MELCOR 2.1 analysis of melt behavior in a BWR lower head during LOCA and SBO accident

Highlights: • MELCOR 2.1 modeling of LOCA and SBO accident on BWR. • Sensitivity analysis of melt behavior in reactor pressure vessel lower head. • Core debris discharge and location at different failure models of the lower head. • Comparison of timings of events between LOCA and SBO accident. - Abstract: Melt behavior in reactor pressure vessel (RPV) lower head is important for estimating melt leakage to reactor cavity (pedestal), molten core concrete interaction (MCCI) and radioactivity release to the environment. In order to expand universal knowledge of severe accident research on boiling water reactors (BWRs), the melt behavior in BWR lower head and leakage paths during hypothetical loss-of-coolant accident (LOCA) and station blackout (SBO) accident were analyzed with MELCOR 2.1. The instrument guide tube (IGT) failure, a potential lower head failure mechanism, was modeled in MELCOR code. When IGTs failure was not considered the lower head failed through creep rupture, resulting in the discharge of all the debris into the reactor cavity. Otherwise, the debris discharged out through the openings at the IGT locations, but a part of debris still remained in the lower plenum as solid state. The pressure vessel wall did not suffer from creep rupture after IGTs failure. The timing of events in SBO accidents were delayed compared with LOCAs, and the oxidation degree of core debris was higher in SBO accidents. In each case, only SS (stainless steel) and a small amount of Zr (zircaloy) were present in molten state in the lower plenum. Most of the discharged debris was in solid state.