Patent Number: 046719040
Section: description

DETAILED DESCRIPTION OF THE INVENTION A method of the type mentioned at the outset is characterized, according to the invention, by the feature that for mixing with the base powder, a rare-earth-containing powder is generated with powder particles which exhibit at least in one surface layer a crystal lattice of the fluorite type with the stoichiometric composition (SE.sub.0.5,U.sub.0.5) O.sub.2.00 and/or generate it during the sintering. A crystal lattice of the fluorite type for UO.sub.2 is shown in FIG. 3.12 on page 30 of Olander: Fundamental Aspects of Nuclear Reactor Fuel Elements, TID-26711-P1, 1976, Energy Research and Development Administration (USA). There, the oxygen ions form a simple cubical lattice. In the crystal lattice of the fluorite type, powder particles generated for the method according to the invention, rare earths (SE) as well as uranium ions each occupy in the statistical average one-half of the lattice locations of the cubical-plane-centered (fcc) partial lattice. In such a crystal lattice of the fluorite type, the rare-earth, (SE), the uranium and also the oxygen ions are heavily diffusion-impeded, so that the formation of (SE.sub.y,U.sub.1-y) O.sub.2.00 -phases with y from 0 to 1 from the base powder is suppressed when the compacts are being sintered. For this reason, also a relatively high density of the sintered oxidic nuclear fuel bodies obtained from the compacts can be achieved, independently of the properties of the basic UO.sub.2 powder used even with a relatively high rare-earth content, especially if Gd.sub.2 O.sub.3 in the powder mixture to be compacted is 2 to 15% by weight, and preferably 4 to 8% by weight of the powder mixture. The share of the rare-earth oxide then has the same value in the sintered bodies obtained. The invention and its advantages will now be explained in greater detail with the aid of three embodiment examples: Rare-earth-oxide-containing powder intended for mixing with the base powder can advantageously be produced by annealing a rare-earth-oxide-containing starting powder with an UO.sub.2 -coating on the starting powder particles. For this purpose, for instance 50 g Gd.sub.2 O.sub.3 starting powder which is spread out in a flat dish, is sprayed, if necessary, repeatedly, with an aqueous solution of uranyl nitrate, again uranyl citrate and/or uranyl oxalate by means of an aerosol spraying device. The dish advantageously is subjected to a shaking movement here, so that the powder particles are provided uniformly with the uranium-containing coating due to their vibration. Subsequently, the dish is heated up, and the uranium-containing coating on the powder particles is dried and, if necessary, freed of crystal water. Thereupon, the Gd.sub.2 O.sub.3 -containing powder with the uranium-containing coating is calcined in a reducing hydrogen atmosphere at 500.degree. to 1000.degree. C. and is then annealed at a temperature of about 1500.degree. C. for developing the crystal lattice of the fluorite type with the stoichiometric composition (SE.sub.0.5,U.sub.0.5) O.sub.2.00, at least at the surface of the outer particles. Thus produced Gd.sub.2 O.sub.3 -containing powder is then mixed with 660 g of the base powder which consists of pure UO.sub.2 and which has a specific surface of 6.5m.sup.2 /g and a mean crystallite diameter of 50 nm and is compacted into compacts which contain 7% by weight Gd.sub.2 O.sub.3. These compacts are then heated in a hydrogen atmosphere with reducing action for 2 to 5 hours to a temperature in the range of 1500.degree. to 1750.degree. C. The sintered bodies obtained have a density which is 96.5% of their theoretical density. The rare-earth-oxide-containing powder intended for mixing with the base powder can advantageously also be produced from a mixture of rare-earth-oxide-containing starting powder and UO.sub.2 powder by milling. For this purpose, for instance UO.sub.2 and Gd.sub.2 O.sub.3 are mixed by hand in a ratio of 60:40 parts by weight which corresponds to a mol ratio of 50:50 and is subsequently milled intensively in a ball mill for 8 to 15 hours. 0.2 to 50 g distilled water, propane diol and/or zinc stearate are added as a milling aid to the mixture in the ball mill per 500 g of mixture. The material to be milled is subjected, after being milled in a tumbling mixer to a build-up granulation, whereby flowable particles are produced which can be mixed and compacted with base powder of pure UO.sub.2. After mixing 100 g of the granulated material with 700 g of base powder of pure UO.sub.2 which has the specific surface of 6.5 m.sup.2 /g and a mean crystallite diameter of 50 nm, the mixture is compacted into compacts with a Gd.sub.2 O.sub.3 content of 5% by weight. The compacts are sintered in a hydrogen atmosphere with reducing action at a temperature of 1500.degree. to 1750.degree. C. for 2 to 5 hours, developing sintered bodies. During this sintering operation, the granulates containing the UO.sub.2 and Gd.sub.2 O.sub.3 pass, at least at their surface, into the crystal lattice of the fluorite type with the composition (Gd.sub.0.5, U.sub.0.5) .sub.2.00. The sintered bodies obtained have a Gd.sub.2 O.sub.3 content of 5% by weight and a sintered density of 97.2% of their theoretical density. The rare-earth-oxide-containing powder intended for being mixed with the base powder of UO.sub.2 can also be produced advantageously by common precipitation from a solution, in which uranium and the rare earth are dissolved. For this purpose, for instance 60 g UO.sub.2 powder and 40 g Gd.sub.2 O.sub.3 which corresponds to a mol ratio of 50:50, are dissolved in nitric acid. Ammonium carbonate solution is added to the nitric acid solution. Then, ammonia (NH.sub.3) and carbon dioxide (CO.sub.2) gas are simultaneously introduced into the solution and a precipitate containing gadolinium and uranium is precipitated. This precipitate has the molar composition of gadolinium and uranium in the ratio 1:1. The precipitate is subjected to a calcining process carried out at a temperature of 500.degree. to 650.degree. C. in a reducing hydrogen atmosphere. The powder particles of the resulting powder have a crystal lattice of the fluoride type with the stoichiometric composition U.sub.0.5 Gd.sub.0.5) O.sub.2.00. powder is mixed with 344 g of base powder of pure UO.sub.2 which has a specific surface of 6.5 m.sup.2 /g and a mean crystallite diameter of 50 nm is compacted and sintered for 2 to 5 hours in a hydrogen atmosphere with reducing action at a temperature in the range of 1500.degree. C. to 1750.degree. C. The sintered bodies obtained have a Gd.sub.2 O.sub.3 content of 9% by weight and a density of 97.5% of their theoretical density. The foregoing is a description corresponding, in substance, to German application P No. 34 06 084.7, dated Feb. 20, 1984, international priority of which is being claimed for the instant application, and which is hereby made part of this application. Any material discrepancies between the foregoing specification and the specification of the aforementioned corresponding German application are to be resolved in favor of the latter.