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 uranium 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 dioxide 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 mixture 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. nium oxide. 5 The composition of the sintering atmosphere can vary greatly depending on the fluoride ion content of BRIEF DESCRIPTION OF THE DRAWING the ceramic and the oxygen to metal ratio of the ce- The practice of the disclosed process will be further ramic before sintering and the ratio desired after sinter- understood by reference to the accompanying drawings ing as well as the density to be achieved for the sintered in which 10 ceramic. A sintering atmosphere in the range of about FIG. 1 presents a graph of the partial pressure of oxy- 0.5 to about 90 percent hydrogen by volume with the gen versus the hydrogen content of the carbon dioxide- balance carbon dioxide can be employed, and a pre- hydrogen atmosphere at various temperatures and ferred range is about 2 to about 20 percent hydrogen FIG. 2 presents a furnace adapted to receive a sinter- by volume with the balance carbon dioxide. Any source ing atmosphere as disclosed in this invention for ce- 15 of hydrogen can be employed such as cylinders and ramie shapes being sintered in the furnace. tanks of hydrogen or a gas such as ammonia which dis- sociates to provide a source of hydrogen. The sintering DETAILED DESCRIPTION OF THE INVENTION atmosphere can contain carrier gases such as nitrogen It has been surprisingly discovered that compacted and argon in addition to the essential constituents of powders of a ceramic comprising uranium oxide con- 20 hydrogen and carbon dioxide. taining compositions, with or without ceramic addi- The mixture of carbon dioxide and hydrogen consti- tives, may be sintered to very high density by heating tuting the sintering atmosphere undergoes a thermody- to a temperature in the range of about 900° to about namic equilibrium of the following reaction: C02+H- 1,500°C in an atmosphere consisting of a mixture of hy- 2 —• C0+H20. The degree of reaction between these drogen and carbon dioxide. The sintering process of 25 two gases is determined by the equilibrium constant of this invention produces a ceramic composition having the above reaction which is a function of the tempera- a controlled ratio of oxygen to metal atoms, a fluoride ture of the atmosphere. The atomic ratio of oxygen to ion content of about 25 parts per million or less and a metal of the ceramic being sintered equilibrates with density up to about 98 percent of theoretical density the partial pressure of oxygen in the system with a fixed and preferably a density in the range of 92 to 96 per- 30 value at a given temperature. The desired oxygen to cent of theoretical density. The sintering process of this metal ratio of the uranium oxide compositions are ob- invention, in addition to controlling the fluoride con- tained by adjusting the partial pressure of oxygen in the tent, density and oxygen to metal ratio of the resulting sintering atmosphere. The partial pressure of oxygen in sintered ceramic, removes other undesired impurities the sintering atmosphere can be reduced by an increase and contamination from the uranium oxide containing ^5 in the hydrogen constituent of the sintering atmosphere compositions such as entrapped hydrocarbons, en- and an increase in the partial pressure of oxygen in the trapped gases, greases, oils, voids, etc. After comple- sintering atmosphere is achieved by an increase in the tion of the sintering, the sintered ceramic is cooled in carbon dioxide constituent of the sintering atmosphere. an atmosphere having an oxygen partial pressure in the In the practice of this invention, an enhanced sinter- range of 10-4 to 10~18 atmospheres of oxygen such as 40 ing rate results during the sintering of the uranium an atmosphere of dry hydrogen, wet hydrogen, dry car- oxide containing compositions due to the higher diffu- bon monoxide, wet carbon monoxide, inert gases such sion rate of the slower moving species of metallic ions as nitrogen, argon, helium, and neon and mixtures of in the sintered composition. By choosing a gas compo- the foregoing including a mixture of hydrogen and car- sition having a mixture of carbon dioxide and hydrogen bon dioxide preferably within the ranges disclosed in determined by the partial pressure of oxygen desired this invention for the sintering step. for the sintering atmosphere at a given sintering tem- The phase "uranium oxide containing compositions" perature, the uranium oxide containing composition is used herein to cover compositions capable of being can be sintered at the desired oxygen to metal ratio giv- sintered in the practice of this invention which include ing enhanced sintering at lower temperatures than 50 practiced in the prior art. FIG. 1 presents a graph of the uranium dioxide (U02) and mixtures of uranium oxide having an oxygen to metal ratio of up to 2.25 which relationship between the partial pressure of oxygen and could include mixtures of uranium dioxide with one or the hydrogen content in the carbon dioxide-hydrogen atmosphere for given temperatures. The water vapor so more of the following: uranium trioxide (UOa), ura- produced from the reaction given above acts as a hy- nium tritaoctoxide (U308), uranium sesquioxide S5 (U 0 ), uranium pentoxide (U 0 ), or uranium tetrox- drolysis agent for removing fluoride ions and when the 2 3 2 5 hydrogen component is within the range set forth ide (U0 ). The invention is also applicable for the fore- 4 above the final fluoride content of the uranium oxide going uranium oxide containing compositions with one composition will be reduced to less than 25 parts per or more ceramic additives including the plutonium ox- million. This represents a significant reduction in the ides such as plutonium dioxide (Pu0 ), gadolinium 2 fluoride content of the uranium oxide as commercially oxide (Gd 0 ), titanium dioxide (Ti0 ), silicon dioxide 2 3 2 available uranium oxide powders suitable for sintering (Si0 ), aluminum oxide (Al O ) and combinations 2 2 s can contain up to 0.5% by weight or more of fluoride thereof. The plutonium dioxide can be present in ions. As stated previously, a certain amount of water amounts up to about 30 percent by weight of the com- vapor is needed to promote the removal of fluoride position; the gadolinium oxide can be present in ^ from compacted uranium oxide structures during sin- amounts up to about 15 percent by weight of the com- tering. The amount of water vapor in the carbon diox- position; the titanium dioxide can be present in ide-hydrogen atmosphere decreases as the gas compo- amounts up to about 5 percent by weight of the compo- 3,s: ,022 5 6 sition is selected from an increasingly carbon dioxide sintering zone is the same as maintained in the preheat rich or an increasingly hydrogen rich mixture. But rela- zone with the atmosphere varying between about 0.5 to tively small amounts of water vapor in the sintering at- about 90 percent by volume hydrogen with the balance mosphere disclosed in this invention achieves a much being carbon dioxide. When a boat reaches the cooling greater reduction in the fluoride content at the same 5 zone the temperature falls as the distance from heating sintering temperature than can be achieved using a sin- elements 14 is increased. In FIG. 2 there are shown two tering atmosphere without the presence of water vapor gas inlets 18 and 19, with gas inlet 18 being located to- (e.g., dry carbon dioxide or dry hydrogen). ward the heating element 14 while gas inlet 19 is lo- The following is representative of the preliminary cated toward the door 17 which could be a lift door processing of the uranium oxide powder into pellet 10 opening into purge chamber 23. Door 24 is opened to form before it is introduced to the furnace, but this is enable removal of boats 11 from purge chamber 23. As in no way meant to be a limitation on the teaching of previously stated the atmosphere in the cooling zone this invention. The uranium oxide employed in the pro- can be an atmosphere in the range of 10-4 to 10"18 at- cess of this invention can be commercial grades of the mospheres of oxygen, with preferred gases being dry ceramic having up to 0.5% by weight fluoride ions or 15 hydrogen, wet hydrogen or a mixture of hydrogen and greater and varying oxygen to metal ratios. The ura- carbon dioxide within the range disclosed in this inven- nium oxide in powder form is pressed in a press at pres- tion. Where dry hydrogen or wet hydrogen is main- sures up to about 10,000 pounds per square inch into tained in the cooling zone, the hydrogen gas is fed into desired shapes, such as small cylinders, cubes, parallel- inlet 19 from a tank 25 with carbon dioxide being fed epipeds, etc. These desired shapes are then granulated 20 into the furnace in gas inlet 18 from tank 26, both gases in a granulator and screened through a screen having being fed into the furnace in amounts sufficient to ulti- openings in the range of about 5 to about 20 mesh. This mately form a mixture as disclosed in this invention. processing sequence increases the flow properties and Where a mixture of hydrogen and carbon dioxide is bulk density of the uranium oxide powder. The granu- maintained in the cooling zone, both hydrogen and car- lated powder is then pressed into desired shapes such 25 bon dioxide are fed into inlet 19 from tanks 25 (hydro- as small cylinders, cubes, parallelepipeds, etc., in a gen) and 27 (carbon dioxide) with inlet 18 being press with applied pressure up to 40,000 pounds per closed. If a different mixture of carbon dioxide and hy- square inch. Representative dimensions of cylindrical drogen is desired in the cooling zone than in the sinter- shapes of pellets made by this invention are pellets having zone, the cooling zone mixture is fed into inlet 19 ing a diameter of about one-half inch and a height of 30 from tank 25 (hydrogen) and tank 27 (carbon dioxide) about one-half inch. and the sintering zone mixture is fed into inlet 18 from The invention can be applied in a batch process tank 26 (carbon dioxide) and tank 28 (hydrogen). where the pellets of uranium oxide are placed in a suit- From the figure it can be seen that the sintering atmo- able container in a cold furnace, heated to temperature sphere flows in the direction opposite the direction the under the hydrogen-carbon dioxide atmosphere as de- 35 ceramic boats are being moved through the furnace. scribed in this invention and maintained at this temper- This insures maintaining the desired oxygen to metal ature for a period of from about 1 to about 5 hours fol- ratio and the removal of fluoride content from the sin- lowed by cooling the pellets in an atmosphere having tered uranium oxide structure. The sintering atmo- an oxygen partial pressure in the range of 10~4 to 10~18 sphere can flow in the same direction in which the ce- atomospheres of oxygen with a preferred value being in ramie boat is being moved if desired but a higher flow the range of 10~12 to 10~18 atmospheres of oxygen. rate of gas is preferred for this arrangement. The flow Another embodiment of the process of this invention rate of gas through an experimental tube furnace would as shown in FIG. 2 has placing of the uranium oxide be in the range of about 1 to about 10 cubic feet per pellets 12 in a series of ceramic boats 11 placed in a hour in a furnace having a heating chamber 22 of 1 !4 furnace generally represented by number 10 having a inches in diameter and 3 feet in length. gas outlet 16 going to an exhaust. The furnace has three The heating means of the furnace can be commer- zones of temperature as generally indicated in the cially available heating elements such as resistance drawing and designated a preheating zone, a sintering heating elements and induction coils. The furnace walls zone and a cooling zone. The boats 11 are pushed can be made of ceramic such as alumina or high tem- through the furnace 10 by introduction of additional perature metals such as high temperature stainless boats 11. Each boat 11 passes through door 21 to the steels. purge chamber 20 with door 21 then being closed fol- A stoichiometric uranium oxide product, for exam- lowed by lifting of door 13 so the boat can be pushed ple, a uranium dioxide product in which the ratio of ox- into furnace cavity 22 by a device such as a pushing rod j5 ygen atoms to metal atoms is substantially within the (not shown). The first region through which the boat range of about 1.98:1 to about 2.10:1 and preferably passes is a preheating zone which has an increasing about 2.00:1, may be produced by cooling the sintered temperature as the boat moves closer to heating ele- material in an atmosphere such as dry hydrogen, wet ments 14. In the preheating zone the atmosphere of the hydrogen or a mixture of carbon dioxide and hydrogen. furnace 10 is a mixture of hydrogen and carbon dioxide 60 It is possible to select a mixture of hydrogen and carbon as taught in the practice of this invention. It is thought dioxide such that preheating, sintering and cooling can that an appreciable amount of the fluoride and oxygen be carried out in the same atmosphere to achieve high to be removed from the pellets 12 is accomplished be- density and an oxygen to metal ratio of substantially fore the ceramic boat reaches the sintering zone. When about 1.98:1 to about 2.04:1. the boat 11 reaches the sintering zone, the temperature 65 The discovery of this invention has the advantage of is maintained in the range of 900° to 1,500°C during sintering uranium oxide structures at lower tempera- which a substantial proportion of the sintering prac- tures of about 900° to about 1,500°C, thus avoiding the ticed in this invention occurs. The atmosphere in the necessity for heating at temperatures in excess of 3,872,022 7 8 1,600°C when utilizing a wet hydrogen atmosphere for 2Vz hours and maintained at that temperature for 4 sintering uranium oxides. The present invention also hours in an atmosphere of 10% hydrogen by volume has the advantage of controlling the partial pressure of with the balance being carbon dioxide, this atmosphere oxygen in the furnace by controlling the relative pro- being maintained throughout the heating cycle. The portions of carbon dioxide and hydrogen maintained in 5 structures are then cooled in the same atmosphere to the furnace. room temperature giving a structure having an average Another feature of this invention is the range of pro- density of 95.7 percent of theoretical density, an oxy- cessing parameters enabling a person skilled in the art gen to uranium ratio of 2.012 and a fluoride ion con- great flexibility in selecting the precise operating patent of 2 parts per million, rameters depending on the properties desired for the 10 sintered uranium oxide containing composition. EXAMPLE 4 The sintering atmosphere disclosed in this invention A batch of uranium dioxide powder (fluoride ion is either comparable or cheaper in cost than gas atmo- content of about 80 parts per million) was mixed with spheres currently being utilized in commercial sintering an additive of 3 percent by weight gadolinium oxide of uranium oxide structures. The sintering atmosphere 15 and the powder are pressed into cylindrical pellets of of this invention enables the use of lower furnace tem- about 0.5 inches in diameter and 0.6 inches in height, peratures with lower operating costs for heating the These pellets were sintered at 1,425°C for 4 hours at furnace and gives a longer furnace life due to less cor- this temperature in an atmosphere of 20% hydrogen by ros.ve conditions associated with fluoride containing volume with the balance of the atmosphere being car- atmospheres at lower furnace temperatures. 20 bofi dioxide Thjs atmosphere was maintained through- The teaching of this invention and the methods by out the heatjng cycle and during cooling t0 room tem_ which it is to be performed will become apparent from perature. The sintered structure has an average density the following examples which are offered to be illustra- of 95% of theoretical density and a fluoride content of tive of the invention but are not to serve as a limitation 5 parts per million of the teaching of this invention. 25 ft ,g tQ fee understood that> a,though the invention has EXAMPLE 1 been described with specific reference to particular ,.,,,.,., , . embodiments thereof, it is not to be so limited, since A batch of cylindrical structures of compacted ura- ch afid alterations therein be made whjch are nium dioxide powder or about 0.5 inches in diameter .... ,, . . , , , • • .. , , , • , , in withi n the ffull intended scope of this invention as de- and 0.6 inches in height is placed in an alumina boat JU , , , , , . ,, , • L x J fined by the appended claims, and charged to a cold furnace. TThL e rfurnace is heated . , . , . - 1 inwr • u j • - • j • .u * What is claimed is: to 1,100 C in 2 hours and maintained at tha t tempera- J R J • J J L. a- c . , . , , ,. „„ , . r . 1. AA method ot producing dense, sintered bodies ot ture rfor 4 hours in an atmosphere of 10% by volume hy- . , • • • •, , ... ,, , , f • , a- a TU a particulate composition comprising uranium oxide, drogen with the balance being carbon dioxide. The v . . , r „ , . b , , same atmosphere is maintained throughout this heating 35 comprising the steps of heat,ng compacted bod.es ot cycle. The structures are then cooled in dry hydrogen the composition comprising uranium oxide, which to room temperature giving a structure having an aver- compos,tion can contain fluoride impurities, to a tern- age density of 95.0 percent of theoretical density and Perature ,n the ranSe °f about 900 t0 about, '50fC an oxygen to uranium ratio of 2.006. The initial fluo- m a sintering atmosphere consisting essentially of an ride ion content ofthe structures before sintering is 570 40 mitlal mlxture of carbon d,ox,de and hydrogen, either parts per million and the fluoride ion content ofthe sin- alone or Wlth an ,nert 8as' whlch m,xture reacts t0 «lve tered structures is 5 parts per million. the Presence of water vapor and carbon monoxide, sintering the bodies in said atmosphere at a temperature EXAMPLE 2 in the range of 900° to 1,500°C, wherein said initial Another batch of cylindrical structures of compacted 45 mixture of carbon dioxide and hydrogen is selected to uranium dioxide powder (about 0.5 inches in diameter 8lve a ratl° of oxygen to uranium atoms in the sintered and 0.6 inches in height) is placed in an alumina boat uranium oxide within the range of about 1.98:1 to and charged to a cold furnace. The furnace is heated about 2.10:1, and thereafter cooling the sintered body to 1,100°C in 2 hours and maintained at that tempera- in a cooling atmosphere having an oxygen partial pres- ture for 4 hours in an atmosphere of 5% by volume hy- 50 sure in the range of 10 4 to 10 18 atmospheres of oxy- drogen and 95% by volume carbon dioxide. The sin- 8en- tered pellets are then cooled to room temperature in 2. A method according to claim 1 in which the sinter- the same atmosphere giving a structure having an aver- ing atmosphere has from about 0.5 to about 90 percent age density of 97.2 percent of theoretical density, a hydrogen by volume and the balance is carbon dioxide, final oxygen to uranium ratio of 2.007, fluoride ion 3. A method according to claim 1 in which the sinter- content of six parts per million, entrapped gas in the ce- ing atmosphere has from about 2 to about 20 percent ramie of 3.7 microliters per gram and a grain size of hydrogen by volume and the balance is carbon dioxide, about 1 micron. Before sintering, the fluoride ion con- 4. A method according to claim 1 in which the ura- tent of the cylindrical structures was 134 parts per mil- 6Q nium oxide is uranium dioxide, lion and the oxygen to metal ratio was 2.085. 5. A method according to claim 1 in which the cooling atmosphere is a mixture of hydrogen and carbon di-

EXAMPLE 3 oxide having about 0,5 to about 90 percent hydrogen Another batch of cylindrical structures of compacted by volume and the balance is carbon dioxide, uranium dioxide powder (about 0.5 inches in diameter 65 6. A method according to claim 1 in which the cool- and 0.6 inches in height) having a fluoride ion content ing atmosphere is dry hydrogen, of 151 parts per million are placed in an alumina boat 7. A method according to claim 1 in which the cool- in a cold furnace. The furnace is heated to 1,300°C in ing atmosphere is wet hydrogen. 3,8' '2,022 9 10 8. A method according to claim 1 in which the cool- compacted bodies contain a mixture of uranium diox- ing atmosphere is dry carbon monoxide. ide and silicon dioxide, the silicon dioxide being pres- 9. A method according to claim 1 in which the cool- ent in an amount up to about 5 percent by weight. ing atmosphere is wet carbon monoxide. 15. The method according to claim 1 in which the 10. A method according to claim 1 in which the cool- 5 compacted bodies contain a mixture of uranium diox- ing atmosphere is an inert gas. ide and aluminum oxide, the aluminum oxide being 11. The method according to claim 1 in which the present in an amount up to about 5 percent by weight. compacted bodies contain a mixture of uranium diox- 16. The method according to claim 1 in which the hy- ide and plutonium dioxide, the plutonium dioxide being drogen in the sintering atmosphere is from dissociated present in an amount up to about 30 percent by weight. ammonia and the sintering atmosphere has a carrier gas 12. The method according to claim 1 in which the compacted bodies contain a mixture of uranium diox- of nitrogen. ide and gadolinium oxide, the gadolinium oxide being 17. A method according to claim 1 in which the sin- present in an amount up to about 15 percent by weight. tering atmosphere includes an inert gas. 13. The method according to claim 1 in which the ]5 18. A method according to claim 17 in which the compacted bodies contain a mixture of uranium diox- inert gas is argon. ide and titanium dioxide, the titanium dioxide being 19. A method according to claim 17 in which the present in an amount up to about 5 percent by weight. inert gas is nitrogen. 14. The method according to claim 1 in which the 20