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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

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Uranium Chemistry and the Fuel Cycle


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    1. 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.

    2. 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

    3. Chemistry overview • Extraction and conversion • Uranium acid-leach

    4. 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

    5. 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

    6. 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

    7. 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

    8. Uranyl chemical bonding • Uranyl (UO22+) linear molecule • Bonding molecular orbitals • sg2 su2 pg4 pu4 • Order of HOMO is unclear • pg<pu<sg<< suproposed • 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

    9. 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

    10. 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

    11. 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

    12. 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

    13. 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

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

    15. 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

    16. 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

    17. 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

    18. 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

    19. 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

    20. 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

    21. 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

    22. 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

    23. 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

    24. Gaseous Diffusion • High proportion of world’s enriched U • 95 % in 1978 • 40 % in 2003 • 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 235UF6and 238UF6 • 235UF6 and is 0.429 % faster on average • why would UCl6 be much more complicated for enrichment?

    25. Gaseous Diffusion • 235UF6impacts 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

    26. 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

    27. 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

    28. 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

    29. 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

    30. 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

    31. 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

    32. 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

    33. 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

    34. 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

    35. 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 MWeneeds 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

    36. Centrifuges Natanz US

    37. 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

    38. 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

    39. 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

    40. 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

    41. 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

    42. 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

    43. 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

    44. 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

    45. 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

    46. 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

    47. 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

    48. 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+