To avoid high energy releases in unprotected loss-of-flow accidents, cores can be designed such that the removal of sodium would add only a small amount of reactivity or even a negative reactivity to the reactor. Reduction in sodium void reactivity can be achieved by changing either the geometry or composition of the core. Pancake, modular, and heterogeneous core configurations were investigated. Heterogeneous cores showed sodium void reactivities in the 2-dollar range with only small penalties in doubling time when compared with the equivalent homogeneous cores. Liquid-metal fast breeder reactors using U-Th fuel in the form of metal, oxide, or carbide show negative sodium void reactivities but doubling times above 30 yr.