Heat up and failure of BWR upper internals during a severe accident

•A higher fidelity model of the BWR upper internals was developed for MELCOR.•The upper internals are predicted to heat up and relocate during an accident.•Oxidation and relocation of debris impacts the subsequent accident progression.•The results are compared to and consistent with the limited past investigations. In boiling water reactors, the shroud dome, separators, and dryers above the core are made of approximately 100,000kg of stainless steel. During a severe accident in which the coolant boils away and exothermic oxidation of zirconium occurs, gases (steam and hydrogen) are superheated in the core region and pass through the upper internals. In this scenario, the upper internals can also be heated by thermal radiation from the hot degrading core. Historically, models of the upper internals have been relatively simple in severe accident codes. The upper internals are typically modeled in MELCOR as two lumped volumes with simplified heat transfer characteristics and no structural integrity considerations, and with limited ability to oxidize, melt, and relocate. The potential for and the subsequent impact of the upper internals to heat up, oxidize, fail, and relocate during a severe accident was investigated. A higher fidelity representation of the shroud dome, steam separators, and steam driers was developed in MELCOR v1.8.6 by extending the core region upwards. The MELCOR modeling effort entailed adding 45 additional core cells and control volumes, 98 flow paths, and numerous control functions. The model accounts for the mechanical loading and structural integrity, oxidation, melting, flow area blockage, and relocation of the various components. Consistent with a previous study, the results indicate that the upper internals can reach high temperatures during a severe accident sufficient to lose their structural integrity and relocate. The additional 100metric tons of stainless steel debris influences the subsequent in-vessel and ex-vessel accident progression.