The EPICURE experimental program provides a complete and high-quality experimental database for evaluating the uncertainties in reactor physics calculations of plutonium-recycling pressurized water reactors. To understand possible discrepancies between experimental values and values calculated with the conventional scheme, a study is performed of the major physics approximations involved in the calculation of heterogeneous Mixed OXide-UO2-fueled cores. This study determines the origin of the significant discrepancies between the calculated and the measured power distributions. An optimized calculation scheme is developed based on the use of Sn transport calculations and on a refined energy group structure. Its application to the analysis of EPICURE experiments results in very satisfactory agreement.