Space-dependent nuclear characteristics, measured by critical experiments on large-size fast breeder reactor (FBR) cores, were reviewed and interpreted. It was observed that radial neutron flux distributions were significantly distorted by perturbations, control rod reactivity interaction effects were large, and the point kinetics was not valid. These physical behaviors are 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 decoupling. This is an important issue that must be taken into account in the nuclear design for large FBR cores.

A new nuclear core design method for large FBR cores is proposed in which neutronic stability is considered at the same time as performance and safety for the optimization of core design. The neutronic stability is improved by reducing the spatial decoupling and by taking into account the spatial higher harmonics.