Patent Number: 044302762
Section: summary

BACKGROUND OF THE INVENTION In the manufacture of UO.sub.2 pellets for use in nuclear reactors, high density pellets are desirable to maximize the amount of fuel per unit volume, but due to the formation of fission products in the fuel some porosity is also desirable in order to contain these fission products. A compromise of these opposing considerations requires that the UO.sub.2 fuel pellets have an optimum density of about 95%. The UO.sub.2 fuel pellets are prepared by sintering pressed UO.sub.2 powder. If the UO.sub.2 powder is of poor quality, the pellets will sinter to less than the optimum density. In order to achieve higher densities, various dopants may be added to the UO.sub.2 powder prior to sintering. While the addition of the dopants may enable the powder to sinter to the optimum density, they are not normally added during commercial fuel fabrication. Without the addition of sintering aids, a fuel pellet made from poorly sinterable powder results in grains of small size. A small grain size generally is equivalent to a fine porosity which readily disappears in use in the reactor, permitting the escape of fission gases during operation. Second, while the pellets have the optimum density initially, they are often not stable in the reactor and gradually become denser when subjected to the heat of the reactor. In pressurized reactors where the fuel rods themselves are not pressurized, this can result in the collapse of the fuel rod, while gaps can be formed as a result of pellet densification in all fuel rods. A different sort of problem arises when highly sinterable UO.sub.2 powders are used to prepare the fuel pellets. These powders, which result if the particle size of the UO.sub.2 is very fine, sinter to a density which is above the optimum density. If the sintering process is stopped when the optimum density is reached, the fuel pellets will simply continue to densify in the reactor. Thus, until now, highly sinterable UO.sub.2 powders could not be used without otherwise reducing the sinterability by introduction of large quantities of scrap or oxidized material ("add-back") to reduce the density which also results in an increase in the quantity of fine porosity. SUMMARY OF THE INVENTION We have discovered that certain dopants can be added during one of the steps in the process of producing ammonium diuranate (ADU), from which UO.sub.2 powder is produced, so as to yield a UO.sub.2 fuel pellet of the optimum density. Pellets produced according to the process of this invention are dimensionally stable in a reactor and have the desirable large grain size. We have also discovered, quite surprisingly, that the addition of these dopants to a highly sinterable UO.sub.2 powder will actually lower the density to which the powder will sinter, yet still produce a dimensionally stable fuel pellet of large grain size. Because the fuel pellets produced according to the method of this invention are dimensionally stable in a reactor, they do not densify and cause the collapse of the fuel rods. In addition, because they have a large grain size, the fission products remain trapped in the fuel pellets and do not escape into the reactor. Also, because the sinterability of the powder is modified by the sintering aid, the fraction of fine pores &lt;2 .mu.m is reduced considerably giving a larger mean pore size. PRIOR ART A paper presented to the American Ceramic Society in Cincinnati, Ohio in May, 1979, by L. F. A. Raven entitled "Gas Retentive Annular Fuel Pellets: Palliative or Panacea for the L.W.R.?" discloses the addition of TiO.sub.2, MgO, Al.sub.2 O.sub.3, CrO.sub.2, and Nb.sub.2 O.sub.5 to UO.sub.2 to increase the grain size of sintered UO.sub.2 while simultaneously increasing the density. DESCRIPTION OF THE INVENTION In the process of this invention, a dopant is added at some step during the formation of ammonium diuranate (ADU), or the resulting UO.sub.2, so that the fuel UO.sub.2 contains about 0.05 to about 1.7 mole% (based on UO.sub.2) of the dopant. The dopants of this invention are compounds of aluminum, calcium, magnesium, titanium, zirconium, vanadium, niobium, or mixtures thereof. The preferred dopant element is either titanium, niobium, or a mixture of about 40 to about 60 mole% calcium with about 40 to about 60 mole% titanium. The compound may be an oxide, a nitrate, an oxalate, a uranate, a chloride, a fluoride, or other suitable compound. If the dopant is added at a step in the process of producing ADU where the uranium is in solution, the dopant should be soluble. If the dopant is added at a later stage to the solid (slurry) ADU or to the UO.sub.2, an insoluble dopant may be used. The preferred compound is a nitrate because uranyl nitrate, UO.sub.2 (NO.sub.3).sub.2, is commonly used in the preparation of ADU and a nitrate dopant would be compatible with that process. Fluoride compounds are also desirable dopants because they have particular compatibility with uranyl fluoride, UO.sub.2 F.sub.2, which is another common precursor of ADU. Thus, it is preferable to add the dopant as a soluble compound to a solution containing the uranium as this results in a more homogeneous mixture of the uranium and the dopant, avoids a solids mixing step, and requires less dopant. The ammonium diuranate from which the UO.sub.2 is prepared can be produced by reacting gaseous UF.sub.6 with water to produce a solution of uranyl fluoride according to the equation: EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF The ADU can be precipitated from the uranyl fluoride solution by the addition of an ammonium compound such as ammonium hydroxide. EQU 2UO.sub.2 F.sub.2 +2NH.sub.4 OH+H.sub.2 O.fwdarw.(NH.sub.4).sub.2 U.sub.2 O.sub.7 +4HF Alternatively, liquid uranyl nitrate, obtained typically from reprocessing scrap sintered pellets by dissolution with nitric acid, could be reacted with ammonium hydroxide to yield ADU as given by: EQU UO.sub.2 (NO.sub.3).sub.2 +6NH.sub.4 OH.fwdarw.(NH.sub.4).sub.2 U.sub.2 O.sub.7 +4NH.sub.4 NO.sub.3 +3H.sub.2 O The dopant could be added either as a soluble compound to the water into which the UF.sub.6 is introduced or into the solution of uranyl fluoride or uranyl nitrate, or to the slurry of the precipitated ADU where an insoluble compound, such as an oxide, could also be used. It is preferable to add the dopant to the uranyl nitrate or fluoride solution in the precipitation vessel as this is the most convenient point of addition. The UO.sub.2 can be prepared from the ADU by calcining at about 500.degree. to 750.degree. C. or other suitable temperature for a time typically of about 1 to 3 hours in a steam/hydrogen atmosphere having a wide range of possible ratios. The production of ADU as an intermediate can be avoided and the UO.sub.2 can be produced more directly by reacting UF.sub.6 with steam to produce a dry uranyl fluoride. The addition of hydrogen to the uranyl fluoride produces the UO.sub.3. EQU U.sub.2 F.sub.2 +H.sub.2 +UO.sub.2 +2HF If this process of producing UO.sub.2 is used, it should be possible to add the dopant to either the steam as an aerosol or to mix the dopant with the resulting UO.sub.2 powder where an insoluble compound could also be used. The preparation of fuel pellets from the powdered UO.sub.2 is a process well known in the art. A common procedure is to grind the UO.sub.2 powder to a particle size of less than 30 microns. The powder is then prepressed ("slugged") and crushed into a form suitable for feed for an automatic press. The resultant material is mixed with a die lubricant, and is pressed to about 40 to about 65% of TD (theoretical density, about 10.96 grams per cc). The pressed powder can then be sintered at about 1400.degree. to 1800.degree. C. for about 1 to about 10 hours, but usually about 5 hours, to produce the fuel pellets. The dopants used in the process of this invention will reduce the density, increase the grain size and mean pore size, and stabilize fuel pellets fabricated from a highly sinterable UO.sub.2 powder. A highly sinterable UO.sub.2 powder may be defined as a powder which will sinter (without the addition of a dopant) to greater than 97% of theoretical density within one hour in H.sub.2 at 1600.degree. C. Highly sinterable powders are physically different from less sinterable powders in that they have a smaller particle size and behave differently under electrophoretic conditions having as they have more positive zeta potentials than powders of poorer sinterability.