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Sintering Uranium Oxide in the Reaction Product of Hydrogen

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Sintering Uranium Oxide in the Reaction Product of Hydrogen United States Patent n^ [it] 3,872,022 De Hollander et al. [45] Mar. 18, 1975 [54] SINTERING URANIUM OXIDE IN THE P. Benedek, et al., Chem. Abstracts, 1962, 8822(e). REACTION PRODUCTS OF E. Otsuka, et al. Chem. Abstracts, Vol. 61, 13159(h), HYDROGEN-CARBON DIOXIDE MIXTURES "Kinetics and Thermodynamics in High-Temperature Gases," Mar. 19, 1970, NASA, SP-239, A & 93. [75] Inventors: William R. De Hollander; Yogesh Nivas, both of San Jose, Calif. Primary Examiner—Donald J. Arnold [73] Assignee: General Electric Company, San Attorney, Agent, or Firm—Ivor J. James, Jr.; Sam E. Jose, Calif. Laub; Samuel E. Turner [22] Filed: May 9, 1973 [57] ABSTRACT [21] Appl. No.: 358,737 Compacted pellets of uranium oxide alone or contain- Related U.S. Application Data ing one or more additives such as plutonium dioxide, [63] Continuation-in-part of Ser. No. 62,353, Aug. 10, gadolinium oxide, titanium dioxide, silica and alumina 1970, abandoned. are heated to a temperature in the range of 900°- 1500°C in the presence of a gas which initially consists [52] U.S. CI 252/301.1 R, 264/.S, 264/65 1, essentially of a mixture of hydrogen and carbon diox- 423/261 ide, either alone or with an inert carrier gas such as [51] Int. CI G04b 35/64, G21 c 3/62 nitrogen and argon, and held at the desired tempera- [58] Field of Search 264/.S, 65; 252/301.1 R; ture in this resultant atmosphere to sinter the pellets. 423/261 The sintered pellets are then cooled in an atmosphere having an oxygen partial pressure in the range of 10~4 18 [56] References Cited to 10~ atmospheres of oxygen such as an atmosphere UNITED STATES PATENTS of dry hydrogen, wet hydrogen, dry carbon monoxide, wet carbon monoxide, inert gases such as nitrogen, ar- 3,258,317 6/1966 Brearton 264/65 gon, helium, and neon and mixtures of any of the fore- 3,422,167 1/1969 Bowman et al 264A5 3,501,411 3/1970 Triggiani et al 264/.S going including a mixture of hydrogen and carbon di- 3,504,058 3/1970 Masselot 264/.S oxide. The ratio of hydrogen to carbon dioxide in the gas mixture fed to the furnace is controlled to give a FOREIGN PATENTS OR APPLICATIONS ratio of oxygen to uranium atoms in the sintered parti- 649,349 10/1964 Belgium 264/0.5 cles within the range of 1.98:1 to about 2.10:1. The 712,078 6/1965 Canada 264/0.5 water vapor present in the reaction products in the 717,459 9/1965 Canada 264/0.5 furnace atmosphere acts as a hydrolysis agent to aid OTHER PUBLICATIONS removal of fluoride should such impurity be present in W. Trinks, et al., Industrial Furnaces, 5th Ed. 1961, the uranium oxide. John Wiley and Sons, Inc., New York, pages 156-160. 19 Claims, 2 Drawing Figures 4 6 7 8 9 10 II 12 14 - logjQ Partial Pressure of Oxygen -» ,400° \300° T s „ \\00° xooo° \8 Z0\ \6 \A \0\ 8 9 1 6 pf ess A pof^1 ov - \oQ\Q o£ f/g- ^ ixtlO' PATENTEDHAfl 181375 . 3,872,022 SHEET 2 OF 2 3*900,551 1 2 SINTERING URANIUM OXIDE IN THE REACTION grain uranium dioxide are superior to the properties of PRODUCTS OF HYDROGEN-CARBON DIOXIDE coarser grain uranium dioxide. The foregoing has made MIXTURES it desirable to lower the sintering temperature of ura- nium dioxides, and more generally of structures rich in This application is a continuation-in-part of Ser. No. 5 uranium oxide, in addition to controlling the oxygen to 62,353 filed Aug. 10, 1970 and now abandoned. metal ratio of the sintered structure and removing un- b Ie PUritieS StmCtUre $UCh 38 BACKGROUND OF THE INVENTION fluorid e ions ^ ^ ^^ This invention relates to the production of sintered Lower sintering temperatures have other desirable uranium oxide containing compositions. One of the 10 features including cost savings as less power is ex- very important utilities of uranium oxide, especially pended in heating the sintering furnace, a longer func- uranium dioxide, is in nuclear power plants as a fuel in tional life for the sintering furnace and its associated the generation of electric power. The uranium dioxide, fixtures, less corrosion of the furnace components and either alone or in a mixture with other ceramics such the possibility of adapting a continuous conveyor belt as gadolinium oxide or plutonium oxide, is compacted 15 or other means of transporting the structures rich in to a given size and shape and sintered to achieve dense uranium oxide through the furnace, bodies for use in a nuclear fuel rod. The uranium pres- lcrT„ _ ....Vr-NTTir,M ent in uranium dioxide must be enriched with the OBJECTS OF 1 HE INVENTION U-235 isotope which is done in a gaseous state, the pre- Accordingly it is an object of this invention to lower ferred practice being to use uranium hexafluoride. 20 the temperature at which the sintering of structures of After enrichment it is necessary to convert the uranium compacted powders rich in uranium oxide is accorn- hexafluoride to uranium dioxide. The resulting ura- plished and still obtain dense structures of the sintered nium dioxide can contain undesired fluoride ion con- ceramic. centrations and an oxygen to metal ratio above the de- A further object of this invention is the use of an at- sired ratio of about 1.98:1 to about 2.10:1. 25 mosphere of a mixture of hydrogen and carbon dioxide The sintering of uranium dioxide structures has been to remove impurities such as fluoride ions from corn- used as the step in the process to attempt to reduce the pacted structures of uranium oxide containing compo- oxygen and the fluoride content of the uranium diox- sitions during sintering. ide. The current practice has been the use of wet hy- A still further object of this invention is the use of an drogen atmospheres at temperatures preferably greater 30 atmosphere of a mixture of hydrogen and carbon diox- than 1,600°C to achieve dense bodies of uranium diox- ide to control the ratio of oxygen to metal atoms during ide. Past experience indicates a certain amount of sintering compacted structures of uranium oxide con- water vapor with the hydrogen is required to remove taining compositions. the fluoride content from compacted ceramic struc- Still another object of this invention is the sintering tures during sintering, but the wet hydrogen process ^5 Qf uranium oxide under an atmosphere of a mixture of has not been satisfactory when the ceramic has high hydrogen and carbon dioxide which reacts to give car- fluoride concentrations. bon monoxide and water vapor and removes fluoride Another method presented in U.S. Pat. No. ions and other undesirable constituents from the ura- 3,375,306 for sintering dense uranium dioxide struc- nium oxide during sintering. tures with or without ceramic additives is to heat the A still further object of this invention is the use of an compressed powder at a temperature of 1,300° to atmosphere of a mixture of hydrogen and carbon diox- 1,600°C in a sintering atmosphere of carbon dioxide or ide for sintering structures of compacted uranium diox- a mixture of carbon dioxide and carbon monoxide and ide powders. cooling the sintered structure in a reducing atmosphere Other objects and advantages of this invention will which varies as the composition of the structure varies. become apparent from the following specification and Where the structure being sintered is uranium dioxide the appended claims, the cooling gas is dry hydrogen, wet hydrogen or a mix- ture of carbon dioxide and carbon monoxide. Where SUMMARY OF THE INVENTION the structure is uranium dioxide with an additive of plu- The above objects, and others, are accomplished ac- tonium dioxide, the cooling gas is steam or carbon di- cording to this invention by providing a striking im- oxide mixed with carbon monoxide. The use of a mix- provement in the sintering of ceramic structures of ture of carbon dioxide and carbon monoxide is more compacted powder rich in uranium oxide involving costly than use of wet hydrogen but enables the use of heating the structures at a temperature in the range of lower temperatures to achieve sintered structures of 55 about 900° to about 1,500°C in a sintering atmosphere high density. However this carbon monoxide-carbon of a mixture of hydrogen and carbon dioxide. The car- dioxide sintering atmosphere does not appreciably de- bon dioxide and hydrogen react to give carbon monox- crease the fluoride content of the uranium dioxide ide and water vapor which enables the removal of un- structures. desirable impurities from the ceramic structures such Sintering temperatures of about 1,600°C or higher 6Q as fluoride ions and the control of the oxygen content produce uranium dioxide structures having large grain of the ceramic. After undergoing the above reaction, sizes with undesirable properties for some fuel applica- the carbon dioxide-hydrogen atmosphere maintained tions. Uranium dioxide structures of smaller grain size over the ceramic controls the partial pressure of oxy- have higher creep rates when compared to the creep gen during sintering and provides water vapor promot- rates of uranium dioxide structures of larger grain size. 65 ing the removal of fluoride ions from the ceramic being A higher creep rate for a uranium dioxide structure is sintered. desirable for better fuel performance. It has also been While this process is especially useful with ceramic determined that other mechanical properties of smaller compositions of uranium dioxide and mixtures of ura- 3,888,733 nium oxides having an oxygen to uranium ratio of up sition; the silicon dioxide can be present in amounts up to 2.25, it is particularly useful with uranium dioxide to about 5 percent by weight of the composition; and containing ceramic compositions with one or more ce- the aluminum dioxide can be present in amounts up to ramic additives such as gadolinium oxide and pluto- about 5 percent by weight of the composition.
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