According to the International Criticality Safety Benchmark Evaluation Project (ICSBEP) 3000 experiments were made for safety/criticity in thermal domain, 700 in fast domain and almost zero in between. The epithermal domain was left behind since the beginning of the nuclear industry because of the need to study thermal neutron for nuclear reactors and fast neutrons for nuclear weapons or fast reactors. However for several years now, a lot of research was carried out to fill this gap, indeed the epithermal domain remains important, when slowing down neutrons pass through this range of energies. This internship follows this trend and aims at modelling the core of a fast reactor (VENUS) on TRIPOLI-4 and then modifying a little part of it to build nuclear configurations that enable the study of epithermal domain.