Development of PWR lower head failure model for severe accident analysis

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 thermo-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 LHTMB (Lower Head Thermal-Mechanical Behavior) analysis module is developed to analyze the thermal-mechanical behavior of lower head by referring to the generalized Hooke's Law, and combined with various failure criteria to judge RPV rupture. The module consists of three parts: thermal-mechanical behavior, failure judgment, and format output. The LHTMB module was verified by the OLHF-1 experiment data and numerical simulation results. 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.