Study of the reduction of UO2 by magnesium or calcium dissolved in molten chlorides

Abstract

The reduction of solid UO2 to uranium by magnesium or calcium dissolved in their molten chlorides has been studied. The rate of reduction per unit area of UO2 surface, at constant temperature and concentration of reductant in the molten chloride, was found to increase with time to a constant value. The rate of reduction per unit area was observed to be proportional to the concentration of reductant in the molten salt. The small increase observed in the reaction rate over the temperature range 750° to 850°C, suggests that the reduction is controlled by transport of the reductant to the reaction site. Solidified salt, containing UO2 pellets which had been partially reduced, was sectioned, polished, and examined microscopically. The products of the reduction reaction form concentric layers around the UO2 pellets. Layers of metallic uranium and oxide containing small amounts of dispersed salt alternated with layers of salt containing small amounts of metallic uranium and oxide. The layers ruptured, presumably because the volume of the products, uranium and oxide, is greater than the volume of the UO2. Therefore, an impervious layer did not form on the oxide surface to inhibit the reduction reaction.