The Source Term Code Package (STCP), an integrated tool for evaluation of source terms, has been widely used for source term estimation. In this work, the individual codes in the IBM-version STCP were modified to be used on workstations which are more readily available. This can be accomplished by replacing the IBM-specific features with the ones which the standard FORTRAN-77 supports. Also, data transfers within the package are facilitated by improving some interface modules. Verification studies show that the modification was successful, except for some increases in running time, as expected. MARCH3 is a constituent of the STCP. This code has been used extensively to analyze severe accidents in light water reactors by a standalone code. Due to the extensive scope of the code, many of the individual models are by necessity simplistic and largely parametric. One of the most uncertain and oversimplified parts is the hotdrop model, which describes an assumed fuel-coolant interaction in the reactor cavity following vessel failure. The fuel-coolant interaction model is very important due to its effect on containment load through steam spike or explosion. In this study, various mechanistic fuel/coolant interaction models are investigated and a dynamic fuel fragmentation and mixing model is coded into the HOTDRP subroutine of the MARCH3 code. As a result, fuel/coolant interaction can be described more realistically and mechanistically.