Analytical applications

RDCH 702: Lecture 9 Separations Part 3Separation methodsSolvent extractionPUREXIon exchangeVolatilityElectrochemistrySpecific actinide separations

Basic concept of separationsOxidation stateIonic radiusDevelopment of advanced separationsTrivalent actinidesNecessary for fuel cycle due to formation of mixtures due to fissionActinides TransuranicsFission productsSe (Z=34) to Dy (Z=66)

Tributyl phosphate (TBP)

9-#PyroprocessesElectrorefining/ElectrochemistryUses molten salt as solventKCl-NaClAvoids problems associated with aqueous chemistryHydrolysis and chemical instabilityReduction of metal ions to metallic stateDifferences in free energy between metal ions and saltThermodynamic data at hand or easy to obtainSequential oxidation/reductionCations transported through salt and deposited on cathodeDeposition of ions depends upon redox potential

9-#Electrochemical SeparationsSelection of redox potential allows separationsCan use variety of electrodes for separationFree energiesnoble metalsiron to zirconiumactinides and rare earthsGroup 1 and 2Solubility of chlorides in cadmiumExplored at part of Integral Fast Reactor (IFR) program

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Molten SaltVolatility of chlorides as separation basisGaps between groupsHigh vapor pressures achievable at low temperature

9-#Molten Salt ExampleOxide fuel dispersed in CaCl2 /CaF2 at 800 C Inclusion of Ca metal reduces U salt to a metalReduced metals dissolved in receiver alloyExample Cu - 40% Mg - Ca Uranium exceeds solubility limits receiver alloy and precipitatesPu, other actinides, rare-earths, and noble metal fission products accumulate in receiver alloySeparated by distillationAlkali metals (Rb and Cs), alkali-earths (Sr and Ba),and remaining iodine and bromine accumulate in the CaCl2/CaF2 salt.Salt contains CaO from the reduction processCaO is electrolytically reduced to metal for reuse

9-#Molten Salt LiCl-KCl-UCl3 at 450-500 oCChopped metallic fuel added to anode basketsU oxidized at anode to U(III)Reduced back to metal at cathodeTransuranics and fission products oxidizedCo-deposition of actinides with cadmium cathode

9-#Pu ProcessingDOR reduces plutonium dioxide to metal PuO2 and Ca/CaCl2Formation of Pu metal and CaOCaO treated with Cl2

Pu metal purified by electrorefiningPu metal melted in NaCl/KCl at 800CPu oxidized to PuCl3, dissolves in molten saltPuCl3 migrates to cathode and reducedPu metal drips from cathode and collects in annular region outside cupring of pure Pu produced

9-#Ionic liquidsRoom temperature ionic liquid (ILs) composed of ions that are liquid below 373 K Composed of a bulky, unsymmetrical organic cation and an organic or inorganic anionRange of possible pairs, can be task specificLow vapor pressureAbility to dissolve organic and inorganic compoundsConductive Wide electrochemical windowIonic liquids are tunable to obtain properties needed for particular applicationSolubilityReaction RateElectrochemical windowSpecific metal ion interaction

9-#8Introduction: Ionic liquidsEthylammonium nitrate electrochemistry in 1914

Initial efforts focused on chloroaluminate anion (AlCl4-)Electroplate aluminum J. Electrochem. Soc. 98, 203 (1951)Detailed studies, identification of limitations with chloroaluminate Moisture sensitivity Requires non-ambient conditionsInorg. Chem. 17, 2728 (1978)Newer ionic liquids have non-hydrolyzing or stable anionsMoisture stable ionic liquids developedJACS, 965 (1992)

9-#9Examples of IL cations and anions1018 possible combinationsRoute for task specific propertiesGroup actinide separation

Can functionalize ILs

9-#10Ionic liquids in separationsIonic liquids can replace traditional solvents in the PUREX Studies showed favorable extraction when ILs used in conjunction with extractantsChem. Commun. 1765 (1998)Possible recovery of metals through electrodeposition direct from the organic phase following solvent extraction

From J. Nucl. Radiochem. Sci., 10(1), 1-6 (2009)

9-#11f-element reduction in ionic liquidsHaloaluminates not stable to reduction of An(III) or Ln(III) to metal stateDevelopment of moisture-stable ILs good cathodic stability large ~6V electrochemical windowsApplication based upon the molten salt system (450 - 800C)Ionic liquids eliminateSpecialized corrosion resistant cell Operation at elevated temperaturesProduction of caustic side reactionsElectrochemistry of Sm3+, La3+, Eu3+ and Th4+ in [Me3NBu][TFSI]Reported reduction of Sm, La, and Eu to the metallic stateTh was reported to reduce to Th0 but subsequently was converted to ThO2 by moisture in ionic liquidDalton Trans, 24, 4532-4534 (2002)

Role of water central, useful in dissolution by problematic in full reduction

9-#12Choice of Ionic LiquidA number of ionic liquids initially investigatedbis(trifluoromethanesulfonyl)imide (TFSI) anion selected for study at UNLVFunctionalized TFSI evaluatedpropyl and imidazoliumlarge potential window with Au, Pt, and Glassy carbon electrodesSuitable for reduction of lanthanides and actinides Ionic liquidN-trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imideUsed with actinidesWritten as [Me3NBu][TFSI]

9-#13Electrochemistry in ionic liquidsILs have wider potential window Actinide electrochemistry possibleMust limit water in the systemTFSI is a poor ligandMoves deposition to favorable potentialElectrode can influence windowGlassy Carbon (GC) widest potential window

9-#U TFSI compound synthesisAll reactions were performed in an argon filled glove boxK(TFSI)(s) synthesized by adding H(TFSI) and KH THF solutionK(TFSI) separated by decanting excess THF90% yieldK(TFSI) and UI3(THF)4 dissolved in THFExcess of K(TFSI) added to ensure reaction completionClear pale yellow solution was stirred for 24 hoursFinal orange red solutionContained off white KI solid precipitateTHF solution contained product U(TFSI)3Solid was isolated by evaporating excess THF74% yield

9-#Formation of Uranium DepositsConstant potential of -1.5 V versus NHESEM, EDS, and XRD analysis performedSEM of U deposited on Au foilXRD evaluation of sampleAlpha U metalSEM EDSNo oxygen at 5.5 keV

Au electrodeU-metal deposit

9-#16Direct dissolution of U3O8 into ionic liquidOriginal dissolution conditions: 45 mg U3O8 with 3.27 M HTFSI in Me3NBuTFSI Stirred for weeks with no change

RESULT: dissolution of materialoxidizing gas usedO3, NO2Addition of HTFSIPromote formation of uranium-TFSI complex

Ozone (From compressed air)t = 24 hrs

9-#17Direct dissolution of U3O8 into RTILProposed dissolution mechanism:

U3O8 + 6 HTFSI + O3 3 UO22+ + 6 TFSI- + 3 H2O + O2

Supporting EXAFS data of the original dissolution solution confirmed presence of UO22+ coordinated with five to six equatorial oxygens.

2 O @ 1.75 6 1 O @ 2.38 U --- O multi scattering @ 3.56

9-#18RDCH 702: Lecture 9 Separations Part 3Separation methodsSolvent extractionPUREXIon exchangeVolatilityElectrochemistrySpecific actinide separations

Basic concept of separationsOxidation stateIonic radiusDevelopment of advanced separationsTrivalent actinidesNecessary for fuel cycle due to formation of mixtures due to fissionActinides TransuranicsFission productsSe (Z=34) to Dy (Z=66)

Tributyl phosphate (TBP)

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