The most efficient way to enhance plutonium consumption in light water reactors is to eliminate the production of plutonium all together. This requirement leads to fuel concepts in which the uranium is replaced by an inert matrix. At PSI, studies have focused on employing ZrO2 as inert matrix. Adding a burnable poison to such a fuel proves to be necessary. As a result of scoping studies, Er2O3 was identified as the most suitable burnable poison material. The results of whole-core three-dimensional neutronics analyses indicated, for a present-day 1000 MWe pressurized water reactor (PWR), the feasibility of an asymptotic equilibrium four-batch cycle fuelled solely with the proposed PuO2-Er2O3-ZrO2 inert matrix fuel (IMF). The present paper presents the results of more recent investigations related to `real-life' situations, which call for transition configurations in which mixed IMF and UO2 assembly loadings must be considered. To determine the influence of the introduction of IMF assemblies on the characteristics of a UO2-fuelled core, three-dimensional full-core calculations have been performed for a present-day 1000 MWe PWR containing up to 12 optimized IMF assemblies.