[en] The US Nuclear regulatory Commission has made the development of mechanistic models for severe accident progression a major priority. The purpose of these models is to provide detailed, best-estimate, coupled analyses of all the major phenomena involved in the reactor vessel and coolant system in the course of the accident. To meet this objective, the MELPROG computer code is being developed. This report describes the two-dimensional, pressurized water reactor (PWR) version of the MELPROG computer code, MELPROG/PWR-MOD1. Preliminary BWR work is described in this report. MELPROG is coupled to the TRAC-PF-1 RCS thermal-hydraulics code to provide an integrated analysis of the behavior of core, vessel, and reactor coolant systems during severe accidents. MELPROG treats core degradation and loss of geometry, debris formation, core melting, attack on supporting structures, slumping, melt/water interactions and vessel failure. The key element in MELPROG is the use of detailed modeling for the entire damage progression and failure sequence. Emphasis is also placed on the rates of hydrogen, steam and fission product formation, and transport to containment during the accident