The RMWR concept builds upon the Advanced Boiling Water Reactor and is under active development in theoretical studies, particularly in Japan.
The goal of the RMWR is to depart further from the thermal neutron spectrum in order to achieve a breeder ratio of slightly greater than one, so that after the initial fuel charge no enrichment of the uranium input to the fuel cycle is required.
In contrast to regular light water reactors and in order to achieve a harder neutron spectrum, which is optimal for breeding purposes, the RMWR uses hexagonal fuel assemblies and Y-shaped control rods.
The fuel is MOX (Mixed Oxide), consisting of 18% plutonium, which is surrounded by depleted uranium in the blanket region.
Another RMWR breeder design intends on closing the nuclear fuel cycle by mixing thorium with reprocessed transuranics, which include plutonium, in a thorium containing MOX fuel.