1. 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

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 ºC
H2 at 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 ºC
g-phase prepared under O atmosphere at ºC

5. Uranium-oxygen UO3 hydrates 6 different hydrated UO3 compounds
UO3.2H2O
Anhydrous UO3 exposed to water from 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 K
Thermal expansion induces anharmonic lattice vibration 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
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 to -0.3
Varied if x is positive or negative

12. 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 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. Questions
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