Study of instabilities due to the coupling between neutronics and thermal hydraulics in boiling water reactors

This thesis deals with the instabilities study in the weak flow rate and mean power BWR reactors. The other part of the thesis is the coupling validation of the thermohydraulic and neutronic codes FLICA-4 and CRONOS-2 of the Cea, in the scope of a benchmark study for the BWR reactor type. A first analysis has been done with a simplified model. Then the study has been improved by the use of a real calculation of neutronic kinetic with the CRONOS-2 code. In this framework, the international NEACRP 3-D LWR core transient benchmark, has been used. The results calculated with the coupling FLICA-4-CRONOS-2 are in agreement with the other codes. The instabilities have also been defined by the use of the cross sections given by the benchmark. (A.L.B.)