Patent Number: 043839533
Section: summary

This invention relates to manufacturing techniques and procedures comprising compressing particulate ceramic materials into compacted, coherent and handleable bodies for subsequent sintering to produce integrated units or products, and it particularly relates to a method of forming green or presintered pellets of particulate fissionable nuclear fuel material having increased physical strength and integrity for enduring subsequent handling or processing, such as sintering and grinding to dimensions, and their final utilization. Various materials are used as fissionable nuclear fuels for nuclear reactors including ceramic compounds of uranium, plutonium and thorium with particularly preferred compounds being uranium oxide, plutonium oxide, thorium oxide and mixtures thereof. An especially preferred fissionable nuclear fuel for use in nuclear reactors is uranium dioxide. Uranium dioxide is produced commercially as a fine, fairly porous powder which cannot be used directly as nuclear fuel. It is not a free-flowing powder but clumps and agglomerates, making it difficult to pack in reactor tubes to the desired density. The specific composition of a given commercial uranium dioxide powder may also prevent it from being used directly as a nuclear fuel. Uranium dioxide is an exception to the law of definite proportions since "UO.sub.2 " actually denotes a single, stable phase that may vary in composition from UO.sub.1.7 to UO.sub.2.25. Because thermal conductivity decreases with increasing O/U ratios, uranium dioxide having as low an O/U ratio as possible is preferred. However, since uranium dioxide powder oxidizes easily in air and absorbs moisture readily, the O/U ratio of this powder is significantly in excess of that acceptable for fuel. Although uranium dioxide suitable as a fissionable nuclear fuel can have an O/U ratio ranging from 1.7 to 2.015, as a practical matter, a ratio of 2.00 and suitably as high as 2.015 has been used since it can be consistently produced in commercial sintering operations. In some instances, it may be desirable to maintain the O/U ratio of the uranium dioxide at a level higher than 2.00 at sintering temperature. For example, it may be more suitable under the particular manufacturing process to produce a nuclear fuel having an O/U ratio as high as 2.195, and then later treat the sintered product in a reducing atmosphere to obtain the desired O/U ratio. A number of methods have been used to make uranium dioxide powder suitable as a fissionable nuclear fuel. Formerly, the most common method was to die press the powder into cylindrically-shaped green bodies of specific size without the assistance of binders since the complete removal of binders and their decomposition products was difficult to achieve prior to sintering. The entrainment of binder residues is considered unacceptable in sintered nuclear fuels. In the sintering process, it is desirable to develop strong diffusion bonds between the individual particles without significantly reducing the interconnecting porosity of the body. The use of organic binders inhibits the formation of strong bonds unless a presintering treatment is applied to remove the binder. The higher compacting pressures and sintering temperatures required to develop such bonds sharply reduce the desired porosity. Sintering atmospheres may range from about 1000.degree. C. to about 2400.degree. C. with the particular sintering temperature depending largely on the sintering atmosphere. For example, when wet hydrogen gas is used as the sintering atmosphere, its water vapor accelerates the sintering rate thereby allowing the use of correspondingly lower sintering temperatures such as a temperature of about 1700.degree. C. The sintering operation is designed to densify the bodies and bring them down to the desired O/U ratio or close to the desired O/U ratio. Conventional organic or plastic binders are unsuitable for use in powder fabrication of nuclear fuel since they tend to contaminate the interior of the sintered body with impurities such as carbon, and their removal requires a separate binder removal treatment or operation. In addition, upon decomposition, these binder materials often leave deposits of organic materials in the equipment utlized to sinter the article, thereby complicating the maintenance procedures for the equipment. U.S. Pat. No. 4,061,700, issued Dec. 6, 1977 to Gallivan, and assigned to the same assignee as this application, discloses a group of new fugitive binders that produce improved sintered bodies of nuclear fuel materials for nuclear reactors by powder ceramic techniques without contaminating the resultant fuel or manufacturing systems, and which permit, through sintering, the formation of strong bonds between the sintered particles without deleteriously affecting porosity. The improved fugitive binders of said U.S. Pat. No. 4,061,700 comprise a compound or its hydration products containing ammonium cations and anions selected from the group consisting of carbonate anions, bicarbonate anions, carbamate anions and mixtures of such anions, preferably a binder selected from the group consisting of ammonium bicarbonate, ammonium carbonate, ammonium bicarbonate carbamate, ammonium sesquicarbonate, ammonium carbamate and mixtures thereof. The binders disclosed in this patent are efficient binders for use in nuclear fuels, and further the binders enable the realization of defect free, pressed bodies of nuclear fuel materials and tensile strength in the bodies comparable to strengths achieved with long chain hydrocarbon binders. Further, the binders in this patent leave substantially no impurities in the nuclear fuel material since these binders decompose upon heating into ammonia (NH.sub.3), carbon dioxide (CO.sub.2) and water (H.sub.2 O) (or water vapor) at temperatures as low as 30.degree. C. The disclosure of the aforesaid U.S. Pat. No. 4,061,700, and U.S. Pat. Nos. 3,803,273; 3,923,933; and 3,927,154, also assigned to the same assignee as the subject application, each relating to significant aspects in the subject field of producing nuclear fuel pellets or bodies from particulate fissionable ceramic material, are all incorporated herein by reference. Notwithstanding the significant contributions of the inventions of above patents to this field and their specific advances in that technology, there remains a need to further increase the green or unfired strength and durability of consolidated bodies or pellets of such particulate ceramic nuclear fuel materials prior to their sintering and thereafter, to thereby reduce the high number of rejects and production costs incurred during manufacture resulting from imperfections or flaws attributable to marginal green or unfired strength or physical integrity. SUMMARY OF THE INVENTION This invention comprises a method for producing green or unfired compressed bodies or pellets of particulate fissionable ceramic fuel materials with fugitive binders of the type and materials set forth in the above cited patents, having significantly greater strength and physical integrity prior to firing or in the green stage, and thereafter, and the improved products derived therefrom. In addition to the specific components or ingredients given, this invention comprises a combination of sequenced manufacturing steps or operations including an essential aging period effected or carried out intermediate to certain of such sequenced steps or operations of the overall procedure. The method of this invention enables the practice of a process with an exceptionally low level of rejects or physical imperfections for the formation and subsequent sintering of bodies of fissionable nuclear fuel, comprising the steps of admixing the nuclear fuel material in particulate form with the binder, forming the resulting mixture into a green body having a density ranging from about 30% to about 70% of theoretical density of the nuclear fuel material, heating said green body to decompose substantially all the binder into gases, further heating the body to produce a sintered body and cooling the sintered body in a controlled atmosphere. This invention also provides a composition of matter that is suitable for sintering in the form of a compacted, coherent handleable structure comprising a mixture of a nuclear fuel material and a binder of a compound or its hydration products containing ammonium cations and anions selected from the group consisting of carbonate anions, bicarbonate anions, carbamate anions and mixtures of such anions and preferably a binder selected from the group consisting of ammonium bicarbonate, ammonium carbonate and mixtures thereof. OBJECTS OF THE INVENTION It is an object of this invention to provide an improved method of manufacturing compacted, coherent and handleable bodies or pellets of nuclear fuel from particulate fissionable ceramic material, and the compressed products of such method. It is also an object of this invention to provide a method of improving the strength and physical integrity of green or unfired compressed and integrated bodies or pellets of nuclear fuel comprising particulate fissionable material and a fugitive binder, and the physically enhanced products thereof. It is a further object of this invention to provide a method for forming and sintering a body of nuclear fuel comprising the steps of admixing the nuclear fuel material in fine particulate form with a binder, forming the resulting mixture into a coherent and handleable green body, heating said green body to dispel any binder ingredients and to produce a durable sintered body wherein the number of rejects due to inadequate strength or physical durability of the green or as yet unsintered bodies, and in turn production costs, are significantly reduced. It is another object of this invention to provide a method for preparing particulate admixtures for producing green or unfired compressed bodies or pellets of particulate fissionable nuclear fuel materials admixed with a fugitive binder that are resistant to pressing flaws or deformities whereby the compressed bodies or pellets formed therefrom are substantially uniform throughout in configuration and physical integrity, and substantially free of physical imperfections or irregularities such as end flakes, radial cracks, fractures, chips and the like debilitating defects that impair the physical integrity of the units and cause their inability to meet specifications for nuclear fuel or simply their ultimate failure in physical structure. A still further object of this invention is to provide a method for preparing particulate admixtures of fissionable nuclear fuel material with a binder for producing compressed and sintered bodies or pellets wherein the density of the sintered product is controlled by the inclusion of a pore forming substance such as ammonium oxalate.