Patent Number: 046769358
Section: description

To explain this method in further detail, three embodiment examples will now be described: EXAMPLE 1 UO.sub.2 -powder and PuO.sub.2 -powder are weighed in the ratio of 4:1 and in an amount of 500 g into a mill. To the powder mixture are added 200 g water, which corresponds to a water content of approximately 30%; this amount of water, however, presents a problem in view of the criticality problems with the fission materials uranium 235 and plutonium. Only if the total amount of fissionable material is kept very small, as in this example, or if a criticality-proof geometry (for instance, the diameter of the mill) is observed, will no difficulties be expected. The quantity of powder is milled in the mill for 12 hours. After the milling, the milled material is dried in a drying cabinet or in a vacuum. Subsequently, the material is granulated and the powders are pressed into pellets, and sintered at 1700.degree. C. in inert gas/hydrogen mixtures. The following table shows some characteristic data of such powder mixtures. ______________________________________ Powder Data Pellet Data Compacted Solubility Apparent Sintered of the Pu- Density Density Component in g/cm.sup.3 g/cm.sup.3 HNO.sub.3, % ______________________________________ Powder mixture, 3.0 10.3 67.3 unmilled Powder mixture, milled 2.9 10.5 99.6 with the addition of water ______________________________________ EXAMPLE 2 UO.sub.2 -powder and PuO.sub.2 -powder are weighed into a mill in the ratio of 7:3 (parts by weight) and a total quantity of 500 g. To the powder mixture are added 0.5 g of a polyalcohol, propane diol, which corresponds to a content by weight of 0.1%. With respect to the criticality problems discussed in Example 1 with the fissionable materials uranium 235 and plutonium, no problems are expected in this case, since the ratio of the number of the hydrogen atoms to the number of the fissionable atoms is far below the critical value of 1. Therefore, no limitation as to the geometric data of the mill need be provided in this case. The powder mixture is milled together with the organic milling aid for 12 hours. The milled powder is granulated, pressed into pellets and sintered at 1700.degree. C. in inert gas/hydrogen mixtures. The following table shows characteristic data of the UO.sub.2 /PuO.sub.2 mixture so processed. ______________________________________ Pellet Data Powder Data Sintered Solubility of the Compacted Apparent Density Pu-Component in HNO.sub.3 Density in g/cm.sup.3 g/cm.sup.3 % ______________________________________ 3.7 10.5 99.9 ______________________________________ EXAMPLE 3 A powder mixture of UO.sub.2 and PuO.sub.2 with a 30-% plutonium content is placed, as described in Example 2, in a mill; 0.1% of the organic milling aid is added and milling proceeds for 12 hours. Subsequently, the mill is emptied, the powder is examined for its plutonium content and then taken to interim storage. After interim storage, a nuclear fuel for light-wear reactors with a plutonium content of 3% is to be manufactured with the powder. To this end, the calculated amount of the plutonium-containing milled powder is taken from storage and mixed with the calculated amount of UO.sub.2 -powder in an intensive mixer. The powder mixture obtained is tested for homogeneous distribution of the plutonium and the predetermined plutonium content. If these data are in the predetermined range, the powder mixture is granulated (by either dry or wet processes), pressed into pellets of the desired shape and sintered, for instance, at 1700.degree. C. The distribution of the plutonium in such pellets is, depending on the intensity of the mixing process, sufficiently homogeneous and corresponds in a substantially better manner to the reactorphysical requirements than would be the case if the same cycle would be run with more coarse-grained granulate instead of with the fine-grain powders. The solubility of such pellets in nitric acid corresponds to that which was also found in Example 2. These examples show that it is possible by means of this simple method to achieve the desired solubility of mixed-oxide nuclear fuels in nitric acid. It is advantageous to granulate the milled powder mixture as a piled bed in a rotating vessel by build-up granulation. Suitable rotating vessels are, for example, the vessels shown in FIG. 1 on page 149 of "Zeitschrift Fuer Werkstofftechnik/Journal of Materials Technology" volume 4, No. 3 (1973), i.e., for instance, hollow granulating cylinders or granulating dishes which execute a rotary motion with a horizontal, inclined, tumbling or gyrotary axis of rotation and which can be operated within a glove box. Since the form of the granulating vessel was found not to be important, differently shaped vessels, i.e. for instance, hollow spheres or conical vessels can also be used for granulating. If such granulating vessels are used, which are advantageously closed off during the granulating, the development of dust is small as compared with the dust development which would occur with the customary granulating by pre-compacting the powder in a press and subsequent breaking-up of the pressed material, and which can lead over an extended period of time to considerable radiation exposure in the glove boxes. After a period of granulating of about 120 minutes in a rotating granulating vessel, there is produced from the milled and optionally dried powder mixture a highly flowable, homogeneous, well processable and sinterable build-up granulate. To facilitate the formation of this granulate, fine agglomeration seeds which consist of sintered uranium-plutonium mixed oxide, can be added to the milled powder mixture (material to be milled) prior to the granulating. These seeds can be made of reprocessed powder which comes from previous productions of mixed oxide nuclear fuel pellets. The addition of a binder to the milled powder mixture prior to granulating is not necessary. The speed of rotation of the granulating vessel may be 10 to 70% of the critical speed of rotation, at which the powder charge begins to cling to the inside wall of the rotating granulating vessel. The degree of fillng of the granulating vessel is less than, or at most equal to, 80% of the volume of the granulating vessel. Starting with optionally admixed agglomeration seeds, spherical granulates with an average diameter of less than 300 micrometers can be produced in this manner. The maximum diameter of the granulates is approximately 1 mm. The finest component of the granulate, which has a tendency to form dust, is considerably less than in granulates which are produced by compacting and subsequent breaking-up. Granulates of the last-mentioned type of production have a broad grain spectrum with a high very-fine grain component tending to produce dust. The build-up granulate produced by the method according to the invention can be compressed in a press without pressing aids, for instance, zine stearate, directly into fuel pellets with predetermined dimensions. These pellets are sintered in conclusion in a reducing atmosphere at about 1700.degree. C., as already mentioned. Uranium dioxide powder as known in practice often contains oxygen in slight excess of the two atoms of oxygen to one of uranium and the term uranium dioxide in the claims is intended to include them.