[en] The in-vessel modules of the integrated LWR melt progression analysis code, MELPROG, are described. These in-vessel modules are being developed along with a reactor coolant system model and a containment response model to provide a detailed, best-estimate analysis tool for severe accident analysis in both BWR and PWR designs. The MELPROG code, which is designed to treat melt progression from accident inception up to and through melt ejection from the reactor vessel, consists of eight explicitly coupled modules: Fluids, Pin-Behavior, Structures, Debris Formation/Behavior, Radiation, Vapor Explosions, Fission Product Release and Transport, and Material Ejection. The first version of the code, which includes the first five modules, has been completed and tested on a simulation of an S1D accident. Results of that simulation are presented