Improvement in thermal conductivity of uranium dioxide fuel

In order to improve the thermal conductivity of UO2 fuel, an innovative composite fuel concept of continuous metal phase with a small amount of metal in UO2 pellet, was developed. Metal candidates of W, Mo and Cr were selected, and fabrication process was conceptually designed from thermodynamic calculations. We have experimentally found that a metal phase envelops perfectly UO2 grains, forming continuous channel throughout the pellet, and improving the thermal conductivity of pellet. The fabrication process are constituted by two thermal annealing step. The microstructure characteristics results indicated that the metal phases were oxidized and melted in the first heat treatment step. The melted oxide penetrated along the grain boundary of UO2 and is interconnected with each other. Under the second reduction step, the liquid phases were transformed to solid metals, which were precipitated along the grain boundary. The thermal diffusivity of UO2-W composite is increased by about 40∼80% than that of pure UO2 sample.