Contribution to the qualification of the Neptune system. Application to the follow-up of the Tihange reactor

For the calculations of light water reactors the modular system Neptune has been developed. It includes transport, diffusion, thermohydraulic and kinetic codes and so allows the treatment of the various problems of core physics. The first part of this thesis is devoted to a comparison of the most usually used modulus of the neutron transport code (APOLLO). Two examples are considered: a PWR lattice and a BWR U-Pu mixed assembly. The consequences of the different hypotheses made to solve the Boltzmann's equation by a collision probability method can be appreciated on these practical cases. The second part is a check of the complete calculation scheme against experimental results obtained during the first cycle of Tihange (900 MWe PWR). The core calculation is a 3D-diffusion calculation taking into account the thermohydraulic feedbacks; the macroscopic cross-sections needed by the neutron calculation are obtained by the transport code and tabulated versus the burn-up, the fuel temperature and the water density. The results prove that Neptune can now be considered as a precise and reliable tool