Patent Number: 043839533
Section: description

DETAILED DESCRIPTION OF THE INVENTION This invention comprises the discovery of an improved manufacturing procedure for the formation of coherent pellets or bodies of compacted particulate ceramic materials having markedly enhanced physical strength and resistance to pressing deformities, a minimum of physical impediments or flaws, and which are durable and handleable when subjected to factory production operations. This new procedure for the formation of such firmly integrated or coherent units from particulate ceramic materials admixed with fugitive binders comprises a combination of a specific sequence of manufacturing steps or operations including an essential period for the maturing or reacting of the fugitive binder of blended admixtures of the ingredients prior to significant subsequent handling or processing thereof, such as the subsequent consolidation of the particulate components to a compacted shape and the sintering thereof, or following work thereon including grinding or cutting to shape or given dimensions. This invention entails the admixing of a nuclear fuel or similar ceramic material in powder or relatively fine particulate form with a binder of a compound or its hydration products containing ammonium cations and anions selected from the group consisting of carbonate anions, bicarbonate anions, carbonate anions and mixtures of such anions. The particulate nuclear fuel materials comprise the various materials used as nuclear fuels for nuclear reactors including ceramic compounds such as oxides of uranium, plutonium and thorium with particularly preferred compounds being uranium oxide, plutonium oxide, thorium oxide and mixtures thereof. An especially preferred nuclear fuel for use in this invention is uranium oxide, particularly uranium dioxide. Further the term nuclear fuel is intended to cover a mixture of the oxides of plutonium and uranium and the addition of one or more additives to the nuclear fuel material such as gadolinium oxide (Gd.sub.2 O.sub.3). In carrying out the present process which will be discussed for the preferred use of uranium dioxide, the uranium dioxide powder (or particles) used generally has an oxygen to uranium atomic ratio greater than 2l00 and can range up to 2.25. The size of the uranium dioxide powder or particles ranges up to about 10 microns and there is no limit on lower particle size. Such particle sizes allow the sintering to be carried out within a reasonable length of time and at temperatures practical for commercial applications. For most applications, to obtain rapid sintering, the uranium dioxide powder has a size ranging up to about 1 micron. Commercial uranium dioxide powders are preferred and these are of small particle size, usually sub-micron generally ranging from about 0.02 micron to 0.5 micron. Compositions suitable for use as a binder in the practice of this invention either alone or in mixtures, include ammonium bicarbonate, ammonium carbonate, ammonium bicarbonate carbamate, ammonium sesquicarbonate, ammonium carbamate and mixtures thereof. When mixed with nuclear fuel materials, these binders and the nuclear fuel material are believed to undergo the phenomenon of adhesion forming an ammonium derivative of the carbonate series such as (NH.sub.4).sub.4 [UO.sub.2 (CO.sub.3).sub.3 ]; (NH.sub.4).sub.6 [(UO.sub.2).sub.2 (CO.sub.3).sub.5 (H.sub.2 O).sub.2 ]H.sub.2 O; (NH.sub.4).sub.2 (CO.sub.3).sub.2 (H.sub.2 O).sub.2 ]; (NH.sub.4).sub.3 [(UO.sub.2).sub.2 (CO.sub.3).sub.3)(OH)(H.sub.2 O).sub.5 ]; NH.sub.4 [UO.sub.2 (CO.sub.3)(OH) (H.sub.2 O).sub.3 ]and UO.sub.2 CO.sub.3 H.sub.2 O, or mixtures of these. In the present invention the binder preferably has certain characteristics. It should be substantially comprised 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 free of impurities so that it can be mixed with uranium dioxide powder and pressed and sintered without leaving any undesired impurities after heating with particularly preferred binders being ammonium bicarbonate and ammonium carbonate and mixtures thereof. It has been found that commercially available ammonium bicarbonate contains virtually no impurities and commercially available ammonium carbonate also contains virtually no impurities except for other ammonium compounds as listed in the foregoing paragraph. Thermogravimetric analysis confirms that there is a complete volatilization of ammonium bicarbonate and ammonium carbonate at heating rates typically used for reductive atmospheric UO.sub.2 sintering. Ammonium bicarbonate and ammonium carbonate when heated to the temperature range of decomposition, decompose to form ammonia, carbon dioxide and water at significant rates leaving substantially no contaminates (impurities) in the fuel and no undesirable residues in the sintering furnace. Additionally the ammonium bicarbonate and the ammonium carbonate are used in small particle sizes of 400 mesh or less in order to achieve maximum surface coverage of the binder on the surface of the nuclear fuel material. Ammonium bicarbonate is used as the binder when it is desired to avoid the formation of density reducing pores in the nuclear fuel material. The plasticity of ammonium bicarbonate and ammonium carbonate may be demonstrated by the fact that these compounds can be die pressed to green densities as high as 90% of theoretical density at moderate pressing pressures. The amount of binder added to the nuclear fuel material generally ranges from about 0.5 to about 7.0 weight percent depending on the formability of the nuclear fuel material. For example formable uranium dioxide powders require less of an addition of the binder while less readily formable powders require larger amounts of binder. When the selected binder is ammonium carbonate, the amount of the addition of this binder is dependent upon the desired sintered density of the nuclear fuel material. Homogeneous blending of the binder with the nuclear fuel material is practiced to develop fully the binding action of the binder on the nuclear fuel material. Where porosity or a lower density is not desired, the homogenous blending of the binder with the nuclear fuel material avoids the formation of agglomerates of the binder since such agglomerates can volatize during sintering leaving pores in the sintered nuclear fuel material which pores reduce the density of the nuclear fuel material in sintered bodies. When it is felt that agglomerates of the binder exist in the nuclear fuel material after mixing, a milling process such as jet milling or hammer milling is practiced so that the agglomerates are destroyed. The blended and milled powder may then be predensified by low pressure die pressing followed by granulation through a sizing screen to promote flowability of the mixture. In carrying out this invention, it is preferred in order to achieve an optimum degree of uniformity of blending and freedom from non-homogeneous agglomerates therein that the binder be added to the particulate ceramic material by pneumatically injecting the binder into a mass of the particulate ceramic while suspended or fluidized in a fluid bed system and therein continuing the fluidized blending thereof until a substantially uniform dispersion of the binder about the particles of ceramic fuel material is achieved. A preferred fluid bed system for the addition and mixing of such ingredients is disclosed in U.S. Pat. Nos. 4,172,667, issued Oct. 30, 1979, and 4,168,914, issued Sept. 25, 1979. Blending of the combined particulate binder and ceramic material preferably should be continued for a period of at least about 10 minutes to insure a high degree of homogeneity and to induce the formation of more handleable, small agglomerates of the blended ingredients. In accordance with this invention the blend of such particulate ceramic material with the binder component described above is held for a relatively brief period of greater than 48 hours and preferably at least about 72 hours in a substantially quiescent state to age or mature the binder, prior to proceeding with the usual manufacturing operations or steps including compressing the particulate admixture into a consolidated or compacted coherent mass or body such as a pellet, and the subsequent sintering of such integrated bodies. It appears that during this period, binders of the type or composition specified, undergo a degree of a decomposition reaction and conversion to products that provide an enhanced binding effect upon the ceramic particles which is markedly superior to that afforded by its precursor. Following completion of said aging period or intermission, the matured mixture of particulate nuclear fuel material with the binder can be formed into a green body, generally a cylindrical pellet by a number of techniques such as pressing (particularly die pressing). Specifically, the mixture is compressed into a form in which it has the required mechanical strength for handling and which, after sintering, is of the size which satisfies reactor specification. The aging of the binders of this invention in the nuclear fuel material significantly enhances both the strength and integrity of the resulting green body. The green body can have a density ranging from about 30% to 70% of theoretical, but usually it has a density ranging from about 40% to 60% of theoretical, and preferably about 50% of theoretical. The green body is sintered in an atmosphere which depends on the particualr manufacturing process. Specifically, it is an atmosphere which can be used to sinter uranium dioxide alone in the production of uranium dioxide nuclear fuel and also it must be an atmosphere which is compatible with the gases resulting from any decomposition of binder ingredients. For example, a number of atmospheres can be used such as an inert atmosphere, a reducing atmosphere (e.g. dry hydrogen) or a controlled atmosphere comprised of a mixture of gases (e.g. a mixture of hydrogen and carbon dioxide as set forth in U.S. Pat. No. 3,872,022) which in equilibrium produces a partial pressure of oxygen sufficient to maintain the uranium dioxide at a desired oxygen to uranium ratio. The rate of heating to sintering temperature is limited largely by how fast the by-product gases are removed prior to achieving a sintering temperature and generally this depends on the gas flow rate through the furnace and its uniformity therein as well as the amount of material in the furnace. Specifically, the rate of flow of gas through the furnace, which ordinarily is substantially the same gas flow used in the sintering atmosphere, should be sufficient to remove the gases resulting from decomposition of binder material before sintering temperature is reached. Generally, best results are obtained when the rate of heating to decompose and volatilize all binder materials ranges from about 50.degree. C. per hour to about 300.degree. C. per hour. After decomposition of the binder material is completed and byproduct gases substantially removed from the furnace, the rate of heating can then be increased, if desired, to a range of about 300.degree. C. to 500.degree. C. per hour and as high as 800.degree. C. per hour but not be so rapid as to crack the bodies. Upon completion of sintering, the sintered body is usually cooled to room temperature. The rate of cooling of the sintered body is not critical in the present process, but it should not be so rapid as to crack the sintered body. Specifically, the rate of cooling can be the same as the cooling rates normally or usually used in commercial sintering furnaces. These cooling rates may range from 100.degree. C. to about 800.degree. C. per hour, and generally, from about 400.degree. C. per hour to 600.degree. C. per hour. The sintered uranium dioxide bodies are preferably cooled in the same atmosphere in which they were sintered. To govern the densities of the sintered bodies of ceramic fuel material of this invention, pore formers such as ammonium oxalate or a uranium precursor can be added to the fuel material along with the binders in the practice of this invention. The pore formers, when used, are preferably reduced to a uniformly fine granular form and premixed with the particulate ceramic material. The green or unfired nuclear fuel pelleted product formed by the new process of this invention and exhibiting improved strength and physical integrity is illustrated in FIG. 1 of the drawing. The following comprises an example of a preferred embodiment for the practice of this invention and an illustration of the pronounced improvement in tensile strength of the products produced thereby. Uniformly fine powdered ammonium bicarbonate was introduced into uranium dioxide particles in a ratio of about 2.7 weight percent based on the UO.sub.2 in the fluid bed system and apparatus of U.S. Pat. No. 4,172,667. The particulate admixture of NH.sub.4 HCO.sub.3 and UO.sub.2 was fluidized and agitated within the system for about 10 minutes, whereupon the resultant homogeneous blend of the particles was aged under quiescent or static condition for 72 hours prior to subsequent processing including pressing and compacting the particles into coherent integrated bodies or pellets. The process was thereafter completed by die pressing into cylindrical fuel pellets in accordance with the procedures set forth in U.S. Pat. No. 4,061,700. The pellets produced from the thus aged product exhibited significantly greater strength and integrity in the green or unfired state with a tensile strength increased by a factor of about twofold over unaged admixtures prepared in a like manner except for aging intermission. After the routine firing of the compressed pellets, the aged pellet samples survived the grinding operation to achieve precise dimensions thereof to the extent of about 95% recovery whereas like produced but unaged pellets had a grinding recovery of less than about 50%. Further identically prepared samples of admixtures of ammonium bicarbonate binder with uranium dioxide were aged for several different periods of time, namely 24 hours, 48 hours and 72 hours, and compared with a sample likewise processed but without any binder. The results of this evaluation, measured in tensile strength, psi, are shown in the graph of FIG. 2 of the drawing.