Fuel Cycle Chemistry

RFSS: Lecture 11 Uranium Chemistry and the Fuel CycleReadings: Uranium chapter:http://radchem.nevada.edu/classes/rdch710/files/uranium.pdfChemistry in the fuel cycleUraniumSolution ChemistrySeparationFluorination and enrichmentMetalFocus on chemistry in the fuel cycleSpeciation (chemical form)Oxidation stateIonic radius and molecular size

Utilization of fission process to create heatHeat used to turn turbine and produce electricityRequires fissile isotopes233U, 235U, 239PuNeed in sufficient concentration and geometry233U and 239Pu can be created in neutron flux235U in natureNeed isotope enrichmentRatios of isotopes established234: 0.0050.001, 68.9 a235: 0.7200.001, 7.04E8 a238: 99.2750.002, 4.5E9 aFission properties of uraniumDefined importance of element and future investigationsIdentified by Hahn in 1937200 MeV/fission2.5 neutrons

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U Fuel Cycle Chemistry OverviewUranium acid-leach

Extraction and conversionUnderstand fundamental chemistry of uranium and its applications to the nuclear fuel cycle

#Fuel FabricationEnriched UF6 UO2Calcination, Reduction TubesPellet Control40-60CFuel Fabrication

Other species for fuelnitrides, carbidesOther actinides: Pu, Th

#Uranium chemistryUranium solution chemistrySeparation and enrichment of UUranium separation from oreSolvent extractionIon exchangeSeparation of uranium isotopesGas centrifugeLaser

200 minerals contain uraniumBulk are U(VI) mineralsU(IV) as oxides, phosphates, silicates Classification based on polymerization of coordination polyhedraMineral deposits based on major anionPyrochlore A1-2B2O6X0-1A=Na, Ca, Mn, Fe2+, Sr,Sb, Cs, Ba, Ln, Bi, Th, UB= Ti, Nb, TaU(V) may be present when synthesized under reducing conditionsFrom XANES spectroscopyGoes to B site

Uraninite with oxidation

#Uranium solution chemistry overviewStrong Lewis acid, Hard electron acceptorF->>Cl->Br-I-Same trend for O and N group based on electrostatic force as dominant factorHydrolysis behaviorU(IV)>U(VI)>>>U(III)>U(V)U(III) and U(V)No data in solutionBase information on lanthanide or pentavalent actinides Uranyl(VI) most stable oxidation state in solutionUranyl(V) and U(IV) can also be in solutionU(V) prone to disproportionation Stability based on pH and ligandsRedox rate is limited by change in speciesMaking or breaking yl oxygensUO22++4H++2e-U4++2H2O 5f electrons have strong influence on actinide chemistryFor uranyl, f-orbital overlap provide bonding

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Uranium chemical bonding: oxidation statesTri- and tetravalent U mainly related to organometallic compoundsCp3UCO and Cp3UCO+Cp=cyclopentadiene 5f CO p backbondingMetal electrons to p of ligandsDecreases upon oxidation to U(IV)Uranyl(V) and (VI) compoundsyl ions in aqueous systems unique for actinidesVO2+, MoO22+, WO22+Oxygen atoms are cis to maximize (pp)M(dp)Linear MO22+ known for compounds of Tc, Re, Ru, OsAquo structures unknownShort U=O bond distance of 1.75 for hexavalent, longer for pentavalentSmaller effective charge on pentavalent UMultiple bond characteristics, 1 s and 2 with p characteristics

#Uranium solution chemistryTrivalent uraniumVery few studies of U(III) in solutionNo structural informationComparisons with trivalent actinides and lanthanidesTetravalent uraniumForms in very strong acidRequires >0.5 M acid to prevent hydrolysisElectrolysis of U(VI) solutionsComplexation can drive oxidationCoordination studied by XAFSCoordination number 91Not well definedU-O distance 2.42 O exchange examined by NMRPentavalent uraniumExtremely narrow range of existencePrepared by reduction of UO22+ with Zn or H2 or dissolution of UCl5 in waterU(V) is not stable but slowly oxidizes under suitable conditionsNo experimental information on structureQuantum mechanical predictions

#Hexavalent UraniumLarge number of compounds preparedCrystallizationHydrothermal Determination of hydrolysis constants from spectroscopic and titrationDetermine if polymeric species formPolynuclear species present except at lowest concentrationHexavalent uranium as uranyl in solution

#Uranyl chemical bondingUranyl (UO22+) linear moleculeBonding molecular orbitalssg2 su2 pg4 pu4Order of HOMO is unclearpg< pu< sg 90 % 235U for submarine reactors235U enrichment below 10 % cannot be used for a deviceCritical mass decreases with increased enrichment20 % 235U critical mass for reflected device around 100 kgLow enriched/high enriched uranium boundary

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Uranium enrichmentExploit different nuclear properties between U isotopes to achieve enrichmentMassSizeShape Nuclear magnetic momentAngular momentumMassed based separations utilize volatile UF6 UF6 formed from reaction of U compounds with F2 at elevated temperatureColorless, volatile solid at room temperatureDensity is 5.1 g/mLSublimes at normal atmosphereVapor pressure of 100 torrOne atmosphere at 56.5 COh point groupU-F bond distance of 2.00

#Uranium HexafluorideVery low viscosity 7 mPoiseWater =8.9 mPoiseUseful property for enrichmentSelf diffusion of 1.9E-5 cm2/sReacts with waterUF6 + 2H2O UO2F2 + 4HFAlso reactive with some metalsDoes not react with Ni, Cu and AlMaterial made from these elements need for enrichment

#Uranium Enrichment: Electromagnetic SeparationVolatile U gas ionized Atomic ions with charge +1 producedIons accelerated in potential of kVProvides equal kinetic energiesOvercomes large distribution based on thermal energiesIon in a magnetic field has circular pathRadius (r)m mass, v velocity, q ion charge, B magnetic fieldFor V acceleration potential

#Uranium Enrichment: Electromagnetic SeparationRadius of an ion is proportional to square root of massHigher mass, larger radiusRequirements for electromagnetic separation processLow beam intensitiesHigh intensities have beam spreadingAround 0.5 cm for 50 cm radiusLimits rate of productionLow ion efficiency Loss of materialCaltrons used during Manhattan project

#CalutronDeveloped by Ernest LawrenceCal. U-tronHigh energy useIraqi Calutrons required about 1.5 MW each90 totalManhattan ProjectAlpha4.67 m magnet15% enrichmentSome issues with heat from beamsShimming of magnetic fields to increase yieldBetaUse alpha output as feedHigh recovery

#Gaseous DiffusionHigh proportion of worlds enriched U95 % in 197840 % in 2003Separation based on thermal equilibriumAll molecules in a gas mixture have same average kinetic energylighter molecules have a higher velocity at same energyEk=1/2 mv2For 235UF6 and 238UF6235UF6 and is 0.429 % faster on average why would UCl6 be much more complicated for enrichment?

#Gaseous Diffusion235UF6 impacts barrier more often Barrier propertiesResistant to corrosion by UF6 Ni and Al2O3Hole diameter smaller than mean free pathPrevent gas collision within barrierPermit permeability at low gas pressureThin materialFilm type barrierPores created in non-porous membraneDissolution or etchingAggregate barrierPores are voids formed between particles in sintered barrierComposite barrier from film and aggregate

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Gaseous DiffusionBarrier usually in tubesUF6 introducedGas controlHeater, cooler, compressorGas sealsOperate at temperature above 70 C and pressures below 0.5 atmosphereR=relative isotopic abundance (N235/N238)Quantifying behavior of an enrichment cellq=Rproduct /Rtail Ideal barrier, Rproduct =Rtail(352/349)1/2; q= 1.00429

#Gaseous DiffusionSmall enrichment in any given cellq=1.00429 is best conditionReal barrier efficiency (eB)eB can be used to determine total barrier area for a given enrichmenteB = 0.7 is an industry standardCan be influenced by conditionsPressure increase, mean free path decreaseIncrease in collision probability in poreIncrease in temperature leads to increase velocityIncrease UF6 reactivityNormal operation about 50 % of feed diffusesGas compression releases heat that requires coolingLarge source of energy consumptionOptimization of cells within cascades influences behavior of 234Uq=1.00573 (352/348)1/2 Higher amounts of 234U, characteristic of feed

#Gaseous DiffusionSimple cascadeWasteful processHigh enrichment at end discardedCountercurrentEqual atoms condition, product enrichment equal to tails depletionAsymmetric countercurrentIntroduction of tails or product into nonconsecutive stageBundle cells into stages, decrease cells at higher enrichment

#Gaseous DiffusionNumber of cells in each stage and balance of tails and product need to be consideredStages can be added to achieve changes in tailing depletion Generally small levels of tails and product removedSeparative work unit (SWU)Energy expended as a function of amount of U processed and enriched degree per kg3 % 235U3.8 SWU for 0.25 % tails5.0 SWU for 0.15 % tails Determination of SWUP product massW waste massF feedstock massxW waste assayxP product assayxF feedstock assay

#Gas centrifugeCentrifuge pushes heavier 238UF6 against wall with center having more 235UF6Heavier gas collected near topDensity related to UF6 pressureDensity minimum at center

m molecular mass, r radius and w angular velocityWith different masses for the isotopes, p can be solved for each isotope

#Gas CentrifugeTotal pressure is from partial pressure of each isotopePartial pressure related to massSingle stage separation (q)Increase with mass difference, angular velocity, and radiusFor 10 cm r and 1000 Hz, for UF6 q=1.26

Gas distribution in centrifuge

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Gas Centrifuge More complicated setup than diffusionAcceleration pressures, 4E5 atmosphere from previous exampleHigh speed requires balanceLimit resonance frequenciesHigh speed induces stress on materialsNeed high tensile strengthalloys of aluminum or titanium maraging steelHeat treated martensitic steelcomposites reinforced by certain glass, aramid, or carbon fibers

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Gas CentrifugeGas extracted from center post with 3 concentric tubesProduct removed by top scoopTails removed by bottom scoopFeed introduced in centerMass load limitationsUF6 needs to be in the gas phaseLow center pressure3.6E-4 atm for r = 10 cmSuperior stage enrichment when compared to gaseous diffusionLess power need compared to gaseous diffusion1000 MWe needs 120 K SWU/yearGas diffusion 9000 MJ/SWUcentrifuge 180 MJ/SWUNewer installations compare to diffusionTend to have no non-natural U isotopes

#Laser Isotope SeparationIsotopic effect in atomic spectroscopyMass, shape, nuclear spinObserved in visible part of spectraMass difference in IR regionEffect is small compared to transition energies1 in 1E5 for U speciesUse laser to tune to exact transition specieProduces molecule in excited stateDoppler limitations with methodMovement of molecules during excitationSignature from 234/238 ratio, both depleted

#Laser Isotope Separation3 classes of laser isotope separationsPhotochemicalReaction of excited state moleculeAtomic photoionizationIonization of excited state moleculePhotodissociationDissociation of excited state moleculeAVLISAtomic vapor laser isotope separationMLISMolecular laser isotope separation

#Laser isotope separationAVLISU metal vaporHigh reactivity, high temperatureUses electron beam to produce vapor from metal sampleIonization potential 6.2 eVMultiple step ionization238U absorption peak 502.74 nm235U absorption peak 502.73nmDeflection of ionized U by electromagnetic field

#Laser Isotope SeparationMLIS (LANL method) SILEX (Separation of Isotopes by Laser Excitation) in AustraliaAbsorption by UF6Initial IR excitation at 16 micron235UF6 in excited stateSelective excitation of 235UF6Ionization to 235UF5Formation of solid UF5 (laser snow)Solid enriched and use as feed to another excitation

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