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RFSS: Lecture 11 Uranium Chemistry and the Fuel Cycle. Readings: Uranium chapter: http://radchem.nevada.edu/classes/rdch710/files/uranium.pdf Chemistry in the fuel cycle Uranium Solution Chemistry Separation Fluorination and enrichment Metal Focus on chemistry in the fuel cycle

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RFSS: Lecture 11 Uranium Chemistry and the Fuel Cycle


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    1. RFSS: Lecture 11 Uranium Chemistry and the Fuel Cycle • Readings: Uranium chapter: • http://radchem.nevada.edu/classes/rdch710/files/uranium.pdf • 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 • 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 • Fission properties of uranium • Defined importance of element and future investigations • Identified by Hahn in 1937 • 200 MeV/fission • 2.5 neutrons

    2. U Fuel Cycle Chemistry Overview • Extraction and conversion • Uranium acid-leach Understand fundamental chemistry of uranium and its applications to the nuclear fuel cycle

    3. 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

    4. 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 • From XANES spectroscopy • Goes to B site Uraninite with oxidation

    5. Uranium solution chemistry overview • 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) • U(III) and U(V) • No data in solution • Base information on lanthanide or pentavalent actinides • 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 • 5f electrons have strong influence on actinide chemistry • For uranyl, f-orbital overlap provide bonding

    6. 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

    7. Uranium solution chemistry • Trivalent uranium • Very few studies of U(III) in solution • No structural information • Comparisons with trivalent actinides and lanthanides • 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 • U(V) is not stable but slowly oxidizes under suitable conditions • No experimental information on structure • Quantum mechanical predictions

    8. 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 • Hexavalent uranium as uranyl in solution

    9. 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

    10. Uranyl chemical bonding • 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

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

    12. 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 • 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

    13. 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

    14. 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

    15. 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

    16. 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

    17. 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 need for enrichment

    18. 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

    19. Uranium Enrichment: Electromagnetic Separation • Radius of an ion is proportional to square root of mass • Higher mass, larger radius • Requirements 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

    20. 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

    21. 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?

    22. Gaseous Diffusion • 235UF6impacts barrier more often • Barrier properties • Resistant to corrosion by UF6 • 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

    23. 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

    24. 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 • Optimization of cells within cascades influences behavior of 234U • q=1.00573 (352/348)1/2 • Higher amounts of 234U, characteristic of feed

    25. 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

    26. 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

    27. 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

    28. 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

    29. 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

    30. 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

    31. 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

    32. 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

    33. 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

    34. 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