Patent Number: 062513095
Section: claims

1. A method of manufacturing uranium dioxide (UO.sub.2) fuel pellets, the method comprising: annealing U.sub.3 O.sub.8 powder at temperatures of 1000.degree. C. to 150.degree. C. in a non-reducing gas atmosphere in order to make U.sub.3 O.sub.8 polycrystalline aggregates having a large crystal size;  dividing the U.sub.3 O.sub.8 polycrystalline aggregate into its constituent U.sub.3 O.sub.8 single crystals;  forming a mixture comprising 1% to 15% by weight of the U.sub.3 O.sub.8 single crystals with the remainder being UO.sub.2 powder;  making granules of the mixture;  pressing the granules into green pellets;  sintering the green pellets at temperatures above 1600.degree. C. for more than one hour in a reducing gas atmosphere. 2. The method of claim 1, wherein the U.sub.3 O.sub.8 powder is produced by heating defective or sound UO.sub.2 pellets at temperatures of 300.degree. C. to 800.degree. C. in air or in an oxygen containing gas to oxidize UO.sub.2 to U.sub.3 O.sub.8, and by sieving the oxidized products in order to eliminate large U.sub.3 O.sub.8 agglomerates and not-oxidized products. 3. The method of claim 1, wherein the U.sub.3 O.sub.8 powder is produced in a oxygen containing gas by calcining uranium compounds in which uranium has an oxidation state of 4+ to 6+, and by sieving the calcined products in order to eliminate large U.sub.3 O.sub.8 agglomerates and not-oxidized products. 4. The method of claim 1, wherein the non-reducing gas comprises air, nitrogen, inert gases, oxygen and mixtures thereof. 5. The method of claim 1, wherein the U.sub.3 O.sub.8 single crystals have an average size of 2 to 30 .mu.m. 6. The method of claim 1, wherein the U.sub.3 O.sub.8 single crystals further comprise the U.sub.3 O.sub.8 single crystals, U.sub.3 O.sub.8 products composed of less than ten U.sub.3 O.sub.8 single crystals, and mixtures thereof. 7. The method of claim 6, wherein the U.sub.3 O.sub.8 product has an average size of less than 50 .mu.m. 8. The method of claim 1, wherein the mixture further comprises the UO.sub.2 powder, the U.sub.3 O.sub.8 single crystals, and the U.sub.3 O.sub.8 powder. 9. The method of claim 8, wherein the total amount of both the U.sub.3 O.sub.8 single crystals and the U.sub.3 O.sub.8 powder is not larger than 15% by weight of the mixture. 10. The method of claim 1, wherein the reducing gas comprises at least one gas selected from the group consisting of hydrogen, a mixture of hydrogen and nitrogen, a mixture of hydrogen and inert gas, a mixture of hydrogen and carbon dioxide, and a mixture of hydrogen and steam.