Neutronic Space-Time Effects with Thermal-Hydraulic Feedback in Large Homogeneous and Heterogeneous Liquid-Metal Fast Breeder Reactors

Two-dimensional multigroup space-time kinetics calculations with thermal-hydraulic feedback were performed for 1000- and 1800-MW(electric) homogeneous and heterogeneous liquid-metal fast breeder reactors. The initiating transient was due to the asymmetric withdrawal of a single control rod. It was found that the point kinetics model can, in many cases, be used for predicting integral reactor characteristics. For accurate predictions of local reactor conditions, space-time kinetics calculations are needed. In the case of both homogeneous and heterogeneous cores, for design basis reactivity insertions with scram, smaller reactivity insertion rates will lead to a greater fuel and cladding temperature rise than large reactivity insertion rates. Heterogeneous cores, because of their inherently greater power shape sensitivity, show a larger temperature rise than the homogeneous cores despite the fact that the transient is of much shorter duration because of an earlier reactor trip due to a lower negative Doppler feedback.