Urania sol is used for preparing uranium oxide microspheres for nuclear reactor fuels. A new process for producing concentrated, 1 M U or more, crystalline urania sols by solvent extraction has been developed. This process is based on a time-temperature conductivity-controlled extraction of nitric acid from a hydrolyzing U(IV) nitrate-formate solution using an amine-in-hydrocarbon extractant. The sols contain predominantly crystalline urania, and are more resistant to rapid, spontaneous gelation and other variations in properties than earlier sols made by solvent extraction. Also, earlier solvent extraction processes produced dilute (0.2-0.3 M U) sols that required concentration to 1 M before making the spheres. The preparation of a satisfactory sol depends on ensuring crystallization of the urania, minimizing uranium oxidation, and having a stable U(IV) nitrate-formate feed solution. Feed solutions were made by reducing uranyl nitrate-formate solution with platinum-catalyzed hydrogen at atmospheric pressure. The reduction requires vigorous agitation of the solution and continuous electrometric monitoring of the U(IV)/U(VI) redox potential to minimize harmful side reactions. The studies include both laboratory development and an engineering-scale demonstration.