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DOI: 10.1615/ICHMT.1995.RadTransfProcHeatMassTransfSevNuclReactAcc.90

Robert W. Wright


Core degradation in the initial intact rod geometry that includes the rapid oxidation transient and the resulting hydrogen generation is reasonably well understood. The current information is included in current severe accident codes such as SCDAP/RELAP5, ICARE2, ATHLET/CD, and MELCOR. There is a reasonable understanding of the early metallic melt phase of melt progression and, in blocked core accidents like TMI-2, a less adequate understanding of the late ceramic melt phase. An important early-phase result is that the metallic melt separates by gravity from the ceramic fuel-rod pellets that remain in place. It appears that this separation is maintained throughout the melt progression process, so that the melts later released from the core and from the reactor vessel are nearly pure ceramic (as in TMI-2) or pure metallic, but not a mixture. The late phase information has come primarily from the TMI-2 core examination and also from the MP-1 and the MP-2 experiments in ACRR. Two major phenomenological uncertainties remain regarding melt progression. The first is whether there are any severe accident sequences, possibly dry core BWR sequences, in which the metallic melt drains from the core when formed rather than freezing to block the core as at TMI-2. In such sequences, the characteristics of the melt released from the core (and later from the reactor vessel) on meltthrough would be very different from the nearly pure ceramic melt at TMI-2. The second major question is the phenomenology of the meltthrough from the core of the ceramic melt pool. This phenomenology determines the threshold and the location of pool meltthrough from the core, that in turn determine the released melt mass. Other remaining potentially significant but less important melt progression uncertainties are also discussed.

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