[en] Under the auspices of the NEA Nuclear Science Committee (NSC), the Working Party on Scientific Issues of the Fuel Cycle (WPFC) has been established to co-ordinate scientific activities regarding various existing and advanced nuclear fuel cycles, including advanced reactor systems, associated chemistry and flowsheets, development and performance of fuel and materials, accelerators and spallation targets. The WPFC has different expert groups to cover a wide range of scientific fields in the nuclear fuel cycle. The Expert Group on Fuel Cycle Transition Scenarios Studies was created in 2003 to study R and D needs and relevant technology for an efficient transition from current to future advanced reactor fuel cycles. The objectives of the expert group are to (1) assemble and organise institutional, technical, and economics information critical to the understanding of the issues involved in transitioning from current fuel cycles to long-term sustainable fuel cycles or a phase-out of the nuclear enterprise and (2) provide a framework for assessing specific national needs related to that transition. After reviewing national, regional or worldwide transition scenarios, the expert group performed a benchmark study to compare the existing codes in terms of capabilities, modelling and results. The benchmark was conducted in two phases: (1) depletion calculations for PWR UOX, PWR MOX and fast reactor calculations and (2) transition calculation using various scenario codes (COSI, FAMILY21, VISION, EVOLCODE and DESAE) using three different transition scenarios (once-through, limited plutonium recycling in LWRs and plutonium and minor actinides recycling in fast reactors). The comparison mainly focused on the mass flow and the composition of heavy elements depending on time, i.e. natural uranium needs, enrichment needs, fresh fuel fabrication needs, fuel irradiation, inventory of spent fuel and nuclear materials, reprocessing needs, etc