Uranium Chemistry and the Fuel Cycle Chemistry in the fuel cycle Uranium Solution Chemistry Separation Fluorination and enrichment Metal Focus on chemistry in the fuel cycle Speciation (chemical form) Oxidation state Ionic radius and molecular size Utilization of fission process to create heat Heat used to turn turbine and produce electricity Requires fissile isotopes 233U, 235U, 239Pu Need in sufficient concentration and geometry 233U and 239Pu can be created in neutron flux 235U in nature Need isotope enrichment Why is U important in the fuel cycle: induced fission cross section for 235U and 238U as function of the neutron energy.

Nuclear properties of Uranium Fission properties of uranium Defined importance of element and future investigations Identified by Hahn in 1937 200 MeV/fission 2.5 neutrons Natural isotopes 234,235,238U Ratios of isotopes established 234: 0.005±0.001, 68.9 a 235: 0.720±0.001, 7.04E8 a 238: 99.275±0.002, 4.5E9 a 233U from 232Th need fissile isotope initially

Chemistry overview Uranium acid-leach Extraction and conversion

Fuel Fabrication Enriched UF6 Calcination, Reduction UO2 Pellet Control 40-60°C Tubes Fuel Fabrication Other species for fuel nitrides, carbides Other actinides: Pu, Th

Uranium chemistry Uranium solution chemistry Separation and enrichment of U Uranium separation from ore Solvent extraction Ion exchange Separation of uranium isotopes Gas centrifuge Laser 200 minerals contain uranium Bulk are U(VI) minerals U(IV) as oxides, phosphates, silicates Classification based on polymerization of coordination polyhedra Mineral deposits based on major anion Pyrochlore A1-2B2O6X0-1 A=Na, Ca, Mn, Fe2+, Sr,Sb, Cs, Ba, Ln, Bi, Th, U B= Ti, Nb, Ta U(V) may be present when synthesized under reducing conditions XANES spectroscopy Goes to B site Uraninite with oxidation

Aqueous solution complexes Strong Lewis acid Hard electron acceptor F->>Cl->Br-I- Same trend for O and N group based on electrostatic force as dominant factor Hydrolysis behavior U(IV)>U(VI)>>>U(III)>U(V) Uranium coordination with ligand can change protonation behavior HOCH2COO- pKa=17, 3.6 upon complexation of UO2 Inductive effect Electron redistribution of coordinated ligand Exploited in synthetic chemistry U(III) and U(V) No data in solution Base information on lanthanide or pentavalent actinides

Uranium solution chemistry Uranyl(VI) most stable oxidation state in solution Uranyl(V) and U(IV) can also be in solution U(V) prone to disproportionation Stability based on pH and ligands Redox rate is limited by change in species Making or breaking yl oxygens UO22++4H++2e-U4++2H2O yl oxygens have slow exchange Half life 5E4 hr in 1 M HClO4 5f electrons have strong influence on actinide chemistry For uranyl, f-orbital overlap provide bonding

Uranyl chemical bonding Uranyl (UO22+) linear molecule Bonding molecular orbitals sg2 su2 pg4 pu4 Order of HOMO is unclear pg< pu< sg<< su proposed Gap for s based on 6p orbitals interactions 5fd and 5ff LUMO Bonding orbitals O 2p characteristics Non bonding, antibonding 5f and 6d Isoelectronic with UN2 Pentavalent has electron in non-bonding orbital

Uranyl chemical bonding Linear yl oxygens from 5f characteristic 6d promotes cis geometry yl oxygens force formal charge on U below 6 Net charge 2.43 for UO2(H2O)52+, 3.2 for fluoride systems Net negative 0.43 on oxygens Lewis bases Can vary with ligand in equatorial plane Responsible for cation-cation interaction O=U=O- - -M Pentavalent U yl oxygens more basic Small changes in U=O bond distance with variation in equatoral ligand Small changes in IR and Raman frequencies Lower frequency for pentavalent U Weaker bond

Uranium chemical bonding: oxidation states Tri- and tetravalent U mainly related to organometallic compounds Cp3UCO and Cp3UCO+ Cp=cyclopentadiene 5f CO p backbonding Metal electrons to p of ligands Decreases upon oxidation to U(IV) Uranyl(V) and (VI) compounds yl ions in aqueous systems unique for actinides VO2+, MoO22+, WO22+ Oxygen atoms are cis to maximize (pp)M(dp) Linear MO22+ known for compounds of Tc, Re, Ru, Os Aquo structures unknown Short U=O bond distance of 1.75 Å for hexavalent, longer for pentavalent Smaller effective charge on pentavalent U Multiple bond characteristics, 1 s and 2 with p characteristics

Uranium solution chemistry: U(III) Dissolution of UCl3 in water Reduction of U(IV) or (VI) at Hg cathode Evaluated by color change U(III) is green Very few studies of U(III) in solution No structural information Comparisons with trivalent actinides and lanthanides

Uranium solution chemistry Tetravalent uranium Forms in very strong acid Requires >0.5 M acid to prevent hydrolysis Electrolysis of U(VI) solutions Complexation can drive oxidation Coordination studied by XAFS Coordination number 9±1 Not well defined U-O distance 2.42 Å O exchange examined by NMR Pentavalent uranium Extremely narrow range of existence Prepared by reduction of UO22+ with Zn or H2 or dissolution of UCl5 in water UV-irradiation of 0.5 M 2-propanol-0.2 M LiClO4 with U(VI) between pH 1.7 and 2.7 U(V) is not stable but slowly oxidizes under suitable conditions No experimental information on structure Quantum mechanical predictions

Hexavalent Uranium Large number of compounds prepared Crystallization Hydrothermal Determination of hydrolysis constants from spectroscopic and titration Determine if polymeric species form Polynuclear species present except at lowest concentration

Uranium speciation Speciation variation with uranium concentration Hydrolysis as example Precipitation at higher concentration Change in polymeric uranium species concentration

Uranium purification from ores: Using U chemistry in the fuel cycle Preconcentration of ore Based on density of ore Leaching to extract uranium into aqueous phase Calcination prior to leaching Removal of carbonaceous or sulfur compounds Destruction of hydrated species (clay minerals) Removal or uranium from aqueous phase Ion exchange Solvent extraction Precipitation Use of cheap materials Acid solution leaching Sulfuric (pH 1.5) U(VI) soluble in sulfuric Anionic sulfate species Oxidizing conditions may be needed MnO2 Precipitation of Fe at pH 3.8 Carbonate leaching Formation of soluble anionic carbonate species UO2(CO3)34- Precipitation of most metal ions in alkali solutions Bicarbonate prevents precipitation of Na2U2O7 Formation of Na2U2O7 with further NaOH addition Gypsum and limestone in the host aquifers necessitates carbonate leaching

Recovery of uranium from solutions Ion exchange U(VI) anions in sulfate and carbonate solution UO2(CO3)34- UO2(SO4)34- Load onto anion exchange, elute with acid or NaCl Solvent extraction Continuous process Not well suited for carbonate solutions Extraction with alkyl phosphoric acid, secondary and tertiary alkylamines Chemistry similar to ion exchange conditions Chemical precipitation Addition of base Peroxide Water wash, dissolve in nitric acid Ultimate formation of (NH4)2U2O7 (ammonium diuranate), yellowcake heating to form U3O8 or UO3

Uranium purification Tributyl phosphate (TBP) extraction Based on formation of nitrate species UO2(NO3)x2-x + (2-x)NO3- + 2TBP UO2(NO3)2(TBP)2 Process example of pulse column below

Uranium enrichment Once separated, uranium needs to be enriched for nuclear fuel Natural U is 0.7 % 235U Different enrichment needs 3.5 % 235U for light water reactors > 90 % 235U for submarine reactors 235U enrichment below 10 % cannot be used for a device Critical mass decreases with increased enrichment 20 % 235U critical mass for reflected device around 100 kg Low enriched/high enriched uranium boundary

Uranium enrichment Exploit different nuclear properties between U isotopes to achieve enrichment Mass Size Shape Nuclear magnetic moment Angular momentum Massed based separations utilize volatile UF6 UF6 formed from reaction of U compounds with F2 at elevated temperature Colorless, volatile solid at room temperature Density is 5.1 g/mL Sublimes at normal atmosphere Vapor pressure of 100 torr One atmosphere at 56.5 ºC Oh point group U-F bond distance of 2.00 Å

Uranium Hexafluoride Very low viscosity 7 mPoise Water =8.9 mPoise Useful property for enrichment Self diffusion of 1.9E-5 cm2/s Reacts with water UF6 + 2H2O UO2F2 + 4HF Also reactive with some metals Does not react with Ni, Cu and Al Material made from these elements

Uranium Enrichment: Electromagnetic Separation Volatile U gas ionized Atomic ions with charge +1 produced Ions accelerated in potential of kV Provides equal kinetic energies Overcomes large distribution based on thermal energies Ion in a magnetic field has circular path Radius (r) m mass, v velocity, q ion charge, B magnetic field For V acceleration potential

Uranium Enrichment: Electromagnetic Separation Radius of an ion is proportional to square root of mass Higher mass, larger radius For electromagnetic separation process Low beam intensities High intensities have beam spreading Around 0.5 cm for 50 cm radius Limits rate of production Low ion efficiency Loss of material Caltrons used during Manhattan project

Calutron Developed by Ernest Lawrence Cal. U-tron High energy use Iraqi Calutrons required about 1.5 MW each 90 total Manhattan Project Alpha 4.67 m magnet 15% enrichment Some issues with heat from beams Shimming of magnetic fields to increase yield Beta Use alpha output as feed High recovery

Gaseous Diffusion High proportion of world’s enriched U 95 % in 1978 Separation based on thermal equilibrium All molecules in a gas mixture have same average kinetic energy lighter molecules have a higher velocity at same energy Ek=1/2 mv2 For 235UF6 and 238UF6 235UF6 and is 0.429 % faster on average why would UCl6 be much more complicated for enrichment?

Gaseous Diffusion 235UF6 impacts barrier more often Barrier properties Resistant to corrosion byUF6 Ni and Al2O3 Hole diameter smaller than mean free path Prevent gas collision within barrier Permit permeability at low gas pressure Thin material Film type barrier Pores created in non-porous membrane Dissolution or etching Aggregate barrier Pores are voids formed between particles in sintered barrier Composite barrier from film and aggregate

Gaseous Diffusion Barrier Thin, porous filters Pore size of 100-1000 Å Thickness of 5 mm or less tubular forms, diameter of 25 mm Composed of metallic, polymer or ceramic materials resistant to corrosion by UF6, Ni or alloys with 60 % or more Ni, aluminum oxide Fully fluorinated hydrocarbon polymers purity greater than 99.9 percent particle size less than 10 microns high degree of particle size uniformity

Gaseous Diffusion Barrier usually in tubes UF6 introduced Gas control Heater, cooler, compressor Gas seals Operate at temperature above 70 °C and pressures below 0.5 atmosphere R=relative isotopic abundance (N235/N238) Quantifying behavior of an enrichment cell q=Rproduct /Rtail Ideal barrier, Rproduct =Rtail(352/349)1/2; q= 1.00429

Gaseous Diffusion Small enrichment in any given cell q=1.00429 is best condition Real barrier efficiency (eB) eB can be used to determine total barrier area for a given enrichment eB = 0.7 is an industry standard Can be influenced by conditions Pressure increase, mean free path decrease Increase in collision probability in pore Increase in temperature leads to increase velocity Increase UF6 reactivity Normal operation about 50 % of feed diffuses Gas compression releases heat that requires cooling Large source of energy consumption

Gaseous Diffusion Simple cascade Wasteful process High enrichment at end discarded Countercurrent Equal atoms condition, product enrichment equal to tails depletion Asymmetric countercurrent Introduction of tails or product into nonconsecutive stage Bundle cells into stages, decrease cells at higher enrichment

Gaseous Diffusion Number of cells in each stage and balance of tails and product need to be considered Stages can be added to achieve changes in tailing depletion Generally small levels of tails and product removed Separative work unit (SWU) Energy expended as a function of amount of U processed and enriched degree per kg 3 % 235U 3.8 SWU for 0.25 % tails 5.0 SWU for 0.15 % tails Determination of SWU P product mass W waste mass F feedstock mass xW waste assay xP product assay xF feedstock assay

Gaseous Diffusion Optimization of cells within cascades influences behavior of 234U q=1.00573 (352/348)1/2 Higher amounts of 234U, characteristic of feed US plants K-25 at ORNL 3000 stages 90 % enrichment Paducah and Portsmouth Reactor U was enriched Np, Pu and Tc in the cycle

Gas centrifuge Centrifuge pushes heavier 238UF6 against wall with center having more 235UF6 Heavier gas collected near top Density related to UF6 pressure Density minimum at center m molecular mass, r radius and w angular velocity With different masses for the isotopes, p can be solved for each isotope

Gas Centrifuge Total pressure is from partial pressure of each isotope Partial pressure related to mass Single stage separation (q) Increase with mass difference, angular velocity, and radius For 10 cm r and 1000 Hz, for UF6 q=1.26 Gas distribution in centrifuge

Gas Centrifuge More complicated setup than diffusion Acceleration pressures, 4E5 atmosphere from previous example High speed requires balance Limit resonance frequencies High speed induces stress on materials Need high tensile strength alloys of aluminum or titanium maraging steel Heat treated martensitic steel composites reinforced by certain glass, aramid, or carbon fibers

Gas Centrifuge Gas extracted from center post with 3 concentric tubes Product removed by top scoop Tails removed by bottom scoop Feed introduced in center Mass load limitations UF6 needs to be in the gas phase Low center pressure 3.6E-4 atm for r = 10 cm Superior stage enrichment when compared to gaseous diffusion Less power need compared to gaseous diffusion 1000 MWe needs 120 K SWU/year Gas diffusion 9000 MJ/SWU centrifuge 180 MJ/SWU Newer installations compare to diffusion Tend to have no non-natural U isotopes Gas Centrifuge

Centrifuges US Natanz

Laser Isotope Separation Isotopic effect in atomic spectroscopy Mass, shape, nuclear spin Observed in visible part of spectra Mass difference in IR region Effect is small compared to transition energies 1 in 1E5 for U species Use laser to tune to exact transition specie Produces molecule in excited state Doppler limitations with method Movement of molecules during excitation Signature from 234/238 ratio, both depleted

Laser Isotope Separation 3 classes of laser isotope separations Photochemical Reaction of excited state molecule Atomic photoionization Ionization of excited state molecule Photodissociation Dissociation of excited state molecule AVLIS Atomic vapor laser isotope separation MLIS Molecular laser isotope separation

Laser isotope separation AVLIS U metal vapor High reactivity, high temperature Uses electron beam to produce vapor from metal sample Ionization potential 6.2 eV Multiple step ionization 238U absorption peak 502.74 nm 235U absorption peak 502.73 nm Deflection of ionized U by electromagnetic field

Laser Isotope Separation MLIS (LANL method) SILEX (Separation of Isotopes by Laser Excitation) in Australia Absorption by UF6 Initial IR excitation at 16 micron 235UF6 in excited state Selective excitation of 235UF6 Ionization to 235UF5 Formation of solid UF5 (laser snow) Solid enriched and use as feed to another excitation Process degraded by molecular motion\ Cool gas by dilution with H2 and nozzle expansion

Nuclear Fuel: Uranium-oxygen system A number of binary uranium-oxygen compounds UO Solid UO unstable, NaCl structure From UO2 heated with U metal Carbon promotes reaction, formation of UC UO2 Reduction of UO3 or U3O8 with H2 from 800 ºC to 1100 ºC CO, C, CH4, or C2H5OH can be used as reductants O2 presence responsible for UO2+x formation Large scale preparation UO4, (NH4)2U2O7, or (NH4)4UO2(CO3)3 Calcination in air at 400-500 ºC H2 at 650-800 ºC UO2has high surface area

Uranium-oxygen U3O8 From oxidation of UO2 in air at 800 ºC a phase uranium coordinated to oxygen in pentagonal bipyrimid b phase results from the heating of the a phase above 1350 ºC Slow cooling

Uranium-oxygen UO3 Seven phases can be prepared A phase (amorphous) Heating in air at 400 ºC UO4.2H2O, UO2C2O4.3H2O, or (HN4)4UO2(CO3)3 Prefer to use compounds without N or C a-phase Crystallization of A-phase at 485 ºC at 4 days O-U-O-U-O chain with U surrounded by 6 O in a plane to the chain Contains UO22+ b-phase Ammonium diuranate or uranyl nitrate heated rapidly in air at 400-500 ºC g-phase prepared under O2 6-10 atmosphere at 400-500 ºC

Uranium-oxygen UO3 hydrates 6 different hydrated UO3 compounds UO3.2H2O Anhydrous UO3 exposed to water from 25-70 ºC Heating resulting compound in air to 100 ºC forms a-UO3.0.8 H2O a-UO2(OH)2 [a-UO3.H2O] forms in hydrothermal experiments b-UO3.H2O also forms

Uranium-oxygen single crystals UO2 from the melt of UO2 powder Arc melter used Vapor deposition 2.0 ≤ U/O ≤ 2.375 Fluorite structure Uranium oxides show range of structures Some variation due to existence of UO22+ in structure Some layer structures

UO2 Heat Capacity Room temperature to 1000 K Increase in heat capacity due to harmonic lattice vibrations Small contribution to thermal excitation of U4+ localized electrons in crystal field 1000-1500 K Thermal expansion induces anharmonic lattice vibration 1500-2670 K Lattice and electronic defects

Vaporization of UO2 Above and below the melting point Number of gaseous species observed U, UO, UO2, UO3, O, and O2 Use of mass spectrometer to determine partial pressure for each species For hypostiochiometric UO2, partial pressure of UO increases to levels comparable to UO2 O2 increases dramatically at O/U above 2

Uranium oxide chemical properties Oxides dissolve in strong mineral acids Valence does not change in HCl, H2SO4, and H3PO4 Sintered pellets dissolve slowly in HNO3 Rate increases with addition of NH4F, H2O2, or carbonates H2O2 reaction UO2+ at surface oxidized to UO22+

Solid solutions with UO2 Solid solutions formed with group 2 elements, lanthanides, actinides, and some transition elements (Mn, Zr, Nb, Cd) Distribution of metals on UO2 fluorite-type cubic crystals based on stoichiometry Prepared by heating oxide mixture under reducing conditions from 1000 ºC to 2000 ºC Powders mixed by co-precipitation or mechanical mixing of powders Written as MyU1-yO2+x x is positive and negative

Solid solutions with UO2 Lattice parameter change in solid solution Changes nearly linearly with increase in y and x MyU1-yO2+x Evaluate by change of lattice parameter with change in y δa/δy a is lattice parameter in Å Can have both negative and positive values δa/δy is large for metals with large ionic radii δa/δx terms negative and between -0.11 to -0.3 Varied if x is positive or negative

Solid solutions of UO2 Tetravalent MyU1-yO2+x Zr solid solutions Large range of systems y=0.35 highest value Metastable at lower temperature Th solid solution Continuous solid solutions for 0≤y≤1 and x=0 For x>0, upper limit on solubility y=0.45 at 1100 ºC to y=0.36 at 1500 ºC Also has variation with O2 partial pressure At 0.2 atm., y=0.383 at 700 ºC to y=0.068 at 1500 ºC

Solid solutions of UO2 Tri and tetravalent MyU1-yO2+x Cerium solid solutions Continuous for y=0 to y=1 For x<0, solid solution restricted to y≤0.35 Two phases (Ce,U)O2 and (Ce,U)O2-x x<-0.04, y=0.1 to x<-0.24, y=0.7 0≤x≤0.18, solid solution y<0.5 Air oxidized hyperstoichiometric y 0.56 to 1 at 1100 ºC y 0.26-1.0 1550 ºC Tri and divalent Reducing atmosphere x is negative fcc Solid solution form when y is above 0 Maximum values vary with metal ion Oxidizing atmosphere Solid solution can prevent formation of U3O8 Some systematics in trends For Nd, when y is between 0.3 and 0.5, x = 0.5-y

Solid solution UO2 Oxygen potential Zr solid solution Lower than the UO2+x system x=0.05, y=0.3 -270 kJ/mol for solid solution -210 kJ/mol for UO2+x Th solid solution Increase in DG with increasing y Compared to UO2 difference is small at y less than 0.1 Ce solid solution Wide changes over y range due to different oxidation states Shape of the curve is similar to Pu system, but values differ Higher DG for CeO2-x compared to PuO2-x

Metallic Uranium Three different phase a, b, g phases Dominate at different temperatures Uranium is strongly electropositive Cannot be prepared through H2 reduction Metallic uranium preparation UF4 or UCl4 with Ca or Mg UO2 with Ca Electrodeposition from molten salt baths

Metallic Uranium phases a-phase Room temperature to 942 K Orthorhombic U-U distance 2.80 Å Unique structure type b-phase Exists between 668 and 775 ºC Tetragonal unit cell g-phase Formed above 775 ºC bcc structure Metal has plastic character Gamma phase soft, difficult fabrication Beta phase brittle and hard Paramagnetic Temperature dependence of resistivity Alloyed with Mo, Nb, Nb-Zr, and Ti a‐phase U-U distances in layer (2.80±0.05) Å and between layers 3.26 Å b-phase

Intermetallic compounds Wide range of intermetallic compounds and solid solutions in alpha and beta uranium Hard and brittle transition metal compounds U6X, X=Mn, Fe, Co, Ni Noble metal compounds Ru, Rh, Pd Of interests for reprocessing Solid solutions with: Mo, Ti, Zr, Nb, and Pu

Uranium-Aluminum Phase Diagram Uranium-Titanium Phase Diagram

Chemical properties of uranium metal and alloys Reacts with most elements on periodic table Corrosion by O2, air, water vapor, CO, CO2 Dissolves in HCl Also forms hydrated UO2 during dissolution Non-oxidizing acid results in slow dissolution Sulfuric, phosphoric, HF Exothermic reaction with powered U metal and nitric Dissolves in base with addition of peroxide peroxyuranates

Review How is uranium chemistry linked with the fuel cycle What are the main oxidation states of the fission products and actinides Describe the uranium enrichment process What drives the speciation of actinides and fission products in fuel Understand the fundamental chemistry of the fission products and actinides Production Solution chemistry Speciation Spectroscopy

Questions What drives the speciation of actinides and fission products in spent nuclear fuel? What would be the difference between oxide and metallic fuel? Describe two processes for enriching uranium. Why does uranium need to be enriched? What else could be used instead of 235U? What are the similarities and differences between lanthanides and actinides? What are some trends in actinide chemistry?

Questions What are the different types of conditions used for separation of U from ore What is the physical basis for enriching U by gas and laser methods? What chemistry is exploited for solution based U enrichment Describe the basic chemistry for the production of Umetal Why is U alloyed? What are the natural isotopes of uranium Provide 5 reactions that use U metal as a starting reagent Describe the synthesis and properties of the uranium halides How is the O to U ratio for uranium oxides determined What are the trends in U solution chemistry What atomic orbitals form the molecular orbitals for UO22+

Pop Quiz What atomic orbitals form the molecular orbitals for UO22+