system pressure. Finally, the paper corroborates that the use of TRACE and XFEM, within the one-way multi-step simulation strategy, reduces the computational costs and the number of assumptions and approximations needed for feasible and relivable 3D fracture mechanics analyses of the RPV with consideration of the cooling plume effect.