Patent Number: 045227691
Section: description

DETAILED DESCRIPTION OF THE INVENTION This invention comprises an improved method for producing a fissionable nuclear fuel product in pellet form from particulate ceramic material utilizing a fugitive binder which is subsequently removed during the sintering operation. The method includes the process of compression molding of particulate ceramic material comprising uranium dioxide powder combined with a fugitive binder comprising a multifunctional primary amine with carbon dioxide gas 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 particulate 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, and mixtures thereof. The particulate nuclear fuel material in the practice of the invention can also include various additives such as neutron absorbing materials comprising gadolinium to moderate neutron flux densities. Fugitive binders suitable for use in the practice of this invention comprise the amines disclosed and claimed in the above identified applications for patent Ser. Nos. 331,492 and 273,900. These include multifunctional primary amines selected from the group consisting of ethylene diamine; 3.3 diaminodipropylamine; 1.3 diaminopropane; 1.6 diaminohexane; 1.7 diaminoheptane; diethylenetriamine; and 3-dimethylaminopropylamine. Such binder agent are preferable employed in amounts of from about 0.3% to about 4% by weight of the amine compound based upon the weight of the nuclear fuel material. Amounts of amine beyond about 4% do not generally provide a proportionally commenserate benefit in bonding capacity, and may introduce unwanted effects which can compromise any advantages or the costs of including greater amounts of the bonding agent. In accordance with the method of this invention, the amine is added to the particulate ceramic fuel material containing uranium dioxide and the binder blended substantially uniformly through the particulate material. Then carbon dioxide gas is passed through the prepared blend of particulate material containing the amine in an amount of gas of at least about one mole of carbon dioxide gas for each gram mole of NH.sub.2 in the amine. Thereafter, the blend of particulate ceramic material and added amine-containing binder with the carbon dioxide gas is compressed into a coherent compact of suitable dimensions in accordance with procedures and means of the art. The method of this invention enables the effective use of such blends in high speed, continuous production rotary press devices and operations. 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 integrate the ceramic particles into a uniform and continuous body. The sintered product, typically in the form of a pellet, is thereafter ground to specified dimensions for its designated service. Blending of the added amine 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. The blending of the particulate material with the amine, and thereafter the introduction of the carbon dioxide gas, should include a dwell time of at least about 20 minutes and preferable at least about 40 minutes. This dwell period provides for the amine-containing binder with the gas to react directly with the uranium dioxide producing an effective bonding mechanism, and also the modification of the brittle mixture of ceramic particles to render them more plastic and amenable in compression molding. Upon applying the amine and carbon dioxide gas of this invention to the particulate ceramic material, the blend thereof can be compression molded into coherent compacts or handleable pellets by substantially any effective means or device in accordance with the technology of this field such as is described in the prior art, including the above cited patents and pending applications. The coherent compacts or pellets of the compression molded, binder containing particulate ceramic material are then sintered to expel binder material therefrom and integrate the ceramic particles into an essentially continuous body or mass of substantially uniform consistency, and relatively high strength and good resistance to fracture. Water or other suitable solvent may be added to the amine prior to its addition to the particulate ceramic fuel material containing uranium dioxide to reduce the corrosive effect of the amine upon certain materials such as rubber lining which may be present in processing equipment. When water is included, its content in the blend must be adjusted to a final amount of below about 5000 parts per million by weight. Examples of preferred procedures for the practice of the method of this invention are as follows. EXAMPLE 1 Ten kilograms of UO.sub.2 powder and a solution of 11/2% ethylene diamine in 0.4% water, both on a UO.sub.2 weight basis, were blended for 20 minutes in a 1 cubic foot Sweco vibratory mill containing 3/4" aluminum cylinders as media. Carbon dioxide gas was then introduced into the mill at a rate of 1 cubic foot per minute while the mill operated for an additional 30 minutes. (H.sub.2 O added to reduce corrosive attack on particular material used to line vibratory mill is not necessary for all materials.) The foregoing procedure was then repeated except 1 cubic foot of nitrogen gas was substituted in place of the carbon dioxide gas. Compacts were then die pressed from the source UO.sub.2 powder (not treatment), the carbon dioxide treated, and the nitrogen treated UO.sub.2 powder. These compacts were then tested in diametral compression which allows a measure of tensile strength (TS), total elongation E.sub.Total, plastic elonglation E.sub.P, and F.sup.3 which is a measure of crack blunting after failure has occurred. ______________________________________ TS E.sub.Total E.sub.P F.sub.3 ______________________________________ Source UO.sub.2 powder 22 psi 22.mu. 4 0 UO.sub.2 + ethyene diamine + 52 68 39 3.5 H.sub.2 O + CO.sub.2 UO.sub.2 + ethylene diamine + 20 39 18 0.5 H.sub.2 O + N.sub.2 ______________________________________ The UO.sub.2 + ethylene diamine, and CO.sub.2 obviously functions well as a binder by increasing tensile strength, total elongation, plastic elongation, and elongation after failure. EXAMPLE 2 Twenty-one kilograms of UO.sub.2 powder and a solution of 0.6% of ethylene diamine in 0.8% water, both on a UO.sub.2 weight basis, well blended in a 3 cubic foot Sweco vibratory mill containing 3/4" aluminum cylinders. (H.sub.2 O added to reduce corrosive attack on vibratory mill lining.) 0.7% ammonium oxalate was also added as a pore former for creating voids in the sintered pellets which will be fabricated from the powder. The mill operated 5 minutes to blend the ethylene diamine in water solution and ammonium oxalate. Carbon dioxide gas was then introduced into the mill at 6 cubic feet per minute with the mill running. The carbon dioxide gas treatment was then followed by 20 minutes of mill operation with 20 cubic feet of nitrogen flowing which by previous testing should evaporate the final water content to less than 5000 ppm. Diametral tests on compacts made from the powder revealed less than desired plasticity. The powder which was still in the mill was given an additional 6 cfm carbon dioxide flow for 10 minutes with the mill running. Diametral compression testing yielded the following properties: ______________________________________ Tensile Strength 67 psi Total Elongation 77.mu. Plastic Elongation 43.mu. F.sup.3 20.mu. ______________________________________ The source (no additive) UO.sub.2 diametral compression test results from example 1 may be used for comparison in establishing the performance of this binder. EXAMPLE 3 Two hundred fifty grams of uranium dioxide along with two hundred grams of 1/2" aluminum grinding media were introduced into an 8 ounce polyethylene bottle. 2.5% ethylene diamine was then added (6.25 grams) and the bottle placed on a paint shaker for 15 minutes. The aluminum balls were then screened from the blended powder which was next placed in a 6" fritted glass porous funnel. 400 cfh of carbon dioxide gas was then flowed upward through the powder creating the binder. Diametral compression at various time intervals of carbon dioxide treatment are as follows: ______________________________________ Tensile Total Plastic Strength Elongation Elongation F.sup.3 ______________________________________ 15 min CO.sub.2 106 psi 115.mu. 70.mu. 48.mu. 30 min CO.sub.2 154 psi 144.mu. -- 21.mu. 45 min CO.sub.2 177 psi 166.mu. -- 16.mu. 60 min CO.sub.2 240 psi 106.mu. 46" 3.mu. ______________________________________ No water used in this example.