Patent Number: 042319765
Section: description

EXAMPLE 1 Sintered pellets from a mixed uranium-plutonium-oxide particularly for light water reactors: As starting materials a uranium xerogel and a plutonium xerogel are used. The uranium xerogel is produced by a sol-gel process corresponding to the so-called "Julich H-Process"(see L5) and the plutonium xerogel is made according to the so-called "EIR Sol-Gel Process" which is described in: (L7) Bischoff K. et al EIR Report No. 236,1973 PA1 (L8) Wymer R.G. PA1 IAEA, Vienna 1968 PA1 "Laboratory and Engineering Studies of Sol-Gel Processes at ORNL". PA1 (L9) Finney B.C. et al. PA1 ORNL-4802, 1972 PA1 "Sol-Gel Process Engineering-Scale Demonstration of the Preparation of High-Density UO.sub.2 Microspheres". PA1 (L10) Lloyd M.H. et al. PA1 Nucl. Appln. 5, 1968 PA1 "A Sol-Gel Process for preparing dense forms of PuO.sub.2 ". "Sol-Gel Processes for Carbide Preparation" To disperse the feed solutions into droplets of 10 to 50 .mu.m diameter a two-jet nozzle is used as is described e.g. in: The uranium and plutonium xerogels are available in the form of microspheres with a diameter of 5 to 25 .mu.m. It is important that the size distribution of the microspheres in the uranium xerogel and the plutonium xerogel should be the same and that in each xerogel the mean diameter should lie between 10 and 15 .mu.m. 5 g of plutonium oxide xerogel and 162 g of uranium oxide xerogel were well mixed together. From the flowable microspheres mixture green pellets were pressed in a usual pelletisation apparatus in the desired size, e.g., with a diameter of 9 mm, with the density of the pressed green pellets being not less than 4.0 g cm.sup.-3. Thus, the production of green pellets takes place here without addition of any binding material. The green pellets are then calcined at 500.degree. C. in a reducing gas of 80% argon and 20% hydrogen and then are sintered in argon at 1400.degree.-1600.degree. C. to the final pellets consisting of mixed oxides with a typical density of 95--98% of the theoretical density (Reaction sintering). The sintered pellets are here produced according to the previously illustrated Route II. EXAMPLE 2 Sintered pellets from a mixed uranium-plutonium oxide, particularly for LW reactors. A plutonium xerogel and a uranium oxide powder are the starting materials. The plutonium xerogel is produced as described in Example 1. The uranium oxide powder must have certain flowability which in this case is ensured by a grain size distribution in the powder of between 50 and 50 .mu.m. 5 g of the plutonium oxide xerogel described in Example 1 and 162 g of uranium oxide powder are well mixed in a tumble mixer. The production of green pellets from the thus obtained mixture and the subsequent heat treatment take place in the same manner as in Example 1. EXAMPLE 3 Sintered pellets from a mixed oxide of uranium and plutonium with a ratio of plutonium to heavy metal of 0.3, particularly for FB-reactors: Firstly, a uranium-plutonium xerogel is made, preferably by a sol-gel process corresponding to a so-called "Gel Precipitation Process" (See L2). The feed solution, which has been previously prepared for the production of xerogel microspheres, has the same ratio of plutonium to heavy metal as that of the desired sintered pellet, thus here 0.3. It is to be emphasized that in this case the mean size of the xerogel microspheres is not critical since the homogeneity range for uranium and plutonium in no way depends here on the size of the feed microspheres. Expediently, microspheres with a diameter in the range of 20 to 100 .mu.m are produced. To obtain a suitable dispersing of the feed solution in practice advantageously a sextuple two-jet nozzle is selected. The production of green pellets and the subsequent heat treatment are undertaken in the same manner as in Example 1. This production of sintered pellets corresponds to Route I mentioned above. EXAMPLE 4 Sintered pellets from a mixed carbide of uranium and plutonium, particularly for FB reactors: A uranium-plutonium xerogel is used as feed material for the production of green pellets, the microspheres of which contain about 12 weight parts, related to the total weight, of finely divided carbon black. The uranium-plutonium (carbon xerogel is preferably produced according to the "EIR Sol-Gel Process" (see L7) already mentioned in Example 1. 98.1 g of the feed solution containing 0.112 mol/kg of plutonium, 0.637 mol/kg of uranium and carbon in the form of carbon black of 2.397 mol/kg are dispersed into droplets of a diameter of 40 to 200 .mu.m and converted into a xerogel. Since, as in Example 2, the mean size of the microspheres is not critical for the homogeneity, the dispersing of the feed solution can be undertaken with a conventional pressure nozzle. From the uranium-plutonium (carbon) xerogel, green pellets are pressed at a pressure of about 6 tonnes/cm.sup.2. The green pellets are calcined, subjected to a carbon reduction and sintered. The principal advantage in this preferred embodiment lies in that the carbon reduction and the sintering takes place in a single operation, thus only one single process step is required for this. In practice, the green pellets are calcined in flowing gas containing 80% argon and 20% hydrogen at 500.degree. C. and then follows a heat treatment in flowing argon at 1700.degree. to 1800.degree. C. Mixed carbide fuel pellets with a density of a constant 95% of the theoretical density are achieved. This production mode corresponds again to Route I. EXAMPLE 5 Sintered pellets from a mixed oxide of uranium and plutonium particularly for FB reactors: The green pellets are produced from uranium oxide microspheres and plutonium oxide microspheres wherein the microspheres of both types have a diameter of 5-25 .mu.m. The uranium oxide microspheres are produced by the so-called "ORNL Sol-Gel Process" which is described in: The plutonium oxide microspheres are produced by a sol-gel process which is described in: The two kinds of microspheres are mixed in a ratio of one weight part of plutonium moxide microspheres and three weight parts of uranium oxide microspheres and from the mixture of microspheres green pellets are pressed in a conventional pelletising press. The subsequent heat treatment of the green pellets is in the form of a reaction sintering in a flowing gas of 96% argon and 4% hydrogen at 1200.degree. to 1300.degree. C. Uranium-plutonium mixed oxide sintered pellets are obtained with a density of 95-98% of the theoretical density. This process corresponds to the previously illustrated general Route III.