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ELECTROMETALLURGICAL TREATMENT OF ALUMINUM-MATRIX FUELS'

J . L. WillitArgonne National LaboratoryChemical Technology Division9700 South Cass AvenueArgonne, IL 60439(708)252-4384C. C. McPheetersArgonne National LaboratoryChemical Technology Division9700 South Cass AvenueArgonne, IL 60439(708)252-4533

E. C. GayArgonne National LaboratoryChemical Technology D ivision9700 South Cass AvenueArgonne, IL 60439(708) 252-4534J. J. LaidlerArgonne National LaboratoryChemical Technology Division9700 South Cass AvenueArgonne, IL 60439(708) 252-4479

W. E. MillerArgonne National LaboratoryChemical Technology Division9700 South Cass AvenueArgonne, IL 60439(708) 252-4536

contractor of the US. Government under contract No .W-31-109-ENG-38. Accordingly. the US . Governmentretainsa nonexclusive, royalty-free licame to publish orreproduce the published form of this contribution, or

Submitted for Presentation atDOE Spent Nuclear Fuel & Fissile Material ManagementEmbedded T opical MeetingSpent Nuclear Fuel - Treatment TechnologiesReno, Nevada, June 16-20, 1996

'Work supp-orted by th e U.S. Department of Energy, Nuclear Energy Research and Development Program, underContract No. W-3 1-109-Eng. 8.

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DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.

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ELECTRO.LfETXiLCRGICAL TREATMEN T OF ALUMiNIJM-.LIA TRIX FUELS

J. L. WillitArgonne National LaboratoryChemical Technology D ivision9700 South Cass AvenueArgonne, IL 60439(708) 252-4384

E. C. Gay-4rgonne National L aboratoryChemical Technology D ivision9700 South Cass AvenueArgonne, IL 60439(708) 252-4534

W. E. MillerArgonne National LaboratoryChemical Technology Division9700 South Cass AvenueArgonne, IL 60439(708) 252-4536

C. C. McPheetersArgonne National LaboratoryChemical Technology D ivision9700 South Cass AvenueArgonne, IL 60439(708) 252-4533

J. J. LaidlerArgonne N ational LaboratoryChemical Technology D ivision9700 South Cass AvenueArgonne, IL, 0439(708) 252-4479

I. INTRODUCTIONThe U.S. Department of Energy faces a dilemmaconcerning spent aluminum -matrix reactor fuel. Over thenext forty years, 12 8 metric tons of spentaluminum-matrix fuel will be shipped to the SavannahRiver Site from U S . and foreign research reactors. Whenoriginally fabricated, this fuel contained over 55 metrictons of uranium at an average enrichment of -20%. AtArgonne National Laboratory we are developing acost-effective electr ome tallurg ical process for recoveringuranium from these aluminum-m atrix fuels. The uraniumthat can be recovered fiom this fuel has a commercialvalue of over $300 million. Recovering the uranium alsoreduces the amount of high-level waste that must

ultimately be disposed of in a geolog ical repository. Otheroptions for dealing with this fuel are (1) using existingfacilities at Savannah River to recover the uranium or(2) disposition of the fuel, including the u ranium, in ageolagical repository. The first option cannot handle allthe research reactor spent fuel, because around 2005 theprocessing canyons at Savannah River will bedecomm issioned. Direct disposition of the metallic fuelin a geological repository is another possibility, as isdisposition following conversion to a glass or ceramicwaste form. However it is questionable whether the NRCwill allow direct disposal of highly-enriched uraniummetallic fuel in the repository. Also, the high level wastevolume can be reduced by about a factor of ten by removalof the aluminum and uranium from the fuel by some typeof processing. This results in a significant savings indisposal costs.

The electrometallurgical treatment process describedin this paper builds on our experience in treating spentfuel from the Experimental Breeder Reactor (EBR-11).The work is also, to some degree, a spin-off f b mapplying electrometallurgical treatment to spent he1 fiom

the Hanford single pass reactors (SPRs) and h e1 and flushsalt tkom the Molten Salt Reactor Experiment (MSRE).In treating EBR-I1 fuel, we recover the actinides from auranium-zirconium fuel by electrorefining the uran ium outof the chopped fuel. With SPR fuel, uranium iselectrorefined out of the alum inum cladding. Both ofthese processes are conducted in a LEI-KCI molten-saltelectrolyte. In the case of the MS RE, which used afluoride salt-based fuel, uran ium in this salt is . recoveredthrough a series of electrochemical reductions.Recovering high-purity uranium tkom an aluminum-matrix fuel is more challenging than treating SPR orEBR-I1 fuel because the aluminum-matrix fuel is typically-90% (volume basis) aluminum.

Thermodynamic calculations predict thatelectrorefining a uranium-alum inum alloy in a moltenchloride electrolyte will not yield a clean separation ofuranicrn from aluminum . To circumvent these difficultieswe modified our process. The first modification waschanging from a LiCl-KC I electrolyte to a LiF-K Felectrolyte. By switching molten salts we are able toelectrotransport aluminum to the electrorefmer cathode,leaving uranium and metal fission products behind in th ebasket of the anode. The aluminu m obtained will be highpurity and disposed of as low-level waste. Then in asecond electroefmer we will recover high-purity uranium .Fission p roducts will be converted to oxides, thenincor porate d into a glass tha t can be fed into the DefenseWaste Processing Facility (DWPF) at Savannah River orbe incorporated into DWPF-type glass in a small-scaleglass melter.

A diagram of the electrometallurgical process isshown in Figure 1. The process will be performed in aninert atmosphere enclosure located in a shielded facility.The process operations are grouped into three types of

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operations: (1 ) head end steps, ( 2 ) electrorefming andconsolidation steps, and (3) oxidation and glass-formingstzps. This paper will dsicuss the overall processflowsheet with particular em phasis on thethermodyn amics invo. :ed in the aluminum and uraniumelectrorefming steps.

1 sort. cut CompactI

, Fluorapatite

II

Melt, Freere Ai,Pour off SaltEldo re f ineAlurn numMelt1 CastAndes -+

II IvI U,TRU,FPs ]

Melt,F r e e U,PouroffSaltElsctroretineUranium *Gl assMek r , kl OxidationFurnacei

Figure 1. Electrometallurgical Treatment Flowsheet for Aluminum-Matrix Fuels (LLW=low-level waste,TR U-a ms uran ics, Fps=fission products)

11. HEAD-END STEPSIn the first head-end step, the fuel assembly ends areremoved, then the fuel is sorted and compacted. Next, the&el is melted in an enclosed t il t-pour h a c e . Becausesilicon complexes with uranium and enhances theseparation of aluminum and uranium, silicon is added to

the molten fuel at this point. Th en the molten fuel is castinto anodes. In the melting st ep we expect that thevolatile fission produFts (cesium, rubidium, bromine,iodine, xenon, and krypton) will vaporize. These volatilespecies, with the exception of xenon and krypton, will becaptured in a fibrous aluminosilicate ( f i b e h ) rap abovethe molten metal. We used a similar trapping method tocapture volatile metals, iodine, and bromine in th e ANLMelt Refining Process and have successfully trapped100%of volatilized cesium and sodium. After casting ofthe anodes, the trap material is compressed and added to aglass melter at the end of the process. Because the entkeprocess is conducted in an inert enclosure, xenon andkrypton can be vented or trapped in cryogenic traps as part

of the purification system of the inert atmosphereenclosure.

111. ELECTROREFMING AND CONSOLIDATIONSTEPS

A. General Ele ctro refm k PrinciplesElectrorefining is used to recover one metal as a puremetal phase from the fuel and leave the more noble metalsbehind in the anode. Electrorefining in molten halidesalts separates metals, based on the relativethermodynamic stability, as metal halides. In general, thespecies that form the most stable metal halides are the

first to be oxidized at the anode and the last to bereduced at the cathode. The more noble the metal, theless stable the metal halide that is formed. Metals whosehalides are widely separated in terms of thermodynamicstability (> 3 kcaVmol F- or Cl-) are readily separated.Two conditions must be satisfied to obtain a pure deposit

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3f a metal (M ) a the elecnorefiner cathod e. First, thvoltage of the elemorefiner must be limited to a valuesuch that no metal more noble than M is oxidized at theanode. Second, to obtain a high-purity deposit, theconcentration of M X , (where M X n is the halide salt ofmetal M) must be large relative to the other metal halidesformed by oxidation of the spent fuel.

In a molten chloride salt, uranium and aluminumcannot be separated from the uranium-alum inum spentfuel. However, by electrorefming, aluminum can beseparated out of a uranium-alum inum-silicon alloy byswitching to a fluoride salt an d by addi ng silicon t o theuranium-aluminum alloy. Figure 2 shows the relativethermodynamic stability of the metal fluorides in aLiF-KF molten electrolyte after the fuel has been alloyedwith silicon. Toward the top of the diagram are speciesthat form more stable meta l fluorides. As one movesdown the diagram, greater voltages (electrorefmer cathodevs. electrorefmer anode) are required to electrotransport therespective metals from th e anode to the cathode. Th eheavy dashed line represents the break between aluminumand rare earth metals. If the electrorefmer is operated at avoltage corresponding to that of the heavy dashed line,only aluminum will be deposited at the cathode.

90 d e

A turnin urn

s t

Figure 2. Diagram Describing Electrorefming Separationsof Alkaline Earth Fission Products (AEFPs),Rare Earths (RE), Transuranics (TRU), Silicon(Si), and Noble Metal Fission Products(NMFPs)

In aluminurn electrorefining , the alkaline earth fissionproducts acc umulate in'. the electrolyte, and the actinides,rare earths, and noble meta ls remain in the anode. In asimilar fashion, in uranium electrorefining the rare earthand transuranic fission products accumulate in theelectrolyte. and the noble metal fission products remain inthe anode.

Argonne National Laboratory has developed a'?igh-throughput design for both the aluminum and

uranium e1ectrorefners.- X simplified top .iew of theelectrorefiner is shown in Figure 3. The anodes aremounted in a cir cu la r m y n d ro ta ted in the channelbetween two cylindrical cathodes. Dendritic uraniumdeposits at the cathode in this design and we anticipatethat the aluminum cathode deposit will likewise bedendritic. The dendrites are scraped off the cathode byscrapers attached to the anode baske ts. The dendritesthen sink to the bottom of the electorefiner where they arecollected. Whe n all of the aluminum or uranium has beenelectrorefined out of the anodes, the current is turned off,the dendrites are compressed, and the dendrites areremoved from the electrorefmer.

Figure 3 . Simplified Top View of High ThroughputAluminumiUranium Electrorefmer

B . Aluminum ElectrorefmingIn the aluminum electrorefmer, aluminum metal isoxidized at the anode to form K3A IF6, a species that issoluble in the LiF-KF electrolyte. At the cathode,K3A1F6 is reduced to alum inum metal. To obtain acathode deposit of high-purity aluminum, K 3AlF6must bethe least thermodynamically stable metal fluoride in themolten electrolyte until all the aluminum has beenremoved from the anode. To achieve this condition, thecell voltage must be limited to prevent the oxidation of

rare earth silicid es to rare earth fluorides, yet it mu st besufficient to electrorefme all the alum inum. When theactivity of aluminum in the anode has been decreased tothe theoretical equilibrium cell voltage is -0.2 V.The calculated data in Table 1 show the effect of cathodevs. anode voltage on the activity ratio of K3AlF6 vs.CeC13. Th e low activity ratios at 0 an d -0.3 V show thatthere are essentially no rare earth fluorides in the salt atcell voltages less negative than or equal to -0.3 V.Therefore, a low activity ratio in the molten electrolytecan be maintained un til essentially all the aluminum hasbeen removed &om the spent fuel and there will be norare earth metal c ontamination of the aluminum product.

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T.-\BLE L . Calculated R atio of CeF; and KrXIF, Activities 2sFunction of Cathode vs . Anode Electroreriner Voltagecarhode vs. anode (Y ) o v -0.3 v 1 -0.4 v~ c , F , / a ~ ~I F ~ 7.1 x IO-' 6.9 x 10.' 0.47

C . Consolidate Aluminum DendritesBecause some salt will be adhering to the dendriticaluminum, the aluminum will be melted and coalescedinto a single metal phase that melts at a temperatureabove the melting point of the salt. Then by cooling themelt below the melting point of aluminum, we can pouroff the salt, which is still molten. The salt is thenreturned to the aluminum electrorefiner, and theconsolidated aluminum ingot w ill be disposed of aslow-level waste.

D. Uranium ElectrorefiningAfter essentially all the aluminum has been removedfrom the spent fuel, all that remains in the anodes are theactinides, rare earths, and noble metals. Th e next s tep isto recover uranium from these anod es in a secondelectrorefmer. This uranium electrorefmer is identical tothe one used for aluminum electrorefining, excep t that it isfilled with a LiF-KF-UF3 molten-salt electrolyte.Uranium electrodeposition in mo lten fluoride electrolyteshas been reported and was found to be similar toelectrodeposition in molten chloride electrolytes.

Because silicon was added in the initial head-endstep, uranium will be present in the uranium electrorefmeranodes as a uranium s ilicide (e.g., U3Sis, USi,).Therefore, the anode and cathode reactions, respectively,are:

As indicated by Figure 2 , these reactions will occurat a cathod e vs. anode voltage more negative than us ed foraluminum electrorefming. Calculations c o n f m thisexpectation. Whereas alum inum is electrorefmed at-0.2 to -0.3 V, uranium is electrorefined at -0.4 to -0.5 V.After all the uranium has been extracted f?om the anodes,all that remains in the anodes are the noble metal fissionproducts. Because the rare earth fluorides and TRUfluorides are more stable thermodynamically than UF3,they will remain in the molten fluorid e electrolyte.

E. Consolidate Uranium DendritesThe uranium electrorefmer dendritic product isconsolidated by a m elting o peration identical to theoperation used to consolidate the aluminum dendritesobtained in the aluminum electrorefmer. The adheringsalt is returned to the uranium electrorefiner.

F . Periodic Salt Scrubbin iWith continued treatment of spent fuel, there will bea buildup of alkaline earth fluorides in the aluminumelectrorefmer an d of rare earth and TRU fluorides in theuranium electrorefiner. Eventually this buildup w ill resultin an undesirable carryover into the electrorefmer product.It will be necessary to periodically scrub these metalfluorides from the salt or discard the salt. Salt scrubbingis the preferred choice because it will allow a single batchof sal t to be used in each electrorefiner for the entirecam paign . Several approaches are available for thisperiodic scrubbing, including chemical reduction andoxide precipitation.

IV . OXIDATION AND GLASS-FORMING STEPSA. Oxi dize Uranium Electrorefmer Anode Heels andScrubbed Fission ProductsThe scrubbed alkaline earths, rare earths, and TRUsare then converted to oxides along with the metal thatremains in the anode after uranium electrorefming. Theoperation is similar to an oxidation performed at ANLpreviously. The conversion is performed in an airoxi dati on furnace. Th e outpu t fiom the furnace is anoxide pow der with noble metal fine s dispersed throughoutthe oxide.

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B. Melt Fission Product Oxides and Fabricate WasteGlassA sm all glass melter will be used to melt the oxidepowder fi-om the oxidation fiunace, the compressedaluminosilicate trap, and additional glass formers. Weanticipate that we will be able to formulate a glass withinthe specifications of the DW PF glass. To minimize wastevolumes we have been careful throughout the process toadd only glass-forming oxides (fibrous aluminosilicate)and silicon, which is converted to silica in the oxidationfiunace. Alum ina and silica are compone nts of DW PFglass. Th e glass can then be poured into D WPF wastecanisters, which will later be bundled into a wastepackage.

Y. DEVELOPMENT STATUSMany of the process steps in Figure 1 have alreadybeen successfblly dem onstra ted. Uranium electrorefinhgin a high-throughput electrorefmer has been demonstratedat AN L. We have also successfully scrubbed m e earthsout of a molten chloride salt by using chemical reduction

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with lithium. Melting, casting. and consolidation stepswere demonstrated in the Melt Refining Process at ANL.The key process operations that have yet to bedemonstrated are electrorefining aluminum in ahigh-through put electrorefiner and s crubb ing the alkalineearth fission products ou t of the aluminum electrorefmersalt. Demonstrating these two steps is the main focus ofour R& D effort for the next year. We are also beginningto design an engineering-scale high-throughputelectrorefiner for demonstration w ith sim ulate d sp ent fuel.Fabrication and installation of this electrorefmer arescheduled to be complete by this fall.

REFERENCES1 . D. Hampson, R. Frye, and J. Rizzie, Melt Refiningof EBR-I1 Fuel, in Nuclea r Me tallurgy, P. Chiotti(ed), volume 15, pp. 57-76 (1969).2

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E. Gay, W. Miller, and J. Laidler, Proposed HighThroughput Electrorefining Treatment of SpentN-Reactor Fuel, Proceedings of Spring 1996American Nuclear Society Meeting, Reno, NV.W. E. Miller, G. J. Bersteing, R. F. Malecha,M. A . Slawecki, R. C. Paul, and R. F. Fryer,EBR-I1 Plant Equipment for Oxidation of MeltRefining Skulls, Proc. 15th Conf. on Remo teSystems Technology, Chicago, IL, Am. Nucl. SOC.,Hinsdale, IL , pp 43-51 (1967).

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DISCLAIMERThis report was prepared as an account of work sponsored by an agency of theUnited State s Government. Neither the United States Government nor any agencythereof, nor any of their employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy, completeness, or use-fulness of any information, apparatus, product, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference herein to any spe-cific commercial product, process, or service by trade name, trademark, manufac-turer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof.The views and opinions of authors expressed herein do not necessarily state orreflect those of the United States Government or any agency thereof.