A fabrication technique for a UO2 pellet consisting of UO2 grains and a continuous W channel on the grain boundary

A new fabrication process of UO2–W composite fuel has been studied in order to improve the thermal conductivity of the UO2 pellet by the addition of a small amount of W. A fabrication process was designed from the phase equilibria among tungsten, tungsten oxides and UO2. The conventionally sintered UO2 pellet which contains W particles is heat-treated in an oxidizing gas and then in a reducing gas. In the oxidizing heat-treatment W particles are oxidized and liquid tungsten oxide penetrates within the UO2 grain boundary, and in the reducing heat-treatment liquid oxide is transformed to solid tungsten which forms a continuous channel along the UO2 grain boundary. This developed technique can provide a continuous W channel covering UO2 grains for a UO2–W composite fuel even with a small amount of a metal phase – below 6 vol.%. The thermal diffusivity of the UO2–6 vol.%W cermet composite increases by about 80% when compared with that of a pure UO2 pellet.