Patent Number: 062513095
Section: description

DETAILED DESCRIPTION OF THE INVENTION This invention provides the method of manufacturing large-grained UO.sub.2 pellets with the aid of U.sub.3 O.sub.8 single crystals. The method according to this invention comprises two main steps; producing U.sub.3 O.sub.8 single crystals and manufacturing large-grained UO.sub.2 pellets. FIG. 1 shows the manufacturing processes of the U.sub.3 O.sub.8 single crystals and large-grained UO.sub.2 fuel pellets. The step of producing U.sub.3 O.sub.8 single crystals comprises annealing U.sub.3 O.sub.8 powder at temperatures of 1000.degree. C. to 1500.degree. C. for more than 1 hour in a non-reducing gas atmosphere in order to make U.sub.3 O.sub.8 polycrystalline aggregates having a large crystal size, and dividing the U.sub.3 O.sub.8 polycrystalline aggregate into its constituent U.sub.3 O.sub.8 single crystals. The step of manufacturing large-grained UO.sub.2 pellets comprises forming a mixture of UO.sub.2 powder and the U.sub.3 O.sub.8 single crystals, making granules of the mixture, pressing the granules into green pellets, and sintering the green pellets at temperatures above 1600.degree. C. for more than 1 hour in a reducing gas atmosphere. The content of U.sub.3 O.sub.8 single crystals is in the range between 1% and 15% by weight of the mixture. The reducing gas is hydrogen or a hydrogen containing gas. More detailed descriptions of the invention are as follows. The U.sub.3 O.sub.8 powder is made by heating UO.sub.2 pellets at temperatures of 300.degree. C. to 800.degree. C. in an oxidizing gas to oxidize UO.sub.2 to U.sub.3 O.sub.8. During the oxidation UO.sub.2 pellets are spontaneously pulverized to U.sub.3 O.sub.8 powder, for the orthorhombic U.sub.3 O.sub.8 lattice has about 30% larger volume than the cubic UO.sub.2 lattice, so that a large amount of stress builds up. The oxidizing gas is air or an oxygen containing gas. As the heating temperature increases, the amount of stress decreases. Thus the size of the U.sub.3 O.sub.8 powder increases. Defective or sound UO.sub.2 pellets can be so heated and oxidized as to make U.sub.3 O.sub.8 powder. However, it is more economical to use defective UO.sub.2 pellets since defective UO.sub.2 pellets which do not meet pellet specifications are usually produced in a small quantity in the manufacturing processes of UO.sub.2 pellets. The present invention provides a method of reusing defective UO.sub.2 pellets. The fragments of UO.sub.2 pellets are also used in making U.sub.3 O.sub.8 powder. The UO.sub.2 pellet fragments are heated in an oxidizing gas at temperatures of 250.degree. C. to 700.degree. C., which is lower than the temperature for the oxidation of UO.sub.2 pellets. The other method to make U.sub.3 O.sub.8 powder is to calcine uranium compounds, in which uranium has an oxidation state of 4+ to 6+. On calcining a uranium compound in an oxygen containing gas, it changes to U.sub.3 O.sub.8 powder with gaseous products released. The U.sub.3 O.sub.8 powder, which was made from UO.sub.2 pellets, pellet fragments and uranium compounds, may include large U.sub.3 O.sub.8 agglomerates and not-oxidized products, so it is passed through a sieve to eliminate them. The U.sub.3 O.sub.8 powder has an average size of 5 .mu.m to 15 .mu.m, and each particle has many cracks that were formed during the oxidation. A crystal size of the U.sub.3 O.sub.8 powder is very small, and the morphology of the U.sub.3 O.sub.8 powder, as an example, is shown in FIG. 2. When the U.sub.3 O.sub.8 powder is annealed at temperatures of 1000.degree. C. to 1500.degree. C., the particles of U.sub.3 O.sub.8 powder are partially bonded one another and simultaneously very small U.sub.3 O.sub.8 crystals grow. Thus U.sub.3 O.sub.8 polycrystalline aggregates are made, each of which consists of many large crystals without cracks. The size of the crystals in the U.sub.3 O.sub.8 polycrystalline aggregate increases as the annealing temperature and time increase. FIG. 3 shows an example of the morphology of the U.sub.3 O.sub.8 polycrystalline aggregates. The crystals in the U.sub.3 O.sub.8 polycrystalline aggregate have a shape of polyhedrons, and they are found to be weakly bonded one another. Thus the U.sub.3 O.sub.8 polycrystalline aggregate is divided easily into its constituent U.sub.3 O.sub.8 single crystals by mechanical force. FIG. 4 shows an example of the morphology of the U.sub.3 O.sub.8 single crystals. The U.sub.3 O.sub.8 single crystals have an average size of 2 .mu.m to 30 .mu.m. UO.sub.2 powder, which is normally used in producing UO.sub.2 pellets, is mixed with the U.sub.3 O.sub.8 single crystals to form a mixture. The content of the U.sub.3 O.sub.8 single crystals is 1% to 15% by weight of the mixture, and it is preferable for the content to be 2% to 8% by weight. If there is much U.sub.3 O.sub.8 powder to be reused, the U.sub.3 O.sub.8 powder is additionally added to the mixture, in which a total amount of both the U.sub.3 O.sub.8 single crystal and the U.sub.3 O.sub.8 powder is not larger than 15% by weight of the mixture. A green pellet made according to the invention consists of mainly UO.sub.2 powder and of uniformly dispersed U.sub.3 O.sub.8 single crystals. Such a green pellet being sintered at temperatures above 1600.degree. C. in a reducing atmosphere, U.sub.3 O.sub.8 single crystals act as initiators for rapid grain growth. Eventually, the UO.sub.2 pellet comes to have uniformly large grains after sintering. After a green pellet which includes an excessive amount of U.sub.3 O.sub.8 single crystals had been sintered, it was found that the UO.sub.2 pellet did not have a large grain. This is understood by assuming that the grain growth initiated by the U.sub.3 O.sub.8 single crystal proceeds only in a range that is determined by the spacing between the U.sub.3 O.sub.8 single crystals. So it is preferable to keep the amount of the U.sub.3 O.sub.8 single crystals less than about 15% by weight of the green pellet. If the U.sub.3 O.sub.8 polycrystalline aggregate is not completely divided, U.sub.3 O.sub.8 products composed of several single crystals are produced. It was found that a large-grained UO.sub.2 pellet could be produced using the U.sub.3 O.sub.8 products composed of less than about ten single crystals. The U.sub.3 O.sub.8 products have an average size of less than about 50 .mu.m. Therefore, the method according to the invention is conducted using the U.sub.3 O.sub.8 single crystal, the U.sub.3 O.sub.8 products composed of less than ten single crystals, or mixtures of the U.sub.3 O.sub.8 single crystals and the U.sub.3 O.sub.8 products. The UO.sub.2 pellet manufactured according to the invention has a grain size of about 12 .mu.m after sintering at 1700.degree. C. for 4 hours. Without using the U.sub.3 O.sub.8 single crystals, the UO.sub.2 pellet has a grain size of about 8 .mu.m, and a grain size of 12 .mu.m can be achieved only by sintering at 1700.degree. C. for more than 20 hours. The method provided by the present invention has an advantage of increasing the grain size of UO.sub.2 pellets by about 50%. The amount of the fission gas released decreases with increasing the grain size of a UO.sub.2 pellet, so the large-grained UO.sub.2 pellet according to the invention will decrease the amount of the fission gas released during irradiation. In the other method according to the invention, UO.sub.2 single crystals substitute for the U.sub.3 O.sub.8 single crystals. UO.sub.2 single crystals are made simply by reducing the U.sub.3 O.sub.8 single crystals at temperatures of 400.degree. C. to 1200.degree. C. in hydrogen or a hydrogen containing gas. The UO.sub.2 single crystal is identical with the U.sub.3 O.sub.8 single crystal in shape and size. The other method to make UO.sub.2 single crystals comprises reducing the U.sub.3 O.sub.8 polycrystalline aggregates to UO.sub.2 polycrystalline aggregates at temperatures of 400.degree. C. to 1500.degree. C. and dividing the UO.sub.2 polycrystalline aggregate into its constituent UO.sub.2 single crystals. When the green pellet which consists of mainly UO.sub.2 powder and of uniformly dispersed UO.sub.2 single crystals is sintered at temperatures above 1600.degree. C. in a reducing atmosphere, the UO.sub.2 single crystals act as initiators for rapid grain growth. Eventually, the UO.sub.2 pellet comes to have uniformly large grains after sintering. Another method of manufacturing large-grained UO.sub.2 pellets comprises forming a mixture of UO.sub.2 powder and the UO.sub.2 single crystals, making granules of the mixture, pressing the granules into green pellets, and sintering the green pellets at temperatures above 1600.degree. C. for more than 1 hour in a reducing gas atmosphere. The content of the UO.sub.2 single crystals is not larger than 15% by weight of the mixture. The reducing gas is hydrogen or a hydrogen containing gas. The following example illustrates a preferred method of manufacturing large-grained UO.sub.2 fuel pellets in accordance with the present invention. However, these examples should be understood in no way to limit the scope of the invention, which is only defined by the appended claims. EXAMPLE U.sub.3 O.sub.8 single crystals were prepared according to the following procedures. UO.sub.2 pellets were oxidized in flowing air at 400.degree. C. for 3 hours to make U.sub.3 O.sub.8 powder, which was then passed through a 100-mesh sieve to eliminate large U.sub.3 O.sub.8 agglomerates and not-oxidized products. The U.sub.3 O.sub.8 powder had an average size of 8 .mu.m, and its morphology is similar to that shown in FIG. 2. The U.sub.3 O.sub.8 powder was annealed at 1300.degree. C. for 4 hours in flowing air, and then cooled to make U.sub.3 O.sub.8 polycrystalline aggregates, whose morphology is similar to that shown in FIG. 3. The U.sub.3 O.sub.8 polycrystalline aggregate was divided into U.sub.3 O.sub.8 single crystals by mechanical force and intensive stirring. The complete dividing of the U.sub.3 O.sub.8 polycrystalline aggregate made a product consisting of 1 to 2 single crystals, and the partial dividing made a product consisting of 3 to 5 single crystals. The former product, which had an average size of 5.5 .mu.m, is termed `A crystal`, and the latter product, which had an average size of 8 .mu.m, is termed `B crystal`. The morphology of `A crystal` is similar to that shown in FIG. 4. The U.sub.3 O.sub.8 powder was annealed at 1200.degree. C. for 4 hours in flowing air, and then cooled to make U.sub.3 O.sub.8 polycrystalline aggregates, which were then divided into U.sub.3 O.sub.8 products consisting of 6 to 10 single U.sub.3 O.sub.8 crystals. This U.sub.3 O.sub.8 product, which is termed `C crystal`, has an average size of 8 .mu.m. Mixtures were prepared by mixing UO.sub.2 powder with the A crystal, the B crystal, and the C crystal, respectively. The contents of each crystal were 2%, 4%, 6%, and 8% by weight. The powder mixture was pre-pressed under 100 MPa into slugs, which were then broken up into granules. The granules were pressed under 300 MPa into green pellets, which were then sintered at 1700.degree. C. for 4 hours in hydrogen gas in order to make UO.sub.2 pellets. Table 1 shows the grain size of the UO.sub.2 pellets fabricated in accordance with the above procedures. In order to show clearly the effect of the U.sub.3 O.sub.8 single crystals on the grain size, UO.sub.2 pellets were produced using powder mixtures of not containing U.sub.3 O.sub.8 single crystals. The mixtures of UO.sub.2 and U.sub.3 O.sub.8 powder were pressed and sintered, and the results are also shown in Table 1, as a comparative example. TABLE 1 Grain size of UO.sub.2 pellets. contents of crystals in powder type of mixture (% by weight) crystal 0% 2% 4% 6% 8% Remarks A 7.6 .mu.m 7.9 .mu.m 9.3 .mu.m 12 .mu.m 10 .mu.m B 7.6 .mu.m 8.6 .mu.m 10.3 .mu.m 11.9 .mu.m 10.5 .mu.m C 7.6 .mu.m 7.8 .mu.m 8.1 .mu.m 9.6 .mu.m 9.4 .mu.m U.sub.3 O.sub.8 7.6 .mu. m 7.3 .mu.m 6.9 .mu.m 6.9 .mu.m 6.8 .mu.m comparative powder example