The process of the ex-vessel molten core cooling in a pre-flooded reactor cavity during a severe accident of a light water reactor includes complicated phenomena such as melt jet breakup, debris bed formation and cooling and the molten core-concrete interaction. The melt coolability and its impact on the containment consequences are dependent on the interactions among those phenomena. A simplified parametric model, COOLAP-II, covering the melt jet breakup, debris bed formation and cooling was developed for the synthetic assessment of the ex-vessel melt coolability. The model was validated on the melt breakup and initial cooling by comparison with a full-model for fuel-coolant interactions, JASMINE.

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