Creep and Damage Analysis of Reactor Pressure Vessel Considering Core Meltdown Scenario

Abstract

The Fukushima accident shows that In-Vessel Retention (IVR) of molten core debris has not been appropriately assessed, and a certain pressure (up to 8.0MPa) still exists inside the reactor pressure vessel (RPV). Generally, the pressure is supposed to successfully be released, and the externally cooled lower head wall mainly experiences 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 behaviour and damage distribution of the RPV under complex thermal-mechanical loadings. Accordingly, considering the unlikely core melt down scenario for a light water reactor (LWR) a possible failure mode of the reactor pressure vessel (RPV) and its failure time has to be predicted for a determination of the loadings on the containment. Due to the thickness of RPV, the high temperature gradient results in various failure modes, i.e., plastic failure and creep failure. In disclosing the failure mechanism, the finite element model has been developed simulating the thermal processes and the visco-plastic behaviours of vessel wall. An advanced model for creep damage has been established to analyze the fracture time and fracture position of a vessel with an internally heated melt pool. Before the above, the stress and strain distributions along the wall thickness are investigated by ABAQUS software. Finally, the result shows that the calculated stress outside the RPV is lower than the yield stress of the material through most thickness. It is concluded that the RPV can maintain its integrity under IVR with given time, even if there exists the internal pressure of 8MPa.