RFSS: Part 2 Lecture 11 Uranium Chemistry and the Fuel Cycle

1. RFSS: Part 2 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 • Oxide • 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. 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 • UO2 has high surface area

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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+

10. Solid solutions with UO2 • Solid solution • crystal structure unchanged by addition of another compound • mixture remains as single phase • ThO2-UO2 is a solid solution • 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

11. 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

12. 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 structure • 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 • 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

13. U-Zr oxide system

14. 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

15. 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

16. 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

17. 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

18. Uranium-Aluminum Phase Diagram Uranium-Titanium Phase Diagram

19. 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

20. Review • How is uranium chemistry linked with the fuel cycle • What are the main oxidation states uranium • Describe the uranium enrichment process • Mass based • Laser bases • Understand the fundamental chemistry of uranium as it relates to: • Production • Solution chemistry • Speciation • Spectroscopy

21. 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? • Describe the basic chemistry for the production of U metal • What are the natural isotopes of uranium • 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+ • What else could be used instead of 235U as the fissile isotope in a reactor? • Describe two processes for enriching uranium. Why does uranium need to be enriched?

22. Pop Quiz • What atomic orbitals form the molecular orbitals for UO22+? These are made from the oxygen and uranium orbitals. • Provide comments in the blog • Bring answer to next class or e-mail