Uranium Chemistry and the Fuel Cycle

1. Uranium Chemistry and the Fuel Cycle • 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 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. Acid-Leach Process for U Milling U ore Water Crushing & Grinding Slurry H2SO4 Steam NaClO3 40-60°C Acid Leaching Separation Tailings Organic Solvent Solvent Extraction NH4+ Recovery, Precipitation Drying (U3O8)

4. Converting U with Fluorination HNO3 U ore concentrates Solvent extraction purification Conversion to UO3 H2 Reduction UO2 HF UF4 F2 Mg U metal UF6 MgF2

5. 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

6. 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

7. Uranyl chemical bonding • Uranyl (UO22+) linear molecule • Uranyl(VI) most stable oxidation state in solution • 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. Uranium chemical bonding: oxidation states • 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

10. 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

11. 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

12. Micromole/L UO22+

13. Millimole/L UO22+

14. 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

15. 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

16. Uranium purification • TBP extraction • Based on formation of nitrate species • UO2(NO3)x2-x + (2-x)NO3- + 2TBP UO2(NO3)2(TBP)2

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. Calutron

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

31. Gaseous Diffusion • Determination of SWU • P product mass • W waste mass • F feedstock mass • xW waste assay • xP product assay • xF feedstock assay

32. 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

33. 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

34. 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

35. 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

36. 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

37. Gas Centrifuge • 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

38. Centrifuges Natanz US

39. 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

40. 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

41. 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

42. 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

43. Nuclear Fuel: Uranium-oxygen system • A number of binary uranium-oxygen compounds • 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

44. 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

45. 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

46. 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

47. 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

48. 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

49. 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

50. 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