CHEM 312: Part 2 Lecture 12 Uranium Chemistry and the Fuel Cycle
Readings:Uranium chapter: 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: 0.001, 68.9 a 235: 0.001, 7.04E8 a 238: 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 C to 1100 C 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 C H2 at C UO2 has high surface area Uranium-oxygen U3O8 From oxidation of UO2 in air at 800 C
a phase uranium coordinated to oxygen in pentagonal bipyrimid b phase results from the heating of the a phase above 1350 C Slow cooling Uranium-oxygen UO3 Seven phases can be prepared A phase (amorphous)
Heating in air at 400 C 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 C 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 C g-phase prepared under O atmosphere at C Uranium-oxygen UO3 hydrates 6 different hydrated UO3 compounds
UO3.2H2O Anhydrous UO3 exposed to water from C Heating resulting compound in air to 100 C 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 K Thermal expansion induces anharmonic lattice vibration 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 C to 2000 C 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 a/y a is lattice parameter in Can have both negative and positive values a/y is large for metals with large ionic radii a/x terms negative and between 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