This report, prepared by the task group Degraded Core Cooling (DCC) for the CSNI, summarizes the experimental and theoretical knowledge of molten material relocation from a degraded core to the lower plenum of the reactor vessel under severe accident conditions. First, an overview of core melt progression is provided, emphasizing the connection with ultimate material transfer to the lower plenum. The initial core degradation processes from core uncovering to melting of the metallic materials are described, with reference to fuel damage experiments such as PBF-SFD, PHEBUS-SFD, ACRR DF and ST, and CORA. The later stages of core melt progression are described, emphasizing the essential differences between relocation of metallic and ceramic materials. The nature of the crust formation and melt pool growth, leading to melt release to the lower plenum is discussed, with reference to melt progression experiment such as ACRR MP and PHEBUS FP, and the experience gained from the analysis of the TMI-2 accident.
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