Patent Number: 043893554
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Nuclear fuel pellets manufactured to fuel nuclear reactors are produced by starting with a uranium dioxide (UO.sub.2) powder (industry has adopted UO.sub.2 although the stoichiometric starting powder can be represented as UO.sub.2.05-2.15) and blending it with a commercially available organic binder powder. The quantity of organic binder is not crucial but it should be in the range of about 0.1 to 0.3% by weight of the blended mixture, the remainder being about 99.7 to 99.9% UO.sub.2, an amount sufficient to hold the powders together during shaping and pressing. Blending time should be sufficient to produce a homogenous mixture. After blending, the blended mixture is shaped and cold pressed into pressed green pellet compacts, the pressing force being that sufficient to compact the powders to approximately 50% of their theoretical mixture density, a determination based upon compact length, diameter and weight. The compacts are then heated and sintered in a microwave induction furnace in a reducing atmosphere consisting essentially of a nitrogen (N.sub.2) and hydrogen (H.sub.2) gas mixture and, more specifically, about a 75% H.sub.2 -25% N.sub.2 gas mixture. However, any reducing atmosphere would be operable. The sintering temperature is in the range of about 1600.degree. C. to 1800.degree. C., and the compacts are held in the microwave furnace in the reducing atmosphere at the sintering temperature for approximately 2 to 6 hours to achieve a compact density of about 95% of theoretical density. Sintering in a reducing atmosphere reduces the hyperstoichiometric starting powder, UO.sub.2.05-2.15, to UO.sub.2. After sintering, the compacts are cooled to approximately room temperature, the cooling being conducted in the reducing atmosphere. After cooling, the compacts are ground to the desired UO.sub.2 finished pellet product. The nuclear pellet fabrication process above described can also be conducted with additional process steps included after the shaping and pressing step but before the sintering step. However, with the additional steps, the above mentioned pressing force becomes that sufficient to compact the powders to approximately 44% of their theoretical mixture density. The additional steps include forcing the compacts through screens to form a granulate and then cold pressing the granulate into pressed pellet compacts, the granulate pressing force being that sufficient to compact the granulate to approximately 50% of theoretical density. Additionally, U.sub.3 O.sub.8 powder can be blended with the UO.sub.2 and organic binder powders in the first step of the fuel preparation process, the U.sub.3 O.sub.8 powder being approximately 5% by weight of the blended mixture, the remainder being about 0.1 to 0.3% binder and about 94.7 to 94.9% UO.sub.2. The U.sub.3 O.sub.8 constituent of the pellet compacts, upon sintering in the reducing atmosphere, is converted to UO.sub.2. It has been determined that uranium oxide with stoichiometries UO.sub.2 through U.sub.3 O.sub.8 directly suscepts to microwave radiation at approximately 2450 MH.sub.Z, the frequency of the standard kitchen-type microwave oven, heating rapidly to very high, red-hot temperatures. It should be understood that, while a conventional microwave oven was selected for use because of its ready availability, other microwave induction furnaces, conventional and non-conventional, operating at different frequencies would also be operable. Other ceramic materials including alumina, silica, niobia, lithia and graphite do not as readily suscept to microwave radiation at 2450 MH.sub.Z. Testing to ascertain that uranium oxide with stoichiometries UO.sub.2 through U.sub.3 O.sub.8 directly suscepts to microwave radiation was conducted by placing green pressed UO.sub.2 pellet compacts into an alumina tube, placing the tube into a conventional 2450 MH.sub.Z microwave oven, providing a means for introducing a reducing atmosphere into the tube, plugging the ends of the tube with refractory insulating material, and heating the tube, with compacts, in the oven in a reducing atmosphere of a N.sub.2 and H.sub.2 gas mixture. Heating was initially conducted for two minutes at a power setting of 20, which means that 100% of the oven power was delivered 20% of the time. The initial power of 20, sufficient to remove most of any contained moisture, was increased to a power setting of 30 and held for five minutes to remove any remaining moisture. The power was then raised to a setting of 70 for ten minutes and then increased to a 100 setting for fifteen additional minutes. The alumina tube itself was transparent to the microwave radiation allowing for passage of the microwave radiation and heating of the compacts from the inside out rather than from the outside in, a characteristic of normal refractory furnaces. Heating to 1370.degree. C. was achieved after only fifteen minutes of operation. A flickering glow from within the tube began about five minutes into the 70 power setting with the glow becoming constant at the full power setting of 100, a sintering temperature of 1620.degree. C. was measured at the outer surface of the tube. The power setting of 100 was held for an additional fifteen minutes and then the compacts were cooled in the oven to about room temperature, the reducing atmosphere being continuously maintained. Compact density measurements were made and are presented as follows: ______________________________________ THEO- RETI- DI- CAL COM- AME- DEN- DEN- PACT TER LENGTH WEIGHT SITY SITY (#) (cm) (cm) (g) (g/cc) (%) ______________________________________ 1 .70 1.04 2.24 5.59 51.00 unsintered (control) 2 .59 .85 2.12 9.12 83.21 sintered 3 .58 .73 1.72 8.91 81.30 sintered 4 .59 .87 2.01 8.45 77.10 sintered ______________________________________ Additional testing was conducted on UO.sub.2 pellet compacts of the size most commonly encountered in nuclear fuel pellet preparation. The testing was conducted as described above except that the sintering time was increased to four hours and the compacts had an inside diameter of approximately 0.3 cm. Compact density measurements were made and are presented as follows: ______________________________________ THEO- RETI- OUTSIDE CAL COM- DIAM- DEN- DEN- PACT ETER LENGTH WEIGHT SITY SITY (# (cm) (cm) (g) (g/cc) (%) ______________________________________ 1 .97 1.05 7.12 10.14 92.54 2 .97 1.11 7.87 10.52 96.02 3 .98 1.11 7.85 10.43 95.14 4 .98 1.04 7.02 9.93 90.57 5 .97 1.04 7.01 10.07 91.92 6 .98 1.08 7.66 10.41 94.98 7 .98 1.10 7.76 10.43 95.19 8 .97 1.17 7.79 9.92 90.48 9 .98 1.14 7.71 9.86 89.95 10 .97 1.11 7.61 10.41 95.01 ______________________________________ Alumina was selected for use in the sintering chamber because it could withstand the high temperatures generated but not interact with the microwave field. Metallic components could therefore not be considered since they reflect microwaves. During the nuclear fuel preparation process, the compacts need not be sealed in alumina tubes, but, alumina boats, vessels or other material invisible to microwave radiation, could be used as the carrying means for the compacts in the microwave induction furnace. Test compacts of uranium dioxide with organic binder, and, uranium dioxide and U.sub.3 O.sub.8 with organic binder, all suscepted to microwave radiation thus demonstrating the microwave induction sintering furnace to be a viable alternative to the refractory-type sintering furnace. Recycling of scrap uranium dioxide is an important adjunct to the nuclear fuel preparation process. During pellet preparation, there is generated a quantity of sintered uranium dioxide pellets and uranium dioxide powder available for recycle. Sintered pellets that do not meet specifications and uranium dioxide powder or grinder sludge generated during the compact grinding step are conveyed, in an alumina boat, vessel or comparable material invisible to microwave radiation, to a microwave induction furnace for reprocessing and recycling. Additionally, any other scrap uranium dioxide could be added and processed in this manner. Heating of the material to its oxidation temperature of at least 200.degree. C. in the microwave furnace is accomplished by microwave radiation in an oxidizing atmosphere, generally, but not limited to, air, wherein UO.sub.2 is oxidized to a U.sub.3 O.sub.8 powder. Specific heating time and temperature are not critical but heating should be conducted for a time sufficient to change the material to a fine black powder, a process that is a function of material mass but is generally accomplished in an approximate temperature range of 400.degree. to 500.degree. C. in approximately 20 to 40 minutes. In the furnace, the pellets are heated to the point where the outer pellet surface oxidizes to U.sub.3 O.sub.8 and separates from the UO.sub.2 inner pellet due to the differences in density between UO.sub.2 and U.sub.3 O.sub.8, the UO.sub.2 being of greater density. With the introduction of fresh, unoxidized surfaces, the process would continue until the entire pellet was oxidized to a black U.sub.3 O.sub.8 powder. The uranium dioxide powder or grinder sludge is already in powder form, therefore, upon heating in the furnace in the oxidizing atmosphere, the sludge would quickly be converted to the U.sub.3 O.sub.8 powder. The oxidized product leaving the furnace is a fine U.sub.3 O.sub.8 powder suitable, after cooling, for blending back with uranium dioxide and organic binder powders in a nuclear fuel pellet preparation process. Heating in the recycling furnace can be continuous or intermittent. Pellet oxidation, however, is enhanced by microwave heating when the power level is alternated, a power on followed by a power off cycle. After a period of power on followed by a period of power off, the UO.sub.2 pellets heat up extremely rapidly when the microwave radiation is restored. Sintered pellets have been completely fragmented by simply turning the microwave source off and on while the pellets are oxidizing. When the UO.sub.2 material has surpassed the oxidation temperature, its susceptance to microwave radiation is very high. Wnen the microwave radiation is interrupted, the material begins to cool, still oxidizing. The material immediately returns to a glowing red temperature when the microwave radiation is reintroduced. The rapid heat-up and cool-down causes tremendous thermal stresses to exist in the pellet structure. The thermal stresses, along with the stresses introduced due to the difference in density between UO.sub.2 and U.sub.3 O.sub.8, cause pellet fracture in a static condition. Fresh surfaces are exposed upon every cooling cycle allowing oxidation to continue to completion. Laboratory testing has established that samples of UO.sub.2 powder placed in a conventional 2450 MH.sub.Z microwave oven directly suscept to microwave radiation in an oxidizing atmosphere. A UO.sub.2 powder sample weighing approximately 5 grams will be glowing red in less than one minute and oxidation will be occurring at all surfaces in contact with air. Testing has also shown that a sintered pellet placed in the microwave field suscepts within approximately 1 to 2 minutes with the pellet breaking up as oxidation progresses. Oxidation of the pellet is enhanced by setting the power level to a power setting of 50. The pellet fractures due to density differences and induced thermal stresses as the microwaves are turned on and are then shut off. A reduced power setting, providing on/off or intermittent microwave radiation, enhanced the oxidation by setting up thermal stresses and density differences causing pellet fracture and increasing the fresh UO.sub.2 surfaces available for oxidation. While in accordance with the provisions of the statutes there is illustrated and described herein a specific embodiment of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without corresponding use of the other features.