This thesis is part of the research carried out at IRSN on the ageing of vessel steels in pressurized water reactors. Many microstructural changes occur in these steels due to irradiation. These changes are the key to understand and predict the modification of their viscoplastic responses (including hardening) and their rupture properties with irradiation. In this context, the objective of this work is to propose a new model of plastic behavior at the polycrystal scale, using a crystalline plastic behavior law adapted to an irradiated vessel steel. From the applied point of view, the objective is to propose a behavior model that could be directly implemented into a finite element calculation code, in order to estimate the stress and strain fields during the simulation of a CT test (in place of a J2-type model usually used). Many mean-fields homogenization models have been proposed in the literature for the viscoplasticity of polycrystals. For irradiated vessel steels, in view of the recently developed crystalline laws, strong heterogeneities are expected between the different grains of the polycrystal in terms of strains and stresses during mechanical loading. These heterogeneities are associated with increasingly anisotropic grains during loading (different evolutions of critical resolved shear stresses in the grains according to the different sliding systems). With respect to homogenization, this is the non-linearity of the local behavior (grain behavior) associated with the non-uniformity of mechanical fields that makes the complexity of the problem to be treated. The scientific problem of this thesis topic consists in evaluating the relevance of a recent homogenization model in the case of polycrystals in highly anisotropic viscoplasticity (anisotropy of grain behavior).​