Patent Number: 046701982
Section: description

DETAILED DESCRIPTION OF THE INVENTION This invention comprises a method for producing a fissionable nuclear fuel product in pellet form from particulate ceramic material utilizing a unique fugitive binder system which is subsequently removed during the sintering operation. The method relates to compression molding of particulate ceramic material containing uranium dioxide powder combined with the fugitive binder composition of the invention to form coherent compacts of apt dimensions, and thereafter sintering the compacts to produce integrated bodies of fissionable nuclear fuel suitable for use in nuclear reactors. The paticulate fissionable nuclear fuel materials for use in this invention comprise various materials used as nuclear fuels for nuclear reactors, including ceramic compounds such as oxides of uranium, plutonium and thorium. Preferred fuel compounds consist of uranium oxide, plutonium oxide, thorium oxide, the mixtures thereof. The particulate nuclear fuel material in the practice of the invention can also include various additives such as high neutron absorbing materials comprising gadolinium to moderate neutron flux densities. Fugitive binder systems suitable for use in the practice of this invention consist of carbonate and carbamate-free, amine-containing compounds combined with an oxalate ion source. The amine compounds consist of diamines that are soluble in water, including, for example, ethylenediamine, propylenediamine, hexamethylenediamine, trimethylenediamine, tetramethylenediamine and pentamethylenediamine, and the like water soluble diamines. Preferred sources of the oxalate ions comprise ammonium oxalate and oxalic acid. Such oxalates are stable over temperatures ranging up to about 200.degree. C., and any residue thereof decomposes and volatilizes above about 200.degree. C. whereby they are aptly eliminated during sintering of the compressed compacts of fuel materials. In this invention the above diamine compounds and source of oxalate ion are combined, then added to the particulate ceramic fuel material containing uranium dioxide. The combined binder ingredients and product thereof are then blended substantially uniformly through the particulate material. Heat is preferably applied to the blend to foster a reaction between the oxalate ions and the di-valent uranyl ions present on the surface of the uranium dioxide. The rate for this reaction is dependent upon temperature, which is preferably held at about 55.degree. C. to about 85.degree. C. The ingredients and conditions of the reactions of this invention suggest that a polymeric type of binder compound and structure is produced apparently comprising: EQU [.sup.+ NH.sub.3 CH.sub.2 CH.sub.2 NH.sub.3.sup.+ ][.sup.- UO.sub.2 (C.sub.2 O.sub.4).sub.2 (H.sub.2 O).sub.2.sup.- ][.sup.+ NH.sub.3 CH.sub.2 CH.sub.2 NH.sub.3.sup.+ ]. Ammonium oxalate, which is of low solubility in water, is added to an aqueous solution of the diamine. Hydrolysis of the diamine increases the pH of the solution which facilitates dissolution of the ammonium oxalate with evolution of ammonia gas at elevated temperatures. Preferably a mixture of the oxalate source and the diamine is heated to 50.degree. C. for about 21/2 hours followed by 2 hours at 85.degree. C. The low temperature heating period of less than 50.degree. C. is to prevent excessive foaming from ammonia gas evolution. During the heating period approximately 90 percent of the ammonia content is removed. Cooling the solution an additional 3 hours reduces the ammonia content to less than 500 ppm. Removal of ammonia from solution is desirable in the overall process, since this prevents precipitation of ammonium oxalate crystals, which subsequently act as a poreformer due to volitalization during the later sintering operation. The combined diamine and oxalate ingredients, or products thereof, are added to the particulate ceramic nuclear fuel material comprising uranium dioxide to thereby form a stable uranyl oxalate. The combined and interacted ingredients including the formed binder composition are thereafter dried to remove excessive moisture by any suitable means such as the application of heated nitrogen gas moderately increased in temperature up to about 150.degree. C. It is during this drying process that any ammonium oxalate, if present, precipitates from solution and subsequently acts as a poreformer to produce low sinter density fuel. This unique carbonate and carbamate-free binder system imparts a high degree of plasticity, has been found to be stable and resistant to reaction with water and carbon dioxide, and in turn deterioration. Moreover it has also been found to be stable at relatively elevated temperatures of up to about 85.degree. C., over extended periods. At higher temperatures of about 200.degree. C. it partly decomposes for its effective elimination in the subsequent sintering step. Accordingly, the unique binder system of this invention is not degraded when blends thereof with particulate nuclear fuel material are stored or otherwise retained for long periods of time, even under adverse conditions of high humidity and high temperature. Proportions of such binder forming ingredients for the compositions of this invention preferably comprise the use of a diamine compound identified above in amounts of from about 1% to about 3% by weight based upon the weight of the nuclear fuel material. The oxalate ion source is included in amounts of from about 0.5% to about 1.5% by weight of the nuclear fuel material. Typically the diamine aqueous solution/oxalate ion source combination will comprise about 30 to about 50 (preferably about 40) percent by weight of the diamine with about 25 to about 60 (preferably about 40) percent by weight of water and about 10 to about 25 (preferably about 20) percent by weight of the oxalate ions. This combination is added to the particulate nuclear fuel material in amount of about 2 percent to about 6 percent, by weight thereof. Quantities of such binder ingredients in excess of the above upper amounts generally do not provide a proportionally commensurate benefit in bonding capacity, and may introduce unwanted effects that compromise any advantages or the costs of including greater amounts of these ingredients. As noted hereinabove, the diamine compound is dissolved in water to facilitate the action of the less soluble ammonium oxalate in the system. Diamine water solutions of any practical concentration for achieving the interaction and addition to the particulate ceramic nuclear fuel can be employed. Nevertheless, excesses of water which must be removed later in the operation are not expedient. However, diamine compound water solutions of, for example, about 30% to about 40% solids added initially are generally appropriate. Excessive water is removed following the addition to the particulate nuclear fuel material and the reaction period. Suitable means for water removal comprise passing nitrogen gas at a temperature of up to about 150.degree. C. through the particulate mixture of such ingredients and their reaction product for a typical period of about 20 to about 60 minutes, or longer if appropriate. A binder system prepared with the diamine solution and oxalate in accordance with this invention is blended uniformly with the particulate ceramic nuclear fuel material containing uranium dioxide, and the resultant mixture can thereafter be compressed into a coherent compact of suitable dimensions pursuant to the procedures and means of the art. The method of this invention is reproducible and imparts a high degree of plasticity to the particulate ceramic. Moreover it enables the prompt use, or long delayed use, such as resulting from extended storage or shipment, of the blend of fugitive binder and particulate fuel material for compressing into compacts. Also, the lasting plasticity attributable to the invention is adequate for the effective use of the blends in high speed, continuous production rotary press devices and operations for long periods after preparing the blend to provide unfired coherent compacts. Further, with this binder system, a die lubricant is not needed in the pressing operation. The "green" (unfired) coherent compacts thus formed are then sintered in accordance with the practices and procedures of the art to expel binder material and then integrate the ceramic particles into a uniform and continuous body. The sintered product, typically in the form of a cylindrical pellet, is thereafter ground to specified dimension for its designated service. Blending of the added binder can be effected with any appropriate "dry" mixing apparatus including low shear blenders such as fluidized bed, slab and ribbon blenders, and high shear or intensive blenders such as vibratory mills, ball mills and centrifugal mills. A preferred blending apparatus comprises vibratory mills of the type described in pages 8-29 to 8-30 of Perry and Chilton's 5th edition of Chemical Engineering Handbook, McGraw-Hill Book Co. Examples of a preferred procedure for the practice of this invention, and of the fugitive binders thereof, are as follows: Several examples of the binder system of this invention, comprising the following formulations, were prepared as follows. Oxalic acid or ammonium oxalate were combined with a solution of ethylenediamine to give the following concentrations after ammonia removal. ______________________________________ Ex- Den- % % % % % ample pH sity EDA Oxalate Water Ammonia CO.sub.2 ______________________________________ 1 11.30 1.126 38.8 16.0 45.2 0.086 0.47 2 11.39 1.122 40.7 16.0 43.3 0.045 0.55 3 11.36 1.121 42.8 17.7 39.5 0.053 0.67 4 11.47 1.118 42.7 16.5 40.8 0.066 0.55 ______________________________________ % Ammonia Close to Limit of Detection or 0.02% % CO.sub.2 - Close to Limit of Detection or 0.5% Each sample batch was approximately 15 gallons in volume which provides 8 to 9 small batches of binder solution for each production blend of uranium dioxide powder. The ethylenediamine and oxalate ion binder solution was added to discrete 180 kg batches of uranium dioxide powder fuel material and blended thoroughly. During the blending process the binder and the fuel were heated to about 55.degree.-75.degree. C. to foster the reaction of the binder ingredients with the di-valent uranyl ions present on the surface of the uranium dioxide particles. Approximately 10,000 kgs of the blended uranium dioxide powder fuel were compressed into compacts for evaluation. Fuel pellets comprising sintered compacts of the uranium dioxide powder blended with the ethylenediamine and oxalate ion binder system of the above examples were ground to size and the reject rate due to a defective or weak structure was determined. The overall yield of acceptable pellets is shown below: ______________________________________ Ethylenediamine-Uranyl-Oxalate Binder Process Grinder Yield, % Grinder Yield, % Hydramet Press Rotary Press ______________________________________ 97.2 96.1 97.8 94.3 97.2 96.2 96.8 95.6 97.2 95.2 96.5 96.1 ______________________________________ For comparison, fuel pellets produced in the same manner with the carbamate-carbonate containing binder of the same assignee's application for patent Ser. No. 517,588 filed July 27, 1983, (abandoned) had the following acceptance rate when ground under the same conditions. ______________________________________ Uranyl Oxalate Carbonate Binder Process Rotary Press Grinder Yield, % ______________________________________ 25.5 80.6 62.3 ______________________________________ The components of each category of these two distinctive type of binder systems are also given below for comparison. ______________________________________ Comparison of Uranyl Oxalate Carbonate Binder And Ethylenediamine Uranyl Oxalate Binder ______________________________________ Uranyl Oxalate Carbonate Binder (2) (1) % CO.sub.2 % Oxalate % NH.sub.3 H.sub.2 O, ppm ______________________________________ 0.27 0.82 0.32 2596.00 0.26 0.90 0.35 2629.00 0.28 0.91 0.35 2873.00 ______________________________________ Ethylenediamine Uranyl Oxalate Binder (3) % CO.sub.2 % Oxalate % NH.sub.3 H.sub.2 O, ppm % EDA ______________________________________ LT-LD 0.36 LT-LDD 3349.00 0.91 LT-LD 0.37 LT-LDD 3394.00 0.94 LT-LD 0.39 LT-LDD 3654.00 0.98 ______________________________________ (1) Primarily Particulate AO (2) % CO.sub.2 either in the form of Carbamate of Carbonate (3) Oxalate not in particulate form LT = Less than LD = Limit of detection = 0.100% LDD = Limit of detection = 0.05% The absence of ammonia in the final binder structure of the uranyl oxalate binder indicates particulate ammonia oxalate is not present. On the other hand, chemical analysis of powder containing the uranyl oxalate carbonate binder shows the presence of ammonia. Thus, the oxalate ion in the latter case is acting as a poreformer and not a binder. Carbamate and carbonate ions are not present in the uranyl oxalate binder as indicated by the lack of CO.sub.2. (See the above chemical analysis). Equilibrium with atmospheric CO.sub.2 after or during processing is not a problem with the uranyl oxalate binder. The presence of CO.sub.2 in powder containing the uranyl oxalate carbonate binder indicates that equilibrium with atmospheric CO.sub.2 can produce differences in material properties. The above indicates how the uranyl oxalate is chemically and physically a more stable binder structure.