The conditions leading to onset of thermal convection in a horizontal porous layer are determined analytically using the method of linear stability of small disturbances. The lower boundary is treated as a rigid surface and the upper boundary as a free surface. The critical internal and external Rayleigh numbers are determined for both stabilizing and destabilizing boundary temperatures. The predicted critical external Rayleigh number in the limit of no heat generation is in agreement with the critical number predicted for a porous medium heated from below.

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A series of in-pile experiments that addresses the phenomenology associated with Late-Phase processes in Light Water Reactors (LWRs) has been performed in the Annular Core Research Reactor (ACRR) at Sandia National Laboratories. The Melt Progression (MP) experiments were designed to provide information as part of the effort to develop and verify computer models for the analysis of LWR core damage during severe accidents. The MP-2 experiment is the second experiment in the series. The MP experiments examine the formation and movement of ceramic molten pools that form in the disrupted regions of a reactor core and migrate through the disrupted and intact regions of the core toward the core boundaries. The late phase of a reactor accident evolves as a consequence of early phase core degradation processes that include cladding oxidation, melting, core blockage formation, and general loss of fuel rod geometry. The MP-2 experiment assembly consisted of three regions: (1) a rubble bed composed of enriched U02 and Zr02 that simulated the severely disrupted regions of the reactor core, ( 2 ) a composite ceramic/metallic crust which represented the blockage formed by the early phase melting, relocation, and refreezing of mostly metallic core components, and (3) an intact rod stub region that remained in place below the blockage region. The test assembly was fission heated in the central cavity of the ACRR at an average rate of -0.2 K/s ultimately achieving a peak temperature in the molten pool of -3400 K. Melting of the debris bed ceramic components was initiated near the center of the bed. The molten material relocated downward under the influence of gravity, refreezing to form a ceramic crust near the bottom of the rubble bed. As ACRR power levels were increased over time, the crust gradually remelted and reformed at progressively lower positions in the bed until late in the experiment when it penetrated into and attacked the ceramic/metallic blockage. The metallic components of the blockage region melted and relocated downward to the bottom of the intact rod stub region prior to the time at which the ceramic melt penetrated the blockage region from above. The ceramic pool had penetrated halfway into the blockage region at the end of the experiment. The measurements of thermal response and material relocation are discussed and compared to the results of the computer simulations. Postexperiment examination of the assembly with the associated material interactions and metallurgy are also discussed in detail together with the analyses and interpretation of the results.

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