Two fast breeder reactor (FBR) concepts, the tank type and the loop type, have been studied as possible reactor designs to be used for a demonstration FBR (DFBR). The basic principle of the DFBR design is to ensure plant safety through a defense-in-depth methodology. Improvements in the seismic and thermal stress designs have been attempted for both reactor concepts. The system design study strives to maximize the reliability of the safety-related systems and to rationalize commercialization of the plant.