The FLWR is a boiling water reactor type with a core consisting of hexagonal-shaped fuel assemblies with a triangular-lattice fuel rod configuration, which has been proposed in order to ensure a sustainable energy supply in the future based on well-established light water reactor technologies. This paper proposes a new concept of fuel assembly design named FLWR/MIX. The first stage of FLWR is designed to conserve plutonium effectively with a fissile plutonium conversion ratio of around 1.0, keeping negative void reactivity characteristics. Enriched UO2 fuel rods are arranged in the peripheral region of the assembly, surrounding the mixed oxide (MOX) fuel rods in the central region. Performance evaluation shows that the FLWR/MIX concept is effective for controlling the void reactivity characteristics in the tight-lattice fuel rod configuration and is promising under the framework of the UO2 and MOX fuel technologies and related infrastructures that have been established for the current LWR-MOX utilization.