CA2264613A1 - Thermal process for the conversion of uranium hexafluoride - Google Patents

Abstract

A single-step process for producing solid uranium oxide and gaseous HF from UF6 which comprises bringing together two gaseous reactant streams, one of said streams comprising UF6 optionally admixed with oxygen as O2, and the second reactant stream comprising a mixture of hydrogen as H2 or as a hydrogen-containing compound and oxygen as an oxygen-containing compound, said gaseous reactant streams being brought together at a temperature and composition such that the UF6 is converted rapidly by flame reaction into readily separable solid uranium oxide and a gaseous HF product.

Description

101520253035CA 02264613 1998-ll-13WO 97/45371 PCT/US96/19749THERMAL PROCESS FOR THE CONVERSIONOF URANIUM HEXAFLUORIDEField of the InventionThe present invention relates to a thermal processfor the conversion of uranium hexafluoride (UF5) to givesolid uranium oxide (essentially U02) and hydrogenfluoride (HF). The process represents an efficient wayof converting UF6 to U02 with recovery of the attendantfluorine as HF by a single-step procedure involving twogaseous feed streams.Related ApplicationThis is a continuation-in—part of U.S. applicationSerial No. 08/635,190, filed April 19, 1996, the contentsof which are incorporated herein by reference.Background to the InventionThe separation of uranium isotopes for defense needsand the preparation of commercial nuclear fuels havemainly been by processes that produce enriched anddepleted uranium (i.e., enriched or depleted in theuranium—235 isotope) as UF5. Much of the enriched UF6 isconverted to U02 by processes selected to give the ceramicsinterability needed for preparation of nuclear fuelpellets. The much larger amounts of depleted UF5 from theenrichment process are mostly stored as solid UF6 in steelcylinders.As of July, 1995, the U.S. Department of Energy hadover 600,000 MT of depleted UF6 (containing over 400,000MT of uranium) stored in approximately 66,000 cylinders.Long term strategies for management of this depleteduranium require conversion of the UF6 to uranium oxides.However, while procedures for converting UPS to uranium1015202530CA 02264613 1998-ll-13W0 97/45371 PCT/US96/19749oxides are known, the currently available procedures arenot particularly efficient or economical for convertingdepleted UF6 to solid uranium oxides, notably UOWsuitable for disposal, storage, or further use. Morespecifically, the UF6 conversions for nuclear fuels weredeveloped to prepare U02 with well controlled ceramicproperties and are not optimum for the much larger—scaleconversions of depleted uranium. Furthermore, because ofthe need to control their ceramic properties and thethermodynamic limitations, the known commercialconversion processes are complex with multiple processstages and include the formation of intermediate solidssuch as UOJQ or UF4. Additionally, the fluorine by-products of these conversion processes are usuallyradioactive wastes with high disposal costs.Uranium oxides are thermally and chemically stable,non—volati1e and essentially insoluble in rain and groundwater, and are the preferred compositions of uranium forlong term storage or disposal. The most stable oxide inthe environment is U§%, but other oxides ranging from U02to U45 and U03 and combinations of oxides are alsoacceptable products. Uranium and fluorine are veryreactive elements chemically and, the conversion of UF6 touranium oxides also produces a fluorine compound as aproduct. While some fluorine compounds have littlecommercial value and represent waste products withconsequent disposal costs, hydrogen fluoride is avaluable commercial chemical with many uses to provide amarket for the fluorine in the depleted UP?The conversion of UF6 into uranium oxides and HFrequires reaction with oxygen as 02 or compoundscontaining oxygen and with hydrogen as H2 or compoundscontaining hydrogen. One process used to prepare nuclear210202530CA 02264613 1998-ll-13W0 97/45371 PCT/US96/19749fuel U02 from enriched UF6 uses gross excesses of watervapor and H2 in two steps with the two principal reactionsbeing:UF6(g) + 2}gO(g) => UO2F2(s) + 4HF(g) (1)andUO2F2(s) + H2(g) => UO2(s) + 2HF(g) (2)where (g) and (s) represent, respectively, gas and solid.The preferred sequence is to perform reaction (1) atabout 250°C followed by reaction (2) at about 650°C.The complete conversion of the uranium solids to UO2as represented by reaction (2) requires good contact ofthe solids and gas over a relatively long period of time,on the order of an hour or more. Other known conversionprocesses are precipitation of ammonium diuranate orammonium uranyl carbonate from reaction of UF5 withaqueous solutions. All of these processes requiremultiple steps and give dilute aqueous fluoride solutionsof little or no value as the fluoride product.Nevertheless, these conversion reactions are chosenbecause they give the resulting uranium oxide the ceramicproperties needed for fabrication of nuclear fuels.Numerous U.S. patents have been issued directedtowards processes for the conversion of UF5 to uraniumoxides. See, for example, U.S. patent 4,830,841 and theU.S. patents listed therein which describe procedures forconverting UPS to uranium dioxide in furnaces, rotarykilns, fluidized beds or the like. As representative ofsuch art, it is noted that U.S. patent 4,830,841 itselfis concerned with a process for preparing UO2 from UF6 byreacting UF6 with steam to produce submicron uranylfluoride powder, fluidizing a bed of uranium oxide31015202530CA 02264613 1998-ll-13W0 97/45371 PCT/US96/19749 ._material with a mixture of steam, hydrogen and inert gasat about 580°C to about 700°C, and introducing thesubmicron uranyl fluoride powder into the fluidized bedof uranium oxide material so that the uranyl fluoridepowder is agglomerated, densified, fluidized,defluorinated and reduced to a fluoride—containinguranium oxide material which is removed from thefluidized bed and then contacted with hydrogen and steamat elevated temperature to obtain U02 essentially free offluoride.Another U.S. patent 3,260,575 describes thepreparation of ceramic U02 fuel material by a single—stepprocess comprising the reaction of UPS with astoichiometric excess, generally at least 1.5 times thestoichiometric amount, and preferably much larger (e.g.,30-40 times the stoichiometric amount) of a gaseousmixture of hydrogen and oxygen—bearing gas at atemperature above llO0°C and a pressure not exceeding 20torr., i.e. 20 mm Hg absolute. The patent specifies thata temperature of at least llO0°C is required to avoid theformation of UF4 and that a pressure less than 20 torr iscritical, fluoride intermediates being produced alongwith U02 at higher pressures.Another process directed towards the recovery ofanhydrous hydrogen fluoride from UF6 gas is disclosed inU.S. patent 5,346,684 (equivalent to EP 529768 Al). Thatprocess involves reacting UF6 in a primary reactor withsteam to produce a uranyl fluoride intermediate and agaseous mixture of hydrogen fluoride and water. Theuranyl fluoride is then fed to a secondary reactor andreacted with water to produce a Ugx product for disposaland a gaseous mixture of water, HF and oxygen. Thegaseous mixtures from the two reactors are combined and41015202530CA 02264613 1998-ll-13WO 97/45371 PCT/US96ll9749then distilled to obtain an anhydrous HF product. Anazeotrope of water and HF is vaporized and returned tothe primary reactor.Another prior process for converting UF6 to uraniumoxides involves feeding the UPS into a molten metal bathwhere the UF6 is broken down into recoverable componentsincluding uranium oxide and HF.Notwithstanding the extensive prior efforts referredto above, there remains a substantial need for improvedprocedures for converting UF6, particularly depleted UFWinto solid U02 in a form suitable for storage, disposal oruse. The primary object of the invention is to providesuch a process. Other objects, including, for example,the provision of HF in concentrated aqueous solution orin other highly useful form, such as anhydrous HF, willalso be evident from the description of the inventionwhich follows.Summarv of the InventionThe invention provides a single—step process forefficiently converting UF5 into solid U02 and gaseous orcondensed phase HF. The process involves bringingtogether two gaseous reactant streams, one of the streamscomprising UF5 optionally admixed with oxygen as 02, andthe second reactant stream comprising a mixture ofhydrogen as H2 or as a hydrogen—containing compound andoxygen as an oxygen-containing compound, the gaseousreactant streams being brought together at a temperature,pressure and composition such that the UF6 is convertedrapidly by flame reaction into readily separable soliduranium oxide (essentially U02) and a gaseous HF product.While the composition of the two gaseous reactant streamscan be varied, as discussed hereafter, care should be51015202530CA 02264613 1998-ll-13wo 97/45371 PCT/US96/19749taken to avoid using mixtures which might create anexplosive potential. For example, the second reactantstream preferably does not include the combination of H2and 02 because of the possibility of an explosion.The present process is primarily intended for usewith depleted UF6. However, the process can also be usedwith natural assay or enriched UF6. In either case, asolid oxide consisting essentially of U02 and a gaseous HFproduct are obtained. These are readily separated fromeach other with the gaseous HF product being condensed toprovide a highly concentrated aqueous HF solution oranhydrous HF. This fluoride product can be used directlyby the chemical industry for the manufacture of fluorine(F2) or replacement refrigerants (e.g., non-chlorofluorocarbons) or by the uranium industry for themanufacture of UF4 and UF6. The solid UO2, which isreadily collectable, is easily recovered for storage oruse.The process is not sensitive to the pressure of thereactants and the operating pressure of the system can bevaried to accommodate sizing and throughput requirements.However, it is particularly important to the usefulnessof the process that the gaseous reactant streams arebrought together and reacted at a pressure which isessentially atmospheric or above, although in somecircumstances, it may be desirable to operate at apressure slightly below atmospheric.An important distinguishing feature of the presentprocess over prior procedures is that it does not involvethe use of fluidized beds, molten metal or the like toobtain the desired products. In essence, the presentprocess simply involves bringing the two gaseous reactantstreams together so that a flame reaction occurs and61015202530CA 02264613 1998-ll-13WO 97/45371 PCT/US96/129749 _collects the resultant solid oxide product and gaseous HFproduct.Description of Preferred EmbodimentsPreferably the feed stream comprising UF6 contains atleast part of the oxygen needed for the reaction, thebalance, if any, of the oxygen being included in thesecond feed stream. The oxygen in the UF6 feed streamshould only be in the form of O2 to avoid the potentialfor premature reactions. It is also preferred that theUF6 feed stream does not contain H2.The hydrogen in the second reactant stream can be inthe form of H2 or as a hydrogen-containing compound suchas Ego, NH3 and/or CH4. Preferably all of the hydrogenneeded for the reaction is in the second feed stream.Desirably, the second feed stream comprises a mixture ofH2 and H50 or a mixture of H2 and CO2.While the composition of each gaseous reactantstream can be varied, care must be taken in preparingeach stream to avoid the possibility of creating anexplosion potential or premature reaction. For example,undesirable mixtures include H2 and O2, UF6 and H50, UPSand H2, NH, and 02, CH, and 02. Typical useful mixturesinclude, without limitation thereto, UF6 and 02; (H2and/or NH3) and H53, (NH3 and/or H2) and CO2; and CH, andH20.One or both feed streams may also include an inertgas such as argon or nitrogen.The reaction can be carried out over a relativelywide temperature range such as between 900°C to l500°C orhigher, e.g. up to 2000°C, with a temperature of aroundllOO°C being generally preferred. The reactiontemperature or, stated otherwise, the temperature of the71015202530CA 02264613 1998-ll-13W0 97/45371 PCT/US96/19749 ._ .reaction product, can be controlled by varying the amountof oxygen in the UF6 feed stream and the flow rate of thegases used for the reaction.The reaction can be carried out over a relativelybroad pressure range such as between atmospheric and 250psia. It may be useful in some cases to operate atslightly below atmospheric, e.g. at about to to 13 psia,and possibly as low as 5 psia, but atmospheric oressentially atmospheric pressure is preferred. If atemperature in excess of 2000°C is used, the pressure ispreferably decreased below atmospheric to minimize theformation of undesired by-products (e.g. UFJ.The ratio of hydrogen and oxygen to UF6 can also bevaried to control the amount of water vapor in thegaseous HF product. However, generally speaking, theamounts of oxygen and hydrogen in the feed gases, i.e.,the total amount of oxygen and hydrogen used in theprocess, should be less than about 1.5 times thestoichiometric amounts needed for reaction with the UF6 toform U02 and HF. Preferably the hydrogen and oxygen areused near (i.e., around 10-30% in excess of) thestoichiometric amounts.A particularly unique feature of the invention isthe finding that the desired reaction to provide U02 andHF can be accomplished in a very short period of time.Typical reaction times are on the order of fractions of asecond although times in excess of this can be used.Advantageously the process is carried out using aclosed crucible or reactor made of suitable refractorymaterial, e.g. graphite or alumina. High melting pointnickel and nickel alloys that form stable metal fluoridescan also be used. The gaseous reactant streams are fedseparately to the reactor. Within the specified810202530CA 02264613 1998-ll-13wo 97/45371 PCT/US96/19749 _temperature range of the invention, the reactant streamsreact essentially spontaneously on contact within thereactor to provide solid U02 and gaseous HP. The U02 canbe collected from the bottom of the reactor or filteredfrom the reactor off-gas while the HF may be withdrawnfrom multiple points, e.g. reactor top or side, and, ifdesired, condensed.Advantageously, the reactant streams are fed intothe reactor to obtain intimate mixing of the reactantssuch as through a concentric tube arrangement, with theUF6 gas being fed through, for example, the inner tube andthe other gaseous reactant stream being fed through anouter tube. The desired reaction occurs at the exit ordischarge ends of the concentric tubes where the reactantstreams come together. Alternatively, the gaseousreactant streams can be fed to separate inlets of anozzle arrangement rather than using concentric tubes,the reactant streams being brought together at or justbefore the nozzle discharge. In either case, one or bothof the reactant streams can be preheated, for example, to700—1000°C. The reaction does not require theapplication of additional heat as the overall reaction isan exothermic one. With appropriate selection of thecomposition of the reactant gas streams, an equilibriumreaction temperature of around 900-2000°C is readilyattained. Preheating of the reactant gas streams isgenerally desirable, particularly if oxygen is notincluded in the UF6 reactant gas.A suitable way of practicing the invention comprisesthe use of a burner designed to receive UF6 in one feedstream which may also contain 02 and another separate feedstream comprising'}gO or CO2 which also contains theprincipal source of hydrogen as H2, NH3 or CH4. The O2 and91015202530CA 02264613 1998-ll-13WO 97145371 PCT/US96/19749 _ .Ego and/or other oxygen-containing compounds togethershould preferably supply a slight excess, e.g. 10 to 20percent over the stoichiometric amount, of oxygen overthat needed to form U02. The Hg) and/or other hydrogen-containing compounds together should also supply a slightexcess of hydrogen over that needed to form HF. Theexcess of oxygen forms CO if carbon is present and H53 ifexcess hydrogen is present. While a stoichiometricexcess of hydrogen and/or oxygen is generally preferred,it is recognized that sub-stoichiometric amounts ofhydrogen and/or oxygen can also be used to provide asomewhat different uranium oxide product distribution.As noted earlier, the oxygen feed rate (when 02 isadmixed with UF6) is used to control the temperature ofthe products of reaction. Without oxygen feed, the gasesshould preferably be preheated to control the temperatureof the products of reaction in the desired rangespecified for the present process, i.e. 900—2000°C orhigher.Brief Description of the DrawingsThe invention is further described by reference tothe accompanying drawings wherein:Fig. 1 is a flow diagram showing one arrangement forpracticing the present process;Fig. 2 graphically shows equilibrium compositionsversus product temperature using a reaction arrangementas in Fig. 1;Fig. 3 diagrammatically shows in more detail one wayof supplying the reactant gas streams to the reactionzone;Fig. 4 shows an alternative flow diagram for thepractice of the invention; and10l0202530W0 97/45371CA 02264613 1998-ll-13PCT/US96/19749Fig. 5 diagrammatically illustrates anotherarrangement for bringing the reactant streams togetherfor reaction.with more specific reference to Fig. 1, two gaseousreactant streams (1) and (2) are fed continuously to alance or burner (3) comprising a pair of concentric tubes(4) and (5) positioned within the reaction vessel (6).A pre—heated gaseous mixture of UPS and O,optionally including an inert gas such as N2 or argon, isfed through the inner tube (4) while a pre—heated gaseousmixture of H2, NH3 and/or CH4 and H50 or CO2, optionallywith inert gas such as N; or argon, is fed through theouter tube (5). Optionally, gaseous feeds are heated inthe lance or burner (3) by an external heating source onvessel (6). The two gaseous streams mix and essentiallysimultaneously react at the discharge ends of the tubesto form U02 and HF without any significant accumulation ofintermediate uranium compounds as solids. The reactionproducts discharge into the lower portion of vessel (6)which is preferably provided with means to facilitateseparate discharge of gases and solids. The uraniumoxides are recovered at (7) as dry, granular solidssuitable for known methods of storage, disposal or use.The gaseous products collected at (8) are filtered (notshown) to remove entrained solids and cooled in condenser(9) to allow separation of condensed HF and residual }go(10). By appropriate selection of the composition of thereactant streams, the amount of water formed in thereaction can be controlled to provide the desiredconcentration of HF solution resulting from thecondensation at (10). The resulting condensed producthas many commercial uses as collected in the form ofhighly concentrated aqueous HF or essentially anhydrousll10202530CA 02264613 1998-ll-13wo 97/45371 PCT/US96/19749 _ .HF. The non-condensable gases (11) may be partlyrecycled to feed (1) or (2) or they may be totallydischarged from the system for use or disposal.Figure 2 is discussed hereafter in connection withExample 2. The figure, as earlier noted, shows that theoptimum results, with respect to the production of U02 andHF at atmospheric pressure, are realized at anequilibrium temperature in the range of 900°C to 2000°C,particularly around llOO°C.Figure 3 shows a feed apparatus for introducing apre—heated feed gas containing UF6 and a second pre—heatedfeed gas containing compounds of hydrogen and oxygen.The UF6 feed (12) flows through an inner tube (14)selected to be corrosion resistant for UF5 and O2 and forall the other gaseous feeds. The other gaseous feedmixture (13) flows through the annulus between the innertube and a second, larger tube (15) selected to becorrosion resistant for HF and for the gaseous feedmixtures. A145, nickel, and nickel alloy tubes can beused when they are passivated by a film of solidfluorides. Alternatively, other fluoride compatiblematerials, such as CaF2 and lanthanum hexaboride, may alsobe used.A third, large, graphite tube (not shown) can beplaced around the tube (15) so as to provide for an inertgas flow to prevent excessive temperatures and, moreimportantly, avoid corrosion of the outer surface of tube(15). The UF6 feed (12) includes a controlled supply ofO2 to regulate the reaction temperature and, optionally,N2, argon or other inert gas. The feed (13), as shown,comprises H2, NH2 or CH4 and.}gO or CO2 as the hydrogen andoxygen sources. Optionally, N2, argon or other inert gasmay also be included in feed (13). As in the case of the1210202530CA 02264613 1998-ll-13W0 97/45371 PCT/US96/I9-749 1system shown in Figure 1, the pre—heated gases issuing atthe exits of tubes (14) and (15) flame react essentiallysimultaneously as they come in contact (see "ReactionZone") to form the desired solid U02 and gaseous HF whileleaving Hg) and H2 as excess reactants and inerts (Ar, N5)in the embodiment as shown.Figure 4 shows an arrangement which is useful toavoid excessive temperatures or larger heat fluxes. Inthis embodiment, the pre—heated oxidizing feed gas (16)containing UF6, O2 and N2 or argon and the pre—heatedreducing feed gas (17) comprising a mixture of H2, NH3 orCH4;IgO or CO2; and N; or argon, flow through a lance orburner (18) to mix and react at the lance or burner tip18; inside reactor vessel (20). Part or all of the soliduranium oxide product accumulates at (23) for periodicdischarge from the reactor vessel (20). Gaseous productsand any entrained uranium oxide solids exit continuouslyat (22) into a suitable separating system (not shown).The reactor temperature may be controlled by acombination of means for heating or cooling the reactorby jacket (21), control of the amount of O2 in the feedstream (16), and pre—heat of the feed streams (16) and(17). Preheating the reactor by operation of the reactoras a burner without UF6 and use of an ignition device (19)may also be employed as desired or if necessary toinitiate the process. Generally speaking, thetemperature and composition of the feed streams are suchthat when the two streams are brought together, a flamereaction occurs. If desired, however, the igniter (19)may be positioned at the point where the gaseous streamscome in contact to assist in the initiation of thereaction, although this is not usually necessary,particularly if the gaseous streams have been preheated.131015202530CA 02264613 1998-ll-13W0 97/45371 PCT/US96/ 19749 -Figure 5 shows an alternative to the concentric tubearrangement of Figure 3. Thus, in Figure 5, the reactantstreams (24) and (25) are separately supplied throughinlets (26) in nozzle means (28), the gases coming intocontact adjacent the nozzle discharge where they react toform U02 and HF as in the Figure 3 arrangement.The invention is illustrated by the followingexamples:Example 1An experiment was done with gas feeds to a feedapparatus generally as described in Fig. 1 except thatthe feed arrangement into the reactor comprised threeconcentric tubes rather than two as shown. A mixture ofUF5 at a rate of 250 g/hour and Ar was fed to an innerAlgg tube. A mixture of water vapor at about 31 g/hour,Pg at 19 std liters/hour and Ar was fed into the annulusbetween the inner tube and a middle A145 tube. Ar wasfed into the annulus between the middle tube and an outergraphite tube. The assembly of tubes discharged into thereactor chamber at about 950°C. The uranium collected assolids beyond the tip of the inner tube with anestimated 0.1 second residence time between the end ofthe inner tube and the point of deposition of the uraniumproduct. Analyses of the deposited solids by X-raydiffraction showed essentially pure U02 without detectableamounts of fluoride compounds.Example 2Thermochemical calculations were made using acomputer program [HSC Chemistry for Windows 1.2,Outokumpu Research OY, FORI, Finland] to determine theequilibrium compositions of products and the adiabatic141015202530CA 02264613 1998-ll-13wo 97/45371 PCT/US96/19749 _temperature for reaction without heat transfer. Theequilibrium compositions versus temperature at essen-tially atmospheric pressure (1.0 Bar) are shown in Figure2 for reaction of 1.0 mole of UF6, 0.5 moles of 02, 1.4moles of }go, 2.2. moles of H2 and 2 moles of Ar atatmospheric pressure. The equilibrium composition atll0O°C is more than 99% conversion of the UPS to U02 andHF. With the feed gases pre—heated to 100°C, the producttemperature would be llO8°C. The same equilibriumcomposition shown in Figure 2 would also result if thesame total molar amounts of hydrogen and oxygen were fedusing other combinations of H, Ego and 02. However, theadiabatic temperatures would be less than 500°C for no 02(i.e., all the oxygen as H53) and over 2000°C for no H53(i.e., all the oxygen as 02). This illustrates how the Icomposition of the products and the temperature withoutheat flux can be controlled separately.An important feature of the invention is that the UF6conversion reaction is completed quickly within a veryshort time and distance after the pre—heated UF6 gasstream is mixed with the other reactant gas stream, e.g.a gas stream comprising Hg) and H2, possibly with an inertgas such as argon. Typically, the residence or reactiontime is in the order of fractions of seconds, forexample, 0.1 second using a total gas velocity at the tipof the feed lance or burner of about 3.5 feet per second.Without intending to be limited to any particulartheory of operation, the invention appears to be based ona number of important factors. These include the findingthat, using particular reaction conditions andprocedures, uranium oxide solids can be produced from avery rapid, one-step process for conversion of UFVAnother important finding is that, by using the151015202530CA 02264613 1998-ll-13wo 97/45371 PCT/US96/19749 _conditions described herein, and the indicated feedconfigurations, for example, one pre-heated gaseousstream comprising a mixture of H53 and H2 and another pre-heated feed stream of UPS, with or without oxygen andinert gas, conversion of UPS to essentially U02 solids canbe obtained without significant accumulation of UFNUOZFZ, or other uranium fluoride solids as intermediatecompounds. Another important finding is that theadiabatic product temperature can be controlled bycontrolling the temperatures and compositions of thefeeds.It is known that UF6 gas can be reacted with eitherPg or'}gO gas to give useful conversions as follows:UFs(g) + H2(g) —> 2HF(g) + UF.,(s’) (1)UF6(g) + 2H2O(g) -> 4HF(g) + UO2F2(s) (2)The first reaction is used as the initial step forpreparation of enriched uranium metal from enriched UFVThe UOJQ from reaction (2) is not directly a usefulproduct but can be an intermediate for conversion tocommercial nuclear fuels by reaction (3):UO2F2(s) + H2(g) —> 2HF(g) + UO2(s) (3)This conversion reaction of a solid with a gas ismore difficult than reactions (1) or (2) where onlygaseous reactants are involved. Reaction (3) commonlyrequires long times with good mixing of solids with anexcess of H2. Reaction (1) might be used in combinationwith reaction (4) to prepare U02 from U?“UF4(s) + 2}gO(g) -> 4HF(g) + UO2(S) (4)161015202530CA 02264613 1998-ll-13W0 97/45371 PCT/US96/19749 ,_ y_Reaction (4) is much less practical than reaction(3) because it is less favorable thermodynamically andthe UF4 from reaction (1) is a fused solid of low surfacearea.According to the present invention, the reaction ofan Hflfigo gas mixture with UF6gas at the specifiedconditions and feed compositions results in conversion ofthe UPS in a single process step to essentially uraniumoxide U02 without formation of significant amounts ofintermediate uranium compounds. The overall conversionmay be represented by reaction (5):UF5(g) + 2H§O(g) + H2(g) —> 6HF(g) + UO2(s) (5)While reactions (1) or (2) (or other reactions)might occur on a molecular scale, the mixing of gasesHyflgo and UPS, or modifications thereof according to theinvention, apparently allows reactions (3) or (4) tocomplete the conversion to uranium oxides while theintermediate products are still in a very finely divided(nearly molecular) state.Thermochemical calculations can be used to identifythe limiting requirements for the foregoing reactions.The equilibrium compositions were calculated for eachuranium reactant and a ten percent excess of the otherreactants. The results are given below as percentageconversions of the uranium feed over the most favorablerange of temperature at atmospheric pressure. Theadiabatic product temperatures for these feeds pre-heatedat 700°C were calculated as an indication of the heatingor cooling requirements.17101520253035CA 02264613 1998-ll-13W0 97/45371 PCT/US96/19749 ..ResultingAdiabatic_Reaction No. Equilibrium Conversion Product Temp. °C(for products in the indicated for feeds attemperature range) 700°C)1 ~lO0% for O to 2500°C 18852 >99% for 0 to 750°C 10583 >98% for 850 to l300°C 6394 >98% for 850 to l400°C -2735 >99% for 900 to l300°C 1000These thermochemical calculation results show thatthe reaction of UF6(g) with a mixture of H2(g) and HgD(g)(reaction (5)) provides a well—controlled conversion witha good yield of U02 and HF. The equilibrium conversionsfor reaction (5) are good over a 900 to 2000°C range oftemperatures. Reactants preheated to a reasonabletemperature (700°C) give products within this temperaturerange without reactor heat transfer for control.Additional excesses of H2 or Hg) feeds can be favorable tomore complete conversions. The two products are solid(U02) and gaseous (HF) over wide temperature ranges, thusallowing simple physical separation of the products.The four reactions (1) to (4) are much lessfavorable in one or more respects than reaction (5).Reaction (1) is very exothermic and will typically giveUF4 as a vapor or liquid requiring cooling to prepare theUF4(s) for subsequent reaction to U02 by reaction (4) in asecond reactor. Reaction (4) is extremely endothermicand will require large heat inputs to hold the favorablerange of temperatures. Reactions (2) and (3) are mildlvexothermic and endothermic, respectively. But reactior(2) requires limiting the temperature to avoid thermaldecomposition of the Uofig product. Reaction (3) must be18SUBSTITUTE SHEET (RULE 26)1015202530CA 02264613 1998-ll-13W0 97/45371 PCT/US96/19749carried out with a large excess of H2 to complete thereaction without thermal decomposition of the UOJQ feed.Only reaction (5) can be carried out to give a solidproduct and high conversions without large heat fluxes orlarge excesses of rho or H2 feeds.For the conversion of UF6 by reaction (5), all theoxygen may be supplied asIgO(g). Thermochemicalcalculations indicate that the feeds are preferablypreheated to 700°C so the products can be at an optimumtemperature for complete conversion without requiringheat transfer from the reaction zone. The sameconversion products could be formed with oxygen suppliedby O2(g) as shown by reaction (6):UF6(g) + O2(g) + 3H2(g) -——> 6HF(g> + UO2(s) (6)Reaction (6) is very exothermic and feeds at 25°Cresult in adiabatic product temperatures over 2000°C.This high temperature is undesirable from a practicalviewpoint and is less favorable thermodynamicallyprimarily because UF4(g) becomes a significant product.The most practical conversion of UF5(g) to uraniumoxides and HF involves supplying oxygen as both O2(g) and}gO(g) with the proportions controlled to control theproduct temperature. The reaction for half of the oxygenfron1}gO(g) and half from O2(g) is:UF5(g) + 0.502(9) + H,o(g) + 21-12(9) ---> 6HF(g) + UO2(s) (7)Since large heat fluxes are not needed andtemperatures are limited, the design of the equipment issimplified and a relatively wide variety of materials ofconstruction are acceptable for use. Thus, while ceramicl9101520253035 CA 02264613 1998-ll-13wo 97/45371 PCT/US96/19749 .. .materials such as alumina or graphite are generallypreferred, the invention permits operations wherein thereactor wall may be substantially cooler than thereaction temperature, thus allowing the use of metalssuch as nickel or Monel. Resistance heating elements onthe reactor wall may also be adequate. Alternatively, aninduction heating system may be used to heat the reactor.Adiabatic product temperatures were calculated usingten percent excesses of both oxygen and hydrogen withfeeds at 25°C and the division of oxygen supplied betweenO2(g) and H5D(g) as a variable. The results are:Adiabatic ProductTemperature for productsUF6 O2 }gO H2 of 6HF(g)+UO2+O.2H2O(g)Moles Moles Moles Moles +O.1H () O . . 370°C0.15 . . 637°C0.30 . . 891°C0.45 . . ll46°C.60 . . l389°C.75 . . l63l°C.10 . 2l43°CThe 1l46°C for UF5:O2flgO molar ratio of l:O.45:l.3appears to be near the middle of the optimum temperaturerange. The amount of 02 may be controlled from atemperature measurement. Less 02 is needed if the feedsare preheated above 25°C.Only two gaseous feed streams are needed since Hg)and H2 do not react with each other and UF6 and 02 do notreact below l0OO°C. A large excess of H2 (e.g. greaterthan 50% theoretical) may be acceptable as the excess canbe recycled after condensation of the HF product. The20SUBSTITUTE SHEET (RULE 26)1015202530CA 02264613 1998-ll-13WO 97/45371 PCT/US96/19749 ,. .excess of 02 should be well—controlled and small (up to,for example, 20% excess), particularly if an anhydrous HFproduct is desired to minimize the amount of water in theHF.As will be evident from the foregoing, the processof the invention comprises reaction of two gaseous feedmixtures to give a one-step, efficient conversion of UF6into uranium oxide and HF. The gaseous feed compositionsand temperatures are controlled to give the optimumcompositions of uranium oxide and of gaseous products forrecovery of condensed HF as a chemical of significantcommercial value. In a preferred embodiment, one gaseousfeed comprises UF6, e.g. depleted UF6, together with inertgas and/or part of the oxygen needed for the conversion.The other gaseous feed comprises hydrogen as H” Ego, NH3or CH, and all or part of the required oxygen as HgD orCO2. Increasing the fraction of oxygen supplied as 02will generally increase the reaction temperature or thetemperature of the products after reaction. On the otherhand, increasing the fraction of oxygen supplied as Ego orCO2 will generally decrease the temperature of the productafter reaction. The gaseous feeds may also be preheatedbefore mixing to increase the temperature of the productsafter reaction. This control of temperature is importantto simplify the design of process equipment. Avoidingexcessive temperatures allows the use of nickel or othermetals instead of ceramics. The control of temperatureswithout need for large heat fluxes at the reaction zoneis also an important advantage or simplification.Specific features of the invention include thefollowing:211015202530CA 02264613 1998-ll-13W0 97/45371 PCT/US96/19749(1) The process provides uranium oxides from UF6which have useful properties sufficient to meet therequirements for storage, disposal or use of uranium.(2) The process provides a one-step conversion ofUF6 into uranium oxide and HP. The UF6 starting materialand the other reactants are fed into a reaction zone togive readily separable solid and gaseous products.(3) The process involves the feed of two or moregaseous feeds which are separately stable withcompositions and temperatures that give a favorablethermodynamic equilibrium composition and temperaturewith little or no heat transfer during the reaction. Onegaseous feed is the UF6 with varying amounts of O2 and/orinert diluent gas, such as Ar. The other gaseous feedincludes one or more sources of hydrogen (H2, NH, Igo orCH4) and one or more sources of oxygen Ugo, cog. Thecomposition of each gaseous feed stream, the ratios ofthe two feed streams, and the temperatures of the feedscan be controlled and varied to provide the favorablethermodynamic equilibrium compositions and temperatures.(4) The preferred reaction product temperatures forconversion of UF6 into uranium oxides and HF are mostcommonly in the range of 900° to 2000°C.(5) The preferred reaction pressure for conversionof UF5 into uranium oxide and HF is commonly in the rangeof atmospheric, but elevated pressures can beaccommodated.(6) The process results in a gaseous productcomposition that can be controlled to allow recovery ofHF for commercial use.(7) The reaction temperature may be controlled bycontrolling the fractions of oxygen fed as 02 and fed as}go or CO2. Increasing the fraction added as O2 increases221015202530CA 02264613 1998-ll-13WO 97/45371 PCT/U S96/ 19749 ,.the temperature of the reaction products; increasing thefraction fed as PQO or CO2 decreases the temperature.This control of temperature prevents excessivetemperatures, minimizes or eliminates the need for heattransfer to and from the reaction zone and provides anoptimum temperature for obtaining the compositions ofproducts desired.(8) The product compositions can be controlled andvaried by varying the proportions of total oxygen to UF5and total hydrogen to UF5 in the feeds. Since the oxygenin the feed can be from O2 and H53 or CO2 and the hydrogencan be from H2 and Hg), the control of oxygen/UF6 andhydrogen/UF6 ratios can be independent of the control ofreaction temperature as described in (7).(9) Control of temperature can also be accomplishedby a controlled preheating of the gaseous feeds. Forexample, feeds of UF6 (without 02) and H2-+IgO can bepreheated to about 700°C to give an adiabatic reactiontemperature within the preferred range of temperatures.(10) The feed apparatus and procedures allow thegaseous feeds to be fed and reacted directly to finalproduct without excessive corrosion or erosion by thefeeds and without handling and plugging problems causedby bulk solids of uranium intermediates such as Uogg andUF4. The conversion of UF6 to uranium oxides is completedwithout need for a mixing of a gaseous feed with uraniumsolids of an intermediate composition (e.g. UOJQ or UFQ.The mixing and reaction of a gas with bulk solids iscommonly slower and more difficult than the mixing andreaction of gases. The elimination of such a processoperation is a major difference between the presentinvention and prior art procedures.2310152025CA 02264613 1998-ll-13wo 97/45371 PCT/U S96/ 19749 ,.(11) The two principal products in the presentprocess are a solid (uranium oxide, primarily as U02) ofvery low volatility and gaseous HF which can be condensedwhen cooled. This allows easy and efficient separationof the uranium oxide product and the HF product by simplephysical separation (e.g., filtration).(12) The control of reaction temperatures and heatfluxes by controlling feed composition (morespecifically, the oxygen content of the UF6 feed) greatlysimplifies the design of the process equipment. Somematerials of construction and temperature controlprocedures are more practical for the process describedthan for highly exothermic or highly endothermic processreactions. Mildly exothermic process reactions might becontained by cooled metal reactor walls without the needfor ceramics. Low heat fluxes through the reactor wallsallow simpler heat transfer and temperature controldesigns.(13) The process may be carried out continuouslyunder substantially adiabatic conditions with the oxidesolids and gaseous HF periodically or continuouslyremoved from the reactor.It will be appreciated that various modificationsmay be made in the invention as described above withoutdeviating from the spirit and scope thereof as defined inthe following claims wherein:24

Claims (24)

WHAT IS CLAIMED IS:

1. A single-step process for producing solid uranium oxide and gaseous HF from UF6 which comprises bringing together two gaseous reactant streams, one of said streams comprising UF6 optionally admixed with oxygen as O2, and the second reactant stream comprising a mixture of hydrogen as H2 or as a hydrogen-containing compound and oxygen as an oxygen-containing compound, said gaseous reactant streams being brought together at a temperature and composition such that the UF6 is converted rapidly by flame reaction into readily separable solid uranium oxide and a gaseous HF product.

2. The process of claim 1 wherein the reaction is carried out at essentially atmospheric pressure.

3. The process of claim 1 wherein the UF6 is depleted UF6 and the uranium oxide product comprises UO2.

4. The process of claim 1 wherein the solid uranium oxide is separated from the gaseous HF product and the gaseous HF product is condensed to recover HF.

5. The process of claim 1 wherein the gaseous feed streams provide a reaction product having a temperature within the range of 900°C to 2000°C.

6. The process of claim 5 wherein the gaseous feed streams are preheated to a temperature sufficient to yield products in the temperature range of 900°C to 2000°C.

7. The process of claim 1 or claim 6 wherein the reactor is heated by external means to a temperature sufficient to yield products in the temperature range of 900°C to 2000°C.

8. The process of claim 1 wherein the gaseous feed stream comprises UF6 and O2 and the temperature of the reaction product is controlled by controlling the amount of O2 in said UF6 feed stream.

9. The process of claim 1 wherein the ratio of hydrogen or oxygen to UF6 is regulated to control the amount of H2O vapor in the gaseous HF product.

10. The process of claim 1 wherein one or both of the feed streams also includes an inert gas.

11. The process of claim 1 wherein one gaseous feed stream comprises UF6 and O2 and the other gaseous feed stream comprises NH3 and H2O.

12. The process of claim 1 wherein one gaseous feed stream comprises UF6 and O2 and the second gaseous feed stream comprises H2 and H2O.

13. The process of claim 1 wherein one gaseous feed stream comprises UF6 and O2 and the second gaseous feed stream comprises H2 and CO2.

14. The process of claim 11, 12 or 13 carried out at atmospheric pressure.

15. The process of claim 1 wherein the gaseous reactant streams are reacted at atmospheric pressure and a temperature of 900° to 2000°C for less than a second.

16. The process of claim 15 wherein the process is carried out continuously with continuous removal of solid uranium oxide and gaseous HF product.

17. The process of claim 15 wherein the reaction is carried out at essentially atmospheric pressure.

18. The process of claim 1 wherein the reaction is carried out at a pressure above atmospheric but below 250 psia.

19. The process of claim 1 wherein the reaction is carried out at a pressure slightly below atmospheric in the range of 5 to 13 psia.

20. The process of claim 1 wherein the solid uranium product is separated from the HF by physical means internal to the reactor.

21. The process of claim 1 wherein the solid uranium product is separated from the HF by physical means external to the reactor.

22. The process of claim 1 where the oxygen and hydrogen feed gas proportions are near the stoichiometric amounts to fully convert UF6 to UO2.

23. The process of claim 1 where the oxygen and hydrogen feed gas proportions are in excess of stoichiometric but generally less than 1.5 times the stoichiometric amounts.

24. The process of claim 1 wherein the oxygen and hydrogen feed gas proportions are less than the stoichiometric amounts required to fully convert UF6 to UO2.