[en] The MELPROG computer code is being developed to provide mechanistic treatment of Light Water Reactor (LWR) accidents from accident initiation through vessel failure. This paper describes a two-dimensional (r-z) debris meltdown model that is being developed for use in the MELPROG code and discusses validation experiments. Of interest to this study is melt progression in particle beds that can form in both the reactor core and the lower plenum during severe LWR accidents. Key results are (1) a dense metallic crust is created near the bottom of the bed as molten materials flow downward and freeze; (2) liquid accumulates above the blockage as solid continues to melt and if Zirconium is present, the pool grows rapidly as the molten Zr dissolves both UO₂ and ZrO₂ particles; (3a) if the melt wets the solid, a fraction of the melt flows radially outward under the action of capillary forces and freezes near the radial boundary; and (3b) in a nonwetting system, all of the melt flows into the bottom of the bed. Solutions are qualitatively similar to the post-accident configuration of the Three-Mile Island (TMI-2) core. When the models discussed here are implemented in the MELPROG code, we will be able to conduct a very detailed TMI-2 calculation