[en] The calculation of the neutron flux is an important data that is used to determine the dynamic of the core of a Pressurized Water Reactor (PWR). However the transport equation which gives the neutron flux, cannot be solved in three dimensions over the whole core, in evolution because of the power of the current computers, which are too slow. So some simplifications are necessary to calculate this flux. Two-levels schemes are used, where, in a first step, some macroscopic cross sections libraries are generated by solving the transport equation using infinite lattice calculations on two dimensions assemblies. These sections are generally homogenized on the whole assembly and condensed to two energy groups. In a second step, the whole core calculation is carried out using the diffusion equation, with the cross sections of the libraries previously generated, interpolated at the values of the different parameters. However the core of a PWR is made up of many assemblies, that can contain two types of fuel : Uranium OXyde (UOX) or plutonium and uranium Mixed OXyde (MOX). Moreover all these assemblies have different burnup because each one can be used for three or four cycles depending on the PWR. So that imply some burnup gradients. Thus the hypothesis of the infinite lattice used to generate the cross sections libraries can be highly inaccurate. The first goal of this project is to generate cross sections libraries that take into account the environment and to evaluate the impact of this heterogeneous environment on the core calculation. The flux obtained with the diffusion equation at the end of the core calculation is not accurate enough, du to the homogenization by assembly, to determine and to locate the hotspot factor, which represents an important industrial problematic. The principle of the power reconstruction method (PRM) is to reconstruct the more accurately possible the flux in the pins, with a combination of the diffusion flux and some microscopic flux which take into account the heterogeneities in the assemblies. This method is currently used with the data calculated with the infinite lattice. The second goal of this project is to develop a theory to apply the PRM with environmental data and to establish the PRM at the end of a calculation of the core and observe if the results are improved with the environmental data. (author)