Spatial neutronic decoupling of large fast breeder reactor cores: Application to nuclear core design method

Space-dependent nuclear characteristics, measured by critical experiments o n large-size fast breeder reactor (FBR) cores, were reviewed and interprete d. It was observed that radial neutron flux distributions were significantl y distorted by perturbations, control rod reactivity interaction effects we re large, and the point kinetics was not valid. These physical behaviors ar e enhanced as the spatial neutronic decoupling increases. To obtain stable and benign nuclear characteristics and to make the kinetics as close to the point kinetics as possible, it is necessary to reduce the spatial decoupli ng. This is an important issue that must be taken into account in the nucle ar design for large FBR cores. A new nuclear core design method for large FBR cores is proposed in which n eutronic stability is considered at the same time as performance and safety for the optimization of core design. The neutronic stability is improved b y reducing the spatial decoupling and by taking into account the spatial hi gher harmonics.