[en] Pressurized Water Reactors (PWR) are the most common nuclear reactor used today. The core of a PWR is composed of approximately 200 assemblies immersed in pressurized light water, which can be Uranium Oxyde assemblies (UOX) or Mixed Oxyde assemblies (MOX) coming from the reprocessings of used UOX. Electro-nuclear industries want to calculate the neutron flux inside these reactors, by solving the neutron transport equation, because it controls the dynamic of the core. Actually, the computers' power available today does not allow for a solution to the transport equation over the whole core, in three dimensions, with burnup. This is why reactor physicists use several approximations in order to obtain a solution for the neutron flux. This implies defining a pertinent calculation scheme. Generally, the calculation scheme requires homogenized macroscopic cross sections libraries generated using infinite lattice calculations on assemblies. Several parameters are used for the tabulation of these libraries including the burnup, the temperature and density of the coolant and the fuel, the concentration of boron and of xenon-135. Then the code evaluates the flux distribution in the finite reactor with the diffusion equation using cross sections interpolated from these libraries. However, the infinite lattice hypothesis may not be valid for highly heterogeneous cores, for example a core with burnup gradients or MOX / UOX interfaces. The purpose of this study is to evaluate the physical impact of heterogeneous environment on PWR assemblies. We first define a reference calculation scheme for a 3 x 3 assembly cluster, taking all heterogeneous environment effect into account, with the lattice cell code DRAGON. We later compare this reference with infinite lattice calculations, or with other calculation schemes closer to a full reactor calculation code. Those comparisons will allow us to explain physically the effects of the heterogeneous environment, and also to evaluate the errors in the reactor code committed when this effect is not taken into account. Finally, we will propose solutions to this issue. (author)