Creep deformation and damage behavior of reactor pressure vessel under core meltdown scenario

The Fukushima accident shows that In-Vessel Retention (IVR) of molten core debris has not been appropriately assessed, and a certain pressure (upto 8.0 MPa) still exists inside the reactor pressure vessel (RPV). Generally, the pressure was supposed to be successfully released, and the externally cooled lower head wall mainly experienced the temperature difference which may be more than 1000 °C. Therefore, in order to make the IVR succeed, it is necessary to investigate the creep behavior and damage distribution of the RPV under complex thermal-mechanical loadings. Accordingly, considering the unlikely core meltdown scenario as a severe accident condition, the failure site and time have to be predicted for a safety assessment purpose. Due to heavy thick wall of the RPV, the high temperature gradient inevitably results in various failure modes, i.e., plastic failure and creep failure. In disclosing it, the finite element model (FEM) has been developed for simulating the failure processes and the visco-plastic behavior of vessel wall. In accounting for the most dangerous situation, the critical heat flux (CHF) loading is applied on the RPV as a limit boundary. Subsequently, some failure modes has been found as local melting, plastic damage and creep damage by FEM. Furthermore, it is found that the RPV failure varies with the internal pressures. For the situation of low pressure and high temperature, the failure dominated by creep takes place at the hot focus site. With the further increase of pressure, the thinnest wall thickness is weakest for maintaining the RPV integrity, the damage of which initiates at the outside of RPV by plasticity. Besides, the multiaxial state of stress is found to accelerate the damage evolution on the thinnest site, and the uniaxial failure criterion underestimate the damage accumulation.