The progression of core melting in a nuclear power plant is characterized by various complications due mainly to chemical reactions and interactions occurring between core materials, which modify the main core temperatures, affecting the time when melt is effectively beginning. On top of it the geometry of a Boiling Water Reactor (BWR) is introducing additional challenges to code modelers. Challenges exist because of the necessity to adopt approximations in the core discretization, which might result in incorrect predictions of the actual phenomena. In particular large uncertainties exist for a clear definition of the fuel and control rod interfaces, discretization of the lower core region as well as control rod guide tubes and monitor penetrations in the lower head. The severe accident code SAMPSON, with an advanced characterization of the core and lower plenum region, is able to recreate a more realistic representation of the phenomena occurring during the melt progression. In this work, SAMPSON description of molten fuel relocation and debris spreading/cooling are presented. State of the art simulation by the SAMPSON code of Fukushima Daiichi unit 3 has shown that core melting progression develops until zirconium is contained within the core, triggering large heat release during oxidation. Thereafter core conditions almost stabilize and 35% of the core falls into the lower plenum, where pipe penetration ejection is predicted releasing molten material within the pedestal.

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