Aniso-thermal creep damage modeling for SA-508III steel in nuclear reactor pressure vessels under core meltdown

Aniso-thermal creep is one of the key deformation and failure modes of the nuclear reactor pressure vessel (RPV), which experiences ablated wall thinning due to the core meltdown. In this work, an aniso-thermal Kachanov-Rabotnov creep damage model was developed for a SA-508III steel based on a series of iso-thermal, uniaxial creep tests in a wide temperature range from 600 to 1200 °C. In addition, axially loaded notch bar creep rupture tests at 700 °C were conducted to determine the value of the triaxial stress state parameter in the Kachanov-Rabotnov model, which was found to be 0.16. Aniso-thermal creep tests were also performed for model validation, the maximum deviation in rupture lifetime prediction was within 6 %, and the relative error in creep strain rate remained between 2.5 % and 12 % throughout the test duration. The aniso-thermal creep damage model was then implemented into a multi-axial finite element (FE) framework via a user-defined material subroutine to model the thermodynamic responses of boiling water RPV under the ablated wall thinning situation. The developed aniso-thermal Kachanov-Rabotnov creep damage model and the associated FE modelling methodology have clearly demonstrated a potential capability in RPV structural integrity assessment and life prediction.