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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