Patent Number: 041586816
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to sintering nuclear fuel and more particularly refers to a new method and apparatus for sintering pellets of nuclear fuel oxides and mixtures of nuclear fuel oxides in a reducing furnace atmosphere. 2. Description of the Prior Art Nuclear fuel is understood here to mean uranium, plutonium and thorium, alone or in mixture. For the sake of simplification, however, only uranium dioxide will be mentioned in the following discussion. Nuclear fuel pellets are manufactured in a known manner by pressing powdered UO.sub.2+x in which oxygen is in stoichiometric excess of the dioxide and/or mixtures of UO.sub.2+x containing oxygen in stoichiometric excess and powdered PuO.sub.2 to form pressed blanks of various geometry. These pressed blanks or pellets are produced either without the addition of binder and lubricating agents in pressing tools automatically lubricated with lubricating oil of differing origin, or with the addition of binder and lubricating agents such as, for example, Zn stearate, Zn behenate, paraffins or similar materials. After pressing, the formed blanks or pellets are placed in highly heat-resistant transport containers, called transport boats. The laden boats are pushed through a resistance-heated push-through sintering furnace lined with highly refractory blocks, where the stoichiometric excess oxygen of the UO.sub.2+x is first reduced to stoichiometric UO.sub.2.00 in reducing gases such a hydrogen and/or rare gas/hydrogen or nitrogen/hydrogen mixtures; and the pressed blanks or pellets sintered at temperatures of about 1700.degree. C. to form dense, stable pellets. In the special case of manufacturing sintered UO.sub.2 /PuO.sub.2 bodies for light-water reactors and breeder reactors, a gas mixture which maximally contains only 8% hydrogen is used for the reduction of stoichiometric excess oxygen UO.sub.2+x for safety reasons (possible formation of explosive oxygen-H.sub.2 mixtures). As a result of this lower hydrogen concentration in the gas mixture, the reduction potential of the gas mixture (as expressed as partial free enthalpy of the oxygen and thus, in the system H.sub.2 /H.sub.2 O, proportional to the H.sub.2 /H.sub.2 O ratio) is greatly lowered as compared to pure hydrogen. This lower reduction potential leads to a considerable lengthening of the reduction time and, with the sintering furnace following directly, to an equivalent lengthening of the sintering time. The reaction water produced in the reaction lowers the reduction potential further, as the water concentration in the gas increases. For the gas mixture still to have a reducing effect, the H.sub.2 /H.sub.2 O partial pressure ratio should not become lower than 10:1. In order to compensate for this change of the reduction potential, which itself is again proportional to the amount of oxide reduced per unit time, dry fresh gas can be introduced into the furnace. For throughputs of, say, 12 kg UO.sub.2.2 /hour, a total of 35 m.sup.3 of gas mixture is flushed through the furnace per hour, so that the ratio H.sub.2 /H.sub.2 O does not drop below 10:1. In sintering the UO.sub.2 /PuO.sub.2 fuel pellets, an overall stoichiometric oxygen deficient oxide, caused by the reduction of the Pu(IV) to Pu(III), can be produced at the prevailing high temperatures. Depending on the intended application, whether in a light-water reactor or in a breeder reactor, either a stoichimetric or a stoichiometric deficient oxide is desired. To adjust the respective desired stoichiometry, it is necessary to adjust a respectively different reduction potential in the high-temperature portion of the furnace. This is adjusted by humidifying the fresh gas entering the furnace to previously calculated water concentrations. The requirements which are thus obtained for the process technique with respect to the reduction potential in the reduction and sintering portion of the push-through furnace are therefore contradictory. If the inexpensive nitrogen/hydrogen mixture is used for the reduction and the sintering, one finds excessive contamination of the nuclear fuel by nitrogen. This can be reduced by a heat treatment at T&gt;1000.degree. C. in rare gases or rare gas/hydrogen mixtures. Of necessity this leads to using only the expensive rare gas/hydrogen mixture as the reduction and sintering gas, if only one gas mixture is used for both parts of the furnace. SUMMARY OF THE INVENTION An object of the present invention is to provide a method, in which during the reduction of the pressing blanks containing oxygen in stoichiometric excess, a strong reducing gas can be used, i.e., a gas as dry as possible is flushed through the furnace in large quantities. Another object of the invention is to provide a method of reducing pressing blanks or pellets with the use of a relatively inexpensive gas mixture, particularly in view of the necessarily large amount of gas. A further object of the invention is to provide a method for using a gas mixture in the high-temperature zone, which can be adjusted to a different reduction potential lower than that in the reduction zone dependent on the different applications. A still further object of the present invention is to provide a method for use of a rare gas/hydrogen mixture as the gas mixture for cooling the sintered body to T&lt;1000.degree. C. With the foregoing and other objects in view, there is provided in accordance with the invention a method for sintering nuclear fuel oxides and mixtures of nuclear fuel oxides having oxygen in stoichiometric excess of the dioxides in a reducing furnace atmosphere, which includes passing the nuclear fuel pellets run through a heated reduction furnace with a reducing atmosphere to effect reduction of at least substantially all the excess oxygen, regulating the residence time of the nuclear fuel oxides in the reducing furnace to produce reduced nuclear fuel oxides of desired oxygen content, cooling the reduced nuclear fuel oxides, passing the cooled-down nuclear fuel oxides to an intermediate station, subsequently passing the cooled-down nuclear fuel oxides through a sintering furnace, and independently regulating the residence time of the nuclear fuels oxides passing through the sintering furnace to effect sintering of the nuclear fuel pellets. The nuclear fuel pellets are treated in the two separated furnaces at different temperatures and the gas atmospheres in the two furnaces are independent of each other and have different compositions. In another embodiment the nuclear fuel pellets are treated in the two separated furnaces at different temperatures and the gas atmospheres in the two furnaces are independent of each other and the amounts of gas fed to each furnace are different. The gas atmosphere in the reduction furnace has a humidity concentration different from the humidity concentration in the gas atmosphere in the sintering furnace. In a further embodiment a gas mixture of N.sub.2 and 4 to 8% H.sub.2 is introduced into the reduction furnace to supply the gas atmosphere therein, and further a gas mixture of rare gas and 4 to 8% H.sub.2 is introduced into the sintering furnace to supply the gas atmosphere therein. In one method reducing gas is introduced in the reduction furnace to provide a reducing atmosphere and is discharged from the reduction furnace, and further the discharged gas is cooled to condense condensible constituents entrained by the gas in the reduction furnace, and the condensed constituents separated from the gas. In accordance with the invention there is provided apparatus for sintering nuclear fuel pellets of nuclear fuel oxides and mixtures of nuclear fuel oxides having oxygen in stoichiometric excess of the dioxide includes an externally heated elongated reduction furnace, inlet means to the reduction furnace for the entrance of nuclear fuel pellets to be reduced, outlet means from the reduction furnace for the discharge of reduced pellets from the furnace, a gas inlet to the reduction furnace for the introduction of a reducing gas in the furnace to provide a reducing atmosphere around the pellets in the reduction furnace, a gas outlet from the reduction furnace for the discharge of gas therein, cooling means for cooling the reduced pellets, an intermediate station, a transport canal through which the cooled reduced pellets are transported to the intermediate station, a second transport canal for the transport of pellets in the intermediate station to the entrance of a sintering furnace, outlet means from the sintering furnace for the discharge of sintered pellets from the sintering furnace, a gas inlet to the sintering furnace, independent of the gas entering the reduction furnace, for the introduction of gas around the pellets in the sintering furnace, a gas outlet from the sintering furnace, independent of the gas discharge from the reduction furnace, for the discharge of gas therein, and cooling means for cooling the sintered pellets.