CHEM 312: Part 2 Lecture 12 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

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

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 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

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 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

U-Zr oxide system

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 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

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?

Questions Provide comments in the blog Respond to PDF Quiz 12