Development of MPS method and analytical approach for investigating RPV debris bed and lower head interaction in 1F Units-2 and 3

The onsite investigations of Fukushima Daiichi Nuclear Power Plant (1F) Units-2 and 3 indicate possibilities multiple breaches the Reactor Pressure Vessels (RPVs). In meantime, some analytical works that core materials 1F were once relocated to lower plenum RPVs cooled (quenched) before water inventory boiled-off (dry out) once-cooled debris re-melted. This study utilizes Lagrangian-based Moving Particle Semi-implicit (MPS) method investigate such complex solid-liquid multiphase re-melting debris-vessel wall interactions obtain new insight on RPV failure modes help future retrieval from these reactors. Two major modifications/developments have been carried out based previously developed MPS method. Namely, stabilization rigid-body contact model further improvement speedup algorithm enable large long scale debris-bed analyses real plant scale. numerical accuracy pressure boundary condition at fluid-wall has also improved. Sensitivity with results possibility lateral part may subject not only convective heat transfer metallic melt pool, but by conductive oxidic conglomerate. following initial vessel breach, discharge induces relocation conglomerates, leading concentration source around central (bottom) head. However, uncertainties associated are acknowledged validations necessary drawing insights.