Abstract : Due to safety rules running on fourth generation reactors' core development,neutronics simulation tools have to be as accurate as never before. First part of this reportenumerates every step of fast reactor's neutronics simulation implemented in currentreference code: ECCO. Considering the field of fast reactors that meet criteria of fourthgeneration, ability of models to describe self-shielding phenomenon, to simulate neutronsleakage in a lattice of fuel assemblies and to produce representative macroscopic sections isevaluated. The second part of this thesis is dedicated to the simulation of fast reactors' corewith steel reflector. These require the development of advanced methods of condensationand homogenization. Several methods are proposed and compared on a typical case: theZONA2B core of MASURCA reactor.