[en] The ability of the RELAP4/MOD6-Update 3 computer program to predict the phenomena occurring in a pressurized water reactor during the blowdown-refill phase of a loss-of-coolant accident is independently assessed. Calculations of RELAP4 system models are compared with data from several Semiscale experiments. The sensitivity of the calculated results to core phase slip, break flow, critical heat flux correlation, system nodalization and the initial conditions has been investigated. Areas of RELAP4 deficiencies have been noted. Semiscale Mod-1 modeling techniques for future analyses have been recommended