Abstract: Making uranium dioxide pellets of controlled grain size by treating 50-500 g/l UO.sub.2 F.sub.2 with NH.sub.3 in a first and a second stages to form (NH.sub.4).sub.2 U.sub.2 O.sub.7 precipitate, wherein the NH.sub.3 /U molar ratio is between 3-5 in the first stage and between 6-12 in the second stage. The precipitate is then formed into UO.sub.2 pellets having grain size within the range from 10 to 100 .mu.m.

Abstract: There is disclosed a process for fabricating UO.sub.2 pellets from highly active uranium dioxide powders, comprising the steps of (1) uniformly adding at least one pore-former agent in the range of 0.3 to 1.4% by weight to uranium dioxide powders as a starting material, the pore-former agent decomposing and sublimating below 600.degree. C. and having an average particle size in the range of 5 to 500 .mu.m, (2) compacting the uranium dioxide powders including the pore-former agent to form green pellets, and (3) sintering the green pellets to form sintered UO.sub.2 pellets having large grain size crystals. According to the present invention, UO.sub.2 pellets having the sintered density in the range of 94 to 97% TD are fabricated from highly active UO.sub.2 powder when the pellets have large crystalline grain sizes exceeding 20 .mu.m. Accordingly, the fission gasses generated during irradiation is restrained to the inside pellet thereof to enhance the safety of the irradiation.

Abstract: A method of fabricating uranium dioxide (UO.sub.2) powder from uranium hexafluoride (UF.sub.6) is disclosed, which comprises(1) reacting UF.sub.6 gas with steam with controlling the temperature of reaction between said UF.sub.6 gas and said steam at a predetermined temperature within the range of 200.degree. to 700.degree. C., to form solid uranyl fluoride (UO.sub.2 F.sub.2) and/or uranium oxide with an O/U ratio (oxygen-to-uranium atomic ratio) of 2.7 to 3,(2) dissolving said UO.sub.2 F.sub.2 and/or uranium oxide in water or nitric acid to form an aqueous uranyl solution containing UO.sub.2 F.sub.2 and/or uranyl nitrate (UO.sub.2 (NO.sub.3).sub.2),(3) reacting said aqueous uranyl solution with ammonia to precipitate ammonium diuranate (ADU),(4) filtering said precipitate,(5) drying said precipitate,(6) calcining said dry precipitate, and(7) reducing said calcined precipitate, whereby controlling the characteristics of said UO.sub.2 powder.

Abstract: A process of fabricating UO.sub.2 pellets comprising the steps of filtering and drying a slurry of ammonium diuranate (ADU) including ammonium fluoride (NH.sub.4 F), in order to form ADU powder, and then subjecting the ADU powder to calcining, reducing, compacting and sintering treatments, to form UO.sub.2 pellets.

Abstract: A method of controlling the crystal grain size of UO.sub.2 pellets is disclosed, which comprises(1) providing an aqueous uranyl solution which is free of hydrofluoric acid and nitric acid and which contains uranyl fluoride (UO.sub.2 F.sub.2) and/or uranyl nitrate (UO.sub.2 (NO.sub.3).sub.2) as a uranium component,(2) reacting with ammonia said aqueous uranyl solution to precipitate ammonium diuranate (ADU), while adjusting the ratio of said uranium components to a predetermined value within the range varying from the ratio 100% of uranyl fluoride and 0% of uranyl nitrate to the ratio 0% of uranyl fluoride and 100% of uranyl nitrate, adjusting the uranium concentration of the reaction mixture to 50 to 1,000 gU/liter and also adjusting the rate of contact between said uranium component in said aqueous uranyl solution with ammonia to at least 2 moles NH.sub.3 /min/mole U,(3) calcining and reducing said ADU precipitate to form UO.sub.2 powder, and(4) molding and sintering said UO.sub.2 powder, thus producing UO.