[en] Recently, the capabilities of the CORMLT code, which was designed to predict heatup, degradation, and meltdown of core and Reactor Pressure VEssel (RPV) internals during postulated severe accidents, were enhanced to enable tracking of individual fission product species during core meltdown. In addition, a mechanistic treatment of the release and flow of molten materials was developed to replace the engineering models developed earlier. In the present paper, the improved models are described and predictions of melt progression for a postullated accident sequence (TMLB') are discussed. A key issue in the new modeling is the mechanical behavior of fuel pellet stacks during run-off of molten cladding. One view is that capillary forces result in ''welding'' of porous fuel, thereby promoting free-standing pellet stacks; another is that rubblization and slumping of fuel take place. Results are reported for the first view point and its impact on the timings for core collapse into the bottom-head is addressed