Patent Number: 062513109
Section: description

DETAILED DESCRIPTION OF THE INVENTION An irradiated UO.sub.2 -based fuel pellet is first separated mechanically or physically from a cladding tube. Thereafter, the irradiated fuel pellet is processed through the manufacturing steps shown in FIG. 1. The irradiated UO.sub.2 -based fuel pellet is oxidized in an oxidizing gas so as to make U.sub.3 O.sub.8 -based powder containing fission products. Then, the U.sub.3 O.sub.8 -based powder is mixed with an additive to form a mixture. The mixture is granulated into granules, which are then pressed into a green pellet. The green pellet is sintered in a reducing gas atmosphere, preferably at a temperature of at least 1500.degree. C. for at least 1 hour. It is noted that irradiated (U, Pu)O.sub.2 fuel pellets can be processed in the same way as irradiated UO.sub.2 fuel pellets, since they each have the same crystal structure. The detailed description of the method of manufacturing new UO2-based fuel pellets from irradiated UO.sub.2 -based fuel pellets is as follows. The irradiated UO.sub.2 -based fuel pellet is heated in a furnace, preferably at a temperature in the range of about 300.degree. C. to about 700.degree. C., in an oxidizing gas. The oxidizing gas is preferably selected from the group consisting of air; oxygen; a mixture of air and inert gas; and a mixture of oxygen and inert gas. It has been found that the particle size of the U.sub.3 O.sub.8 -based powder increases with the oxidation temperature, so it is preferred to oxidize irradiated UO.sub.2 -based fuel pellets in air at a temperature in the range of about 350.degree. C. to about 600.degree. C. The irradiated UO.sub.2 -based fuel pellet is spontaneously pulverized to U.sub.3 O.sub.8 -based powder by the above treatment, since the lattice volume expands by about 30% and a large amount of stress is resultantly generated. The U.sub.3 O.sub.8 -based powder is preferably screened to remove large agglomerates or pellet fragments, and the U.sub.3 O.sub.8 -based powder so produced preferably has an average particle size of about 8 .mu.m. An irradiated UO.sub.2 -based fuel pellet has both gaseous fission products and solid fission products. Gaseous fission products include xenon and krypton, and solid fission products include Pu, Ce, Mo, Zr, Nd, Ba and La. While the gaseous fission products are almost completely removed from the irradiated fuel pellet during the above oxidation and pulverization steps, the resulting U.sub.3 O.sub.8 -based powder still contains amounts of the above-noted solid fission products. If the content of fissile materials in the U.sub.3 O.sub.8 -based powder is less than that required by the design specification of the fuel pellet to be manufactured, fresh UO.sub.2 -based powder, enriched or depleted, may be added to the U.sub.3 O.sub.8 -based powder in order to meet the amount of fissile materials required by the design specification. In this regard, it is useful to note that fresh UO.sub.2 powder, fresh PuO2 powder or a mixed powder of UO.sub.2 and PuO2 (i.e., (U, Pu)O.sub.2) may be added for the same purpose. The addition of fresh UO.sub.2 -based powder to the U.sub.3 O.sub.8 -based powder enhances the density of the UO.sub.2 -based fuel pellet which will be manufactured, so there is no restriction on the amount of fresh UO.sub.2 powder which may be added, from the viewpoint of pellet manufacturing. In accordance with the present method, the U.sub.3 O.sub.8 -based powder is mixed with an additive comprising at least one oxide of an element selected from the group consisting of Nb, Ti, V, Al, Mg, Cr, Si and Li. The quantity of the additive is preferably in the range of about 0.02% to about 5% by weight of the mixture of the U.sub.3 O.sub.8 -based powder and the additive. The mixture is thereafter preferably pre-pressed under about 1 ton/cm.sup.2 of pressure into slugs, which are broken up into granules having good flowability. The granules are pressed in a mold, preferably under a pressure of higher than about 2 ton/cm.sup.2, to produce a green pellet of about 40% to 70% TD. Before the pre-pressing or the pressing, a lubricant such as zinc stearate may optionally be added to the mixture to decrease the friction between particles during the pressing. A binder may also be added, if necessary, to increase the strength of the green pellet. If the mixture of the U.sub.3 O.sub.8 -based powder and the additive is flowable enough to be directly pressed, the mixture may be pressed into a green pellet without being granulated. If lubricants or binders were added to the green pellet, the green pellet may, at this point in the process, be heated to a temperature of about 700.degree. C. for a sufficient time to remove the lubricants or binders. If no lubricants or binders were added, this step is not necessary. The green pellet is subsequently heated in a reducing gas atmosphere. In a first variation of the method, heating preferably is conducted at a temperature above about 1500.degree. C. for at least 1 hour. The reducing gas atmosphere is needed to make a stoichiometric UO.sub.2 -based fuel pellet, so the reducing gas preferably comprises a gas selected from the group consisting of hydrogen, a mixed gas of hydrogen and at least one inert gas (such as argon and nitrogen), a mixed gas of hydrogen and steam, and a mixed gas of hydrogen and carbon dioxide. Without the addition of additives in accordance with the present invention, a green pellet comprising the U.sub.3 O.sub.8 -based powder is sintered to a UO.sub.2 -based pellet having a density of only about 80% TD, due to the very low sinterability of the U.sub.3 O.sub.8 -based powder. A UO.sub.2 -based pellet with such a low density cannot be used in a nuclear reactor, because fuel specifications require the pellet density to be at least about 94% TD. With the addition of the additives in accordance with the present invention, a pellet density of at least about 94% TD may be achieved. Another problem in achieving higher pellet density is the formation of micro-cracks in the green pellet during heating in the reducing gas. The reduction of orthorhombic U.sub.3 O.sub.8 -based compounds to cubic UO.sub.2 -based compounds results in a contraction of volume, which in turn causes micro-cracks to be formed in the green pellet since the volume contraction is not accommodated at relatively low temperatures. When the reduction of U.sub.3 O.sub.8 -based compounds to UO.sub.2 -based compounds is performed at a high temperature such that the volume contraction can be accommodated without cracking, the green pellet can be sintered to a higher density. Therefore, in a second variation of the method in accordance with the invention, before the sintering step the green pellet is first heated in a non-reducing gas (preferably to a temperature in the range of about 700.degree. C. to 1100.degree. C.), wherein the reduction of U.sub.3 O.sub.8 -based compounds to UO.sub.2 -based compounds is avoided, and then is sintered by heating to higher temperatures in a reducing gas (preferably to a temperature above about 1500.degree. C. for at least 1 hour). UO.sub.2 -based pellets prepared by this variation on the present method have an even higher density than the UO.sub.2 -based pellet produced only in a reducing gas throughout the sintering step. Preferably, the non-reducing gas comprises at least one gas selected from the group consisting of an inert gas, nitrogen, carbon dioxide, air and steam. A UO.sub.2 -based fuel pellet produced in accordance with the present invention has a density in the range of about 94% TD to about 98% TD. A UO.sub.2 -based fuel pellet which has a density between 94% TD and 96.5% TD is suitable for a light water reactor, and a UO.sub.2 -based fuel pellet which has a density between 96% TD and 98% TD is suitable for a heavy water reactor. Thus, the method provided by the present invention is able to fabricate fuel pellets suitable for both types of reactors. The following examples illustrate preferred methods of fabricating UO.sub.2 -based fuel pellets in accordance with the present invention. However, these examples should be understood to in no way limit the scope of the invention, which is only defined by the appended claims. EXAMPLE 1 A UO.sub.2 fuel pellet irradiated to 35,000 MWD/MTU in a light water reactor has compositions of fissile materials and fission products. Their compositions are calculated with the ORIGEN computer code, and 12 major elements were selected from all the elements contained in the irradiated UO.sub.2 fuel pellet. A simulated UO.sub.2 fuel pellet having the same composition as the irradiated UO.sub.2 fuel pellet was prepared using fresh UO.sub.2 powder and the 12 non-radioactive fission products mentioned above. Fresh UO.sub.2 powder was mixed with the pre-determined amounts of the 12 elements, and the composition of the mixed powder is shown in Table I. TABLE I OXIDES % BY WEIGHT SrO 9.147 .times. 10.sup.-2 Y.sub.2 O.sub.3 5.488 .times. 10.sup.-2 ZrO.sub.2 4.487 .times. 10.sup.-1 MoO.sub.3 4.737 .times. 10.sup.-1 RuO.sub.2 3.678 .times. 10.sup.-1 Rh.sub.2 O.sub.3 4.814 .times. 10.sup.-2 PdO 1.464 .times. 10.sup.-1 TeO.sub.2 5.585 .times. 10.sup.-2 BaCO.sub.3 2.552 .times. 10.sup.-1 La.sub.2 O.sub.3 1.926 .times. 10.sup.-1 CeO.sub.2 9.186 .times. 10.sup.-1 Nd.sub.2 O.sub.3 6.605 .times. 10.sup.-1 UO.sub.2 96.286 The mixed powder was bail-milled, pressed and sintered to make simulated UO.sub.2 fuel pellets. The simulated UO.sub.2 fuel pellets have a density of about 96% TD and have the same composition as the irradiated UO.sub.2 fuel pellet, so it can be used as a substitute for the irradiated UO.sub.2 fuel pellet. The simulated UO.sub.2 fuel pellet does not emit radioactive rays, so it could be treated in an unshielded lab. The simulated UO.sub.2 fuel pellet was oxidized in flowing air at 400.degree. C. for 3 hours to make U.sub.3 O.sub.8 -based powder, which was then passed through a sieve having 425 .mu.m openings to remove large agglomerates. The U.sub.3 O.sub.8 -based powder had an average particle size of 8 .mu.m. The U.sub.3 O.sub.8 -based powder was mixed uniformly with niobium oxide (Nb.sub.2O.sub.5) as an additive, which quantity was 0.5% by weight of the U.sub.3 O.sub.8 -based powder. In parallel, the U.sub.3 O.sub.8 -based powder was mixed uniformly with titanium oxide (TiO.sub.2) as an additive, which quantity was 0.2% by weight of the U.sub.3 O.sub.8 -based powder. The mixed powders were pre-pressed under a pressure of 98 MPa into slugs, which were then broken up into granules of 425 .mu.m or smaller. The granules were mixed with zinc stearate of 0.2% by weight of the granules for lubrication and were pressed into green pellets in a mold under pressures of 392 MPa, 490 MPa and 588 MPa. Green pellets were heated to 1700.degree. C. in reducing gas atmospheres for 4 hours and then cooled-down to fabricate UO.sub.2 -based fuel pellets. During the heating process, the green pellet was sintered and simultaneously reduced from U.sub.3 O.sub.8 to UO.sub.2. The reducing gas was hydrogen for the green pellet containing TiO.sub.2 and was a mixed gas of hydrogen and carbon dioxide for the green pellet containing Nb.sub.2 O.sub.5. Table II shows the densities of the UO.sub.2 -based fuel pellets fabricated in accordance with the above procedures. In order to show clearly the effect of the additives, a U.sub.3 O.sub.8 -based powder containing no additives was processed following the same procedure, and the density is also shown for comparison in Table II. TABLE 11 additives pressing pressure green density reducing gas sintered density sintered density (wt %) (MPa) (g/cm.sup.3) atmosph (vol %) (g/cm.sup.3) (% TD) 0.5% Nb.sub.2 O.sub.5 392 5.56 H.sub.2 + 1% CO.sub.2 10.119 94.39 0.5% Nb.sub.2 O.sub.5 490 5.72 H.sub.2 + 1% CO.sub.2 10.192 95.07 0.5% Nb.sub.2 O.sub.5 588 5.86 H.sub.2 + 1% CO.sub.2 10.261 95.72 0.5% Nb.sub.2 O.sub.5 588 5.88 H.sub.2 + 2% CO.sub.2 10.369 96.72 0.5% Nb.sub.2 O.sub.5 588 5.87 H.sub.2 + 3% CO.sub.2 10.314 96.21 0.2% TiO.sub.2 588 5.96 H.sub.2 10.561 98.52 not added* 588 5.82 H.sub.2 8.0 74.6 *comparative example EXAMPLE 2 U.sub.3 O.sub.8 -based powder was prepared in the same way as in Example 1. The U.sub.3 O.sub.8 -based powder was mixed with niobium oxide (Nb.sub.2 O.sub.5), which contents were 0.3% and 0.5% by weight of the U.sub.3 O.sub.8 -based powder, respectively. In parallel, the U.sub.3 O.sub.8 -based powder was mixed with titanium oxide (TiO.sub.2), which contents were 0.1% and 0.2% by weight of the U.sub.3 O.sub.8 -based powder, respectively. The mixed powders were pre-pressed under 98 MPa into slugs, which were then broken up into granules of 425 .mu.m or smaller. The granules were mixed with zinc stearate at 0.2% by weight of the granules for lubrication and were pressed into green pellets in a mold under 588 MPa. The green pellets were heated to 800.degree. C., 900.degree. C. and 1000.degree. C. in argon gas, respectively, for 1 hour, and subsequently heated in reducing gas atmospheres to 1700.degree. C. for 4 hours. The reducing gas was hydrogen for the green pellet containing TiO.sub.2 and was a mixed gas of hydrogen and carbon dioxide for the green pellet containing Nb.sub.2 O.sub.5. Table III shows the densities of the UO.sub.2 -based pellets fabricated in accordance with the above procedures. In order to show clearly the effect of the additives, a U.sub.3 O.sub.8 -based powder containing no additives was processed following the same procedure, and the density is also shown for comparison in Table III. TABLE III annealing of green pellets additives pressing pressure during heating reducing gas sintered density sintered density (wt %) (MPa) (temp/gas/time) atmosh (vol %) (g/cm.sup.3) (% TD) 0.3% Nb.sub.2 O.sub.5 588 800.degree. C./Ar/1 hr H.sub.2 + 2% CO.sub.2 10.318 96.25 0.3% Nb.sub.2 O.sub.5 588 900.degree. C./Ar/1 hr H.sub.2 + 2% CO.sub.2 10.520 98.13 0.3% Nb.sub.2 O.sub.5 588 1000.degree. C./Ar/1 hr H.sub.2 + 2% CO.sub.2 10.300 96.08 0.5% Nb.sub.2 O.sub.5 588 900.degree. C./Ar/1 hr H.sub.2 + 2% CO.sub.2 10.533 98.25 0.1% TiO.sub.2 588 900.degree. C./Ar/1 hr H.sub.2 10.465 97.62 0.2% TiO.sub.2 588 900.degree. C./Ar/1 hr H.sub.2 10.568 98.58 not added* 588 900.degree. C./Ar/1 hr H.sub.2 + 2% CO.sub.2 8.0 74.6 *comparative example EXAMPLE 3 The U.sub.3 O.sub.8 -based powder was prepared in the same way as in Example 1. The U.sub.3 O.sub.8 -based powder was mixed with 0.5% niobium oxide (Nb.sub.2 O.sub.5) and 0.4% titanium oxide (TiO.sub.2) by weight of the U.sub.3 O.sub.8 -based powder, respectively. The mixed powder was pre-pressed at a pressure of 98 MPa into slugs, which were then broken up into granules of 425 .mu.m or smaller. The granules were mixed with zinc stearate at 0.2% by weight of the granules for lubrication and were pressed into green pellets in a mold under 588 MPa of pressure. The green pellets were heated to 1700.degree. C. in a reducing gas atmosphere for 4 hours and then cooled-down. The reducing gas was a mixed gas of argon and hydrogen, and the hydrogen gas contained steam of 1.5% by volume of the hydrogen gas. Table IV shows the densities of UO.sub.2 -based pellets fabricated in accordance with the above procedures. In order to show clearly the effect of the additives, U.sub.3 O.sub.8 -based powder containing no additives was processed following the same procedure, and the density of the pellet is also shown for comparison in Table IV. TABLE IV additives pressing pressure green density reducing gas sintered density sintered density (wt %) (MPa) (g/cm.sup.3) atmosph (vol %) (g/cm.sup.3) (% TD) 0.4% TiO.sub.2 588 5.95 Ar + 5% H.sub.2 10.270 95.80 0.5% Nb.sub.2 O.sub.5 588 5.88 Ar + 5% H.sub.2 10.230 95.43 not added* 588 5.82 Ar + 5% H.sub.2 7.9 73.7 *comparative example