1 Uranium Chemistry and the Fuel Cycle • Chemistry in the fuel cycle § Uranium à Solution Chemistry à Separation à Fluorination and enrichment à 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 § 233 U, 235 U, 239 Pu § Need in sufficient concentration and geometry • 233 U and 239 Pu can be created in neutron flux • 235 U in nature § Need isotope enrichment Why is U important in the fuel cycle: induced fission cross section for 235 U and 238 U as function of the neutron energy.
64
Embed
1 Uranium Chemistry and the Fuel Cycle Chemistry in the fuel cycle §Uranium àSolution Chemistry àSeparation àFluorination and enrichment àMetal Focus on.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
Uranium Chemistry and the Fuel Cycle• Chemistry in the fuel cycle
§ Uraniumà Solution Chemistryà Separationà Fluorination and
enrichmentà 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
Why is U important in the fuel cycle: induced fission cross section for 235U and 238U as function of the neutron energy.
2
Nuclear properties of Uranium• Fission properties of
uranium§ Defined importance
of element and future investigations
§ Identified by Hahn in 1937
§ 200 MeV/fission§ 2.5 neutrons
• Natural isotopes§ 234,235,238U§ Ratios of isotopes established
à 234: 0.005±0.001, 68.9 aà 235: 0.720±0.001, 7.04E8 aà 238: 99.275±0.002, 4.5E9 a
• 233U from 232Th§ need fissile isotope initially
3
Chemistry overview• Uranium acid-leach • Extraction and conversion
4
Fuel Fabrication
Enriched UF6
UO2Calcination, Reduction
Tubes
Pellet Control40-60°C
Fuel Fabrication
Other species for fuelnitrides, carbides
Other actinides: Pu, Th
5
Uranium chemistry• Uranium solution
chemistry• Separation and
enrichment of U• Uranium separation
from ore§ Solvent extraction§ Ion exchange
• Separation of uranium isotopes§ Gas centrifuge§ Laser
• 200 minerals contain uranium§ Bulk are U(VI) minerals
à U(IV) as oxides, phosphates, silicates § Classification based on polymerization of
coordination polyhedra§ Mineral deposits based on major anion
• Pyrochlore § A1-2B2O6X0-1
à A=Na, Ca, Mn, Fe2+, Sr,Sb, Cs, Ba, Ln, Bi, Th, U
à B= Ti, Nb, Taà U(V) may be present when synthesized
under reducing conditions* XANES spectroscopy* Goes to B siteUraninite with oxidation
6
Aqueous solution complexes• Strong Lewis acid• Hard electron acceptor
§ F->>Cl->Br-I-
§ Same trend for O and N groupà based on electrostatic force as dominant factor
• Hydrolysis behavior§ U(IV)>U(VI)>>>U(III)>U(V)
• Uranium coordination with ligand can change protonation behavior § HOCH2COO- pKa=17, 3.6 upon complexation of UO2
à Inductive effect* Electron redistribution of coordinated ligand* Exploited in synthetic chemistry
• U(III) and U(V)§ No data in solution
à Base information on lanthanide or pentavalent actinides
7
Uranium solution chemistry
• Uranyl(VI) most stable oxidation state in solution§ Uranyl(V) and U(IV) can also be in solution
à U(V) prone to disproportionation § Stability based on pH and ligands§ Redox rate is limited by change in species
à Making or breaking yl oxygens
* UO22++4H++2e-U4++2H2O
• yl oxygens have slow exchange
§ Half life 5E4 hr in 1 M HClO4
• 5f electrons have strong influence on actinide chemistry§ For uranyl, f-orbital overlap provide bonding
8
Uranyl chemical bonding• Uranyl (UO2
2+) linear molecule• Bonding molecular orbitals
§ sg2 su
2 pg4 pu
4
à Order of HOMO is unclear* pg< pu< sg<< su
proposedØ Gap for s based on 6p orbitals interactions
§ 5fd and 5f f LUMO§ Bonding orbitals O 2p characteristics§ Non bonding, antibonding 5f and 6d§ Isoelectronic with UN2
• Pentavalent has electron in non-bonding orbital
9
10
Uranyl chemical bonding• Linear yl oxygens from 5f characteristic
§ 6d promotes cis geometry• yl oxygens force formal charge on U below 6
§ Net charge 2.43 for UO2(H2O)52+, 3.2 for fluoride systems
à Net negative 0.43 on oxygensà Lewis bases
* Can vary with ligand in equatorial plane* Responsible for cation-cation interaction* O=U=O- - -M* Pentavalent U yl oxygens more basic
• Small changes in U=O bond distance with variation in equatoral ligand
• Small changes in IR and Raman frequencies§ Lower frequency for pentavalent U§ Weaker bond
11
Uranium chemical bonding: oxidation states
• Tri- and tetravalent U mainly related to organometallic compounds§ Cp3UCO and Cp3UCO+
à Cp=cyclopentadiene * 5f CO p backbonding
Ø Metal electrons to p of ligands
* Decreases upon oxidation to U(IV)
• Uranyl(V) and (VI) compounds§ yl ions in aqueous systems unique for
actinidesà VO2
+, MoO22+, WO2
2+
* Oxygen atoms are cis to maximize (pp)M(dp)
à Linear MO22+ known for
compounds of Tc, Re, Ru, Os* Aquo structures unknown
§ Short U=O bond distance of 1.75 Å for hexavalent, longer for pentavalentà Smaller effective charge on
pentavalent U§ Multiple bond characteristics, 1 s
and 2 with p characteristics
12
Uranium solution chemistry: U(III)• Dissolution of UCl3 in water• Reduction of U(IV) or (VI) at Hg cathode
§ Evaluated by color changeà U(III) is green
• Very few studies of U(III) in solution• No structural information
§ Comparisons with trivalent actinides and lanthanides
13
Uranium solution chemistry• Tetravalent uranium
§ Forms in very strong acidà Requires >0.5 M acid to prevent hydrolysisà Electrolysis of U(VI) solutions
* Complexation can drive oxidation§ Coordination studied by XAFS
à Coordination number 9±1* Not well defined
à U-O distance 2.42 ŧ O exchange examined by NMR
• Pentavalent uranium§ Extremely narrow range of existence§ Prepared by reduction of UO2
2+ with Zn or H2 or dissolution of UCl5 in water
§ UV-irradiation of 0.5 M 2-propanol-0.2 M LiClO4 with U(VI) between pH 1.7 and 2.7à U(V) is not stable but slowly oxidizes under suitable conditions
§ No experimental information on structure§ Quantum mechanical predictions
14
Hexavalent Uranium• Large number of compounds prepared
§ Crystallization§ Hydrothermal
• Determination of hydrolysis constants from spectroscopic and titration§ Determine if polymeric species form§ Polynuclear species present except at
lowest concentration
15
Uranium speciation• Speciation variation with uranium concentration
§ Hydrolysis as example§ Precipitation at higher concentration
à Change in polymeric uranium species concentration
16
Uranium purification from ores: Using U chemistry in the fuel cycle
• Preconcentration of ore§ Based on density of ore
• Leaching to extract uranium into aqueous phase§ Calcination prior to
leachingà Removal of
carbonaceous or sulfur compounds
à Destruction of hydrated species (clay minerals)
• Removal or uranium from aqueous phase§ Ion exchange§ Solvent extraction§ Precipitation
• Use of cheap materials
§ Acid solution leaching* Sulfuric (pH 1.5)
Ø U(VI) soluble in sulfuricØ Anionic sulfate species
Ø Oxidizing conditions may be neededØ MnO2
Ø Precipitation of Fe at pH 3.8§ Carbonate leaching
à Formation of soluble anionic carbonate species
* UO2(CO3)34-
à Precipitation of most metal ions in alkali solutions
à Bicarbonate prevents precipitation of Na2U2O7
* Formation of Na2U2O7 with further NaOH addition
à Gypsum and limestone in the host aquifers necessitates carbonate leaching
17
Recovery of uranium from solutions• Ion exchange
§ U(VI) anions in sulfate and carbonate solutionà UO2(CO3)3
4-
à UO2(SO4)34-
§ Load onto anion exchange, elute with acid or NaCl • Solvent extraction
§ Continuous process§ Not well suited for carbonate solutions§ Extraction with alkyl phosphoric acid, secondary and tertiary
alkylaminesà Chemistry similar to ion exchange conditions
• Chemical precipitation§ Addition of base§ Peroxide
à Water wash, dissolve in nitric acidà Ultimate formation of (NH4)2U2O7 (ammonium diuranate),
• MLIS (LANL method) SILEX (Separation of Isotopes by Laser Excitation) in Australia
§ Absorption by UF6
§ Initial IR excitation at 16 micron
à 235UF6 in excited state
§ Selective excitation of 235UF6
§ Ionization to 235UF5
§ Formation of solid UF5 (laser snow)
§ Solid enriched and use as feed to another excitation• Process degraded by molecular motion\
§ Cool gas by dilution with H2 and nozzle expansion
42
Nuclear Fuel: Uranium-oxygen system• A number of binary uranium-oxygen compounds
§ UOà Solid UO unstable, NaCl structureà From UO2 heated with U metal
* Carbon promotes reaction, formation of UC§ UO2
à Reduction of UO3 or U3O8 with H2 from 800 ºC to 1100 ºC* CO, C, CH4, or C2H5OH can be used as reductants
à O2 presence responsible for UO2+x formationà Large scale preparation
* UO4, (NH4)2U2O7, or (NH4)4UO2(CO3)3
* Calcination in air at 400-500 ºC* H2 at 650-800 ºC* UO2has high surface area
43
Uranium-oxygen• U3O8
§ From oxidation of UO2 in air at 800 ºCà a phase uranium coordinated to oxygen in
pentagonal bipyrimid§ b phase results from the heating of the a phase
above 1350 ºCà Slow cooling
44
Uranium-oxygen• UO3
§ Seven phases can be prepared• A phase (amorphous)
à Heating in air at 400 ºC* UO4
.2H2O, UO2C2O4.3H2O, or
(HN4)4UO2(CO3)3
Ø 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 UO2
2+
• 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
45
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
à b-UO3.H2O also
forms
46
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
47
48
UO2 Heat Capacity• Room temperature to 1000
K§ Increase in heat
capacity due to harmonic lattice vibrationsà 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
49
Vaporization of UO2
• Above and below the melting point
• Number of gaseous species observed§ U, UO, UO2, UO3, O, and O2
à Use of mass spectrometer to determine partial pressure for each species
à For hypostiochiometric UO2, partial pressure of UO increases to levels comparable to UO2
à O2 increases dramatically at O/U above 2
50
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
à Rate increases with addition of NH4F, H2O2, or carbonates
* H2O2 reaction
Ø UO2+ at surface oxidized to UO2
2+
51
Solid solutions with UO2
• 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
52
Solid solutions with UO2
• Lattice parameter change in solid solution§ Changes nearly linearly with increase in y and x
à MyU1-yO2+x
à Evaluate by change of lattice parameter with change in y* δa/δy
Ø a is lattice parameter in ÅØ 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
à Varied if x is positive or negative
53
Solid solutions of UO2
• Tetravalent MyU1-yO2+x
§ Zr solid solutionsà Large range of systemsà y=0.35 highest valueà Metastable at lower temperature
§ Th solid solutionà Continuous solid solutions for 0≤y≤1 and x=0à For x>0, upper limit on solubility
* y=0.45 at 1100 ºC to y=0.36 at 1500 ºCà Also has variation with O2 partial pressure
* At 0.2 atm., y=0.383 at 700 ºC to y=0.068 at 1500 ºC
54
Solid solutions of UO2
• Tri and tetravalent MyU1-yO2+x
§ Cerium solid solutionsà Continuous for y=0 to y=1à For x<0, solid solution restricted to y≤0.35
* Two phases (Ce,U)O2 and (Ce,U)O2-x
à x<-0.04, y=0.1 to x<-0.24, y=0.7à 0≤x≤0.18, solid solution y<0.5à Air oxidized hyperstoichiometric
* y 0.56 to 1 at 1100 ºC* y 0.26-1.0 1550 ºC
• Tri and divalent§ Reducing atmosphere
à x is negativeà fccà Solid solution form when y is above 0à Maximum values vary with metal ion
§ Oxidizing atmosphereà Solid solution can prevent formation of U3O8
à Some systematics in trends* For Nd, when y is between 0.3 and 0.5, x = 0.5-y
55
Solid solution UO2
• Oxygen potential § Zr solid solution
à Lower than the UO2+x system* x=0.05, y=0.3
Ø -270 kJ/mol for solid solution
Ø -210 kJ/mol for UO2+x
§ Th solid solutionà Increase in DG with
increasing yà Compared to UO2 difference
is small at y less than 0.1§ Ce solid solution
à Wide changes over y range due to different oxidation states
à Shape of the curve is similar to Pu system, but values differ
* Higher DG for CeO2-x
compared to PuO2-x
56
Metallic Uranium• Three different phase
§ , , a b g phasesà 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
57
Metallic Uranium phases
• a-phase§ Room temperature to 942 K§ Orthorhombic § U-U distance 2.80 ŧ 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
b-phase
a‐phase U-U distances in layer (2.80±0.05) Å and between layers
Intermetallic compounds• Wide range of intermetallic compounds and solid solutions in alpha and
beta uranium§ Hard and brittle transition metal compounds
à U6X, X=Mn, Fe, Co, Ni§ Noble metal compounds
à Ru, Rh, Pd* Of interests for reprocessing
§ Solid solutions with:à Mo, Ti, Zr, Nb, and Pu
59
Uranium-Aluminum Phase Diagram
Uranium-Titanium Phase Diagram
60
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
61
Review
• How is uranium chemistry linked with the fuel cycle• What are the main oxidation states of the fission products and
actinides• Describe the uranium enrichment process• What drives the speciation of actinides and fission products in fuel• Understand the fundamental chemistry of the fission products and
1. What drives the speciation of actinides and fission products in spent nuclear fuel? What would be the difference between oxide and metallic fuel?
2. Describe two processes for enriching uranium. Why does uranium need to be enriched? What else could be used instead of 235U?
3. What are the similarities and differences between lanthanides and actinides?
4. What are some trends in actinide chemistry?
63
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?
• What chemistry is exploited for solution based U enrichment• Describe the basic chemistry for the production of Umetal• Why is U alloyed?• What are the natural isotopes of uranium• Provide 5 reactions that use U metal as a starting reagent• 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 UO2
2+
64
Pop Quiz
• What atomic orbitals form the molecular orbitals for UO2