[en] This benchmark proposal builds upon that specified in NEA/NSC/DOC(2003)22 report. In addition to the three phases described in that report, another two phases have now been defined. Additional items for calculation have also been added to the existing phases. It is intended that further items may be added to the benchmark after consultation with its participants. Although the benchmark is specifically designed to provide inter-comparisons for plutonium- and thorium-containing fuels, it is proposed that phases considering simple calculations for a uranium fuel cell and uranium core be included. The purpose of these is to identify any increased uncertainties, relative to uranium fuel, associated with the lesser-known fuels to be investigated in different phases of this benchmark. The first phase considers an infinite array of fuel pebbles fuelled with uranium fuel. Phase 2 considers a similar array of pebbles but for plutonium fuel. Phase 3 continues the plutonium fuel inter-comparisons within the context of whole core calculations. Calculations for Phase 4 are for a uranium-fuelled core. Phase 5 considers an infinite array of pebbles containing thorium. In setting the benchmark the requirements in the definition of the LEUPRO-12 PROTEUS benchmark have been considered. Participants were invited to submit both deterministic results as well as, where appropriate, results from Monte Carlo calculations. Fundamental nuclear data, Avogadro's number, natural abundance data and atomic weights have been taken from the references indicated in the document