Study on the lower head failure in severe accident Part I: Model development and validation

Abstract

Under the in-vessel retention (IVR) scenario, the lower head is loaded by the deadweight of melt and the vessel, the internal pressure, and the temperature field. This thermal–mechanical load will destroy the integrity of the reactor pressure vessel (RPV). The failure of the lower head indicates that the in-vessel accident progression has entered the final stage. If the failure occurs, its location and time have to be evaluated. However, most severe accident analysis codes judge RPV failures based on simple parametric models, which cannot accurately predict the failure time, modes, and locations. In this paper, the Lower Head Thermal-Mechanical Behavior analysis module (LHTMB) and Large Displacement model are developed with reference to the generalized Hooke’s law and the Large Displacement elastic–plastic theoretical model. The failure of the lower head is judged by several failure criteria. The developed failure model of lower head is verified based on the OLHF-1 numerical simulation results and experimental data. The mechanical results show that creep rate is usually highest at the location where the lower head is most prone to failure. Comparison of various failure criteria shows that RPV failure can be determined with high confidence when Larson–Miller life fraction and Hedl-Dorn parameter criteria are satisfied.