Patent Number: 043483399
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing oxidic nuclear fuel bodies with an oxygen-to-metal ratio of 2.0.+-.0.02 at sintering temperatures between 1000.degree. and 1400.degree. C. 2. Description of the Prior Art In the manufacture of nuclear fuel bodies, it is the general practice today to sinter them in a reducing atmosphere at 1700.degree. C. Hydrogen or cracked ammonia is used as the reducing atmosphere. Sintered bodies of nuclear fuel materials must attain high operating reliability during the irradiation in the nuclear reactor; they must therefore meet special requirements. These requirements are in substance: The sintered density of the molded bodies is to be higher than 93% of theoretical density; the microstructures should be stable against densification; and the fission gas liberation should be limited; for reasons of corrosion, the fluorine contents should be less than 10 ppm; and an oxygen-to-metal ratio of 2.0.+-.0.02 should be maintained. Since the present furnace temperatures of 1700.degree. C. represent an extraordinarily high stress for the furnace insulation and the furnace lining as well as for the heating elements, which limits the service life seriously, it would be highly desirable if sintering temperatures were sufficient which are substantially lower. Therefore, various proposals in this respect have been made. According to a proposal of U.S. Pat. No. 3,375,306, the pressed nuclear fuel powder is sintered in gas mixtures of CO.sub.2 and CO up to 95% of its theoretical density at a temperature of 1300.degree. to 1600.degree. C. The reduction of the overstoichiometrically sintered molding is accomplished during the cooling in hydrogen or in mixtures of CO.sub.2 and CO. A method described in U.S. Pat. No. 3,927,154 works likewise with CO.sub.2 /CO mixtures and sintering temperatures in the range of 1000.degree. to 1400.degree. C. This method represents in substance sintering operation with continuously changing oxygen-to-metal ratio in the sintered body. The oxygen potential present in the sintering atmosphere is low, so that from the moment of reaching the sintering temperature on, the oxygen-to-metal ratio of the sintered body decreases slowly and finally should reach the specified value .ltoreq.2.02. Thus, this method depends heavily on the oxygen-to-metal ratio of the starting powder. Unfortunately, these sintering methods have not found acceptance in practice. SUMMARY OF THE INVENTION An object of the present invention is to provide a method and apparatus for effectively sintering nuclear fuels in temperature ranges of 1000.degree.-1400.degree. C., which produce reliably, leads to uniform end products, the grain structure of which can also be adjusted, independently of the oxygen-to-metal ratio of the starting powder of the nuclear fuel to be sintered. With the foregoing and other objects in view, there is provided in accordance with the invention a method for manufacturing oxidic nuclear fuel bodies with an oxygen-to-metal ratio of 2.0.+-.0.02 at sintering temperatures between 1000.degree. and 1400.degree. C., which comprises adding a grain-growth-promoting sintering additive to a nuclear fuel powder with an arbitrary oxygen-to-metal ratio, mixing the additive and fuel powder and pressing the mixture into blanks, passing the blanks into a sintering zone of a furnace and sintering the blanks therein in an oxidizing atmosphere at a temperature within the range of 1000.degree.-1400.degree. C., and subsequently passing the sintered blanks into a reducing zone and treating the blanks therein in a reducing atmosphere at a temperature within the range of 1000.degree.-1400.degree. C. In accordance with the invention, there is provided an apparatus for manufacturing oxidic nuclear fuel bodies with an oxygen-to-metal-ratio of 2.0.+-.0.02 at sintering temperatures between 1000.degree. and 1400.degree. C., which comprises a high-temperature furnace with a continuous passageway through which nuclear bodies are moved, said passageway having an oxidation zone followed by a reduction zone with the two zones separated by a gas lock, transport means for moving the nuclear bodies into and through the oxidation zone, then the gas lock, and then through and out of the reduction zone, an inlet near the entrance of the oxidation zone for the introduction of CO.sub.2 -gas and an outlet at the gas lock for the discharge of CO.sub.2 -gas with the CO.sub.2 -gas passing through the oxidation zone in the same direction as the nuclear bodies, conduit means in the gas lock for removing the discharged CO.sub.2 -gas, a second inlet near the exit of the reduction zone for the introduction of a reducing gas and an outlet at the gas lock for the discharge of reducing gas with the reducing gas passing through the reduction zone in a direction opposite to the direction of the nuclear bodies, an inlet and an outlet to the gas-lock for continuously flushing the gas-lock with an inert gas, and electrical heating means for heating the oxidizing zone and the reduction zone at temperatures between 1000.degree. and 1400.degree. C. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a method for manufacturing oxidic nuclear fuel bodies, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.