Fabrication of Uranium Dioxide Microspheres by Classic and Novel Sol-Gel Processes

Uranium dioxide (UO 2 ) microspheres were fabricated by two sol-gel processes. First used was a classical process variant, as developed at Oak Ridge National Laboratory, consisting of (1) reduction of commercial uranyl to U(IV) nitrate; (2) preparation of a sol by precipitation of uranium hydroxide, its peptization, and solvent extraction of nitrates; and (3) gelation to microspheres by extraction of water through addition of a dewatered 2-ethyl-1-hexanol emulsion. Substantial improvement in microsphere production was achieved by application of a sol-gel process in which ascorbic acid was used as strong complexing agent. In this method, the reduction step was omitted and uranyl (VI) ascorbate sols/hydroxyl sols were formed from a suspension of either a uranium trioxide or a uranyl nitrate solution. Gelation through water extraction yielded perfect microspheres. Other metals can be easy added to these sols. Thermal treatment of the UO 2 microspheres by calcination and reduction in hydrogen atmosphere was designed on the basis of differential thermal analysis and thermogravimetric analysis.