• • • • • • • Readings: Uranium chapter: http://radchem.nevada.edu/classes/r dch710/files/uranium.pdf Chemistry in the fuel cycle Uranium Solution Chemistry Separation Fluorination and enrichment Oxide Metal Focus on chemistry in the fuel.
Download ReportTranscript • • • • • • • Readings: Uranium chapter: http://radchem.nevada.edu/classes/r dch710/files/uranium.pdf Chemistry in the fuel cycle Uranium Solution Chemistry Separation Fluorination and enrichment Oxide Metal Focus on chemistry in the fuel.
• • • • • • •
Readings: Uranium chapter:
http://radchem.nevada.edu/classes/r 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
233 U, 235 U, 239 Pu
233 Need in sufficient concentration and geometry U and 239 Pu can be created in neutron flux 235
U 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
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RFSS: Part 2 Lecture 11 Uranium Chemistry and the Fuel Cycle Fission properties of uranium
Defined importance of element and future investigations
Identified by Hahn in 1937 200 MeV/fission 2.5 neutrons
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•
Nuclear Fuel: Uranium-oxygen system
A number of binary uranium-oxygen compounds
UO
Solid UO unstable, NaCl structure From UO 2
*
heated with U metal Carbon promotes reaction, formation of UC
UO 2
Reduction of UO 3 ºC
*
CO, C, CH 4 , or C 2 H 5 OH can be used as reductants O 2 presence responsible for UO 2+x formation Large scale preparation
* *
UO 4 , (NH 4 ) 2 U 2 O 7 , or (NH 4 ) 4 UO 2 (CO 3 ) 3 Calcination in air at 400-500 ºC
* *
or U 3 O 8 with H H 2 at 650-800 ºC UO 2 has high surface area 2 from 800 ºC to 1100
2
•
Uranium-oxygen
U 3 O 8
From oxidation of UO
2 in air at 800 ºC
a
phase uranium coordinated to oxygen in pentagonal bipyrimid
b
phase results from the heating of the
a
above 1350 ºC
Slow cooling phase
3
Uranium-oxygen
•
UO 3
Seven phases can be prepared
•
A phase (amorphous)
Heating in air at 400 ºC
*
UO 4 .
2H 2 O, UO 2 C 2 O 4 .
3H 2 O, or (HN 4 ) 4 UO 2 (CO 3 ) 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 UO 2 2+
b
-phase
Ammonium diuranate or uranyl nitrate heated rapidly in air at 400-500 ºC
g
-phase prepared under O 2 6-10 atmosphere at 400-500 ºC
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Uranium-oxygen
• •
UO 3
hydrates 6 different hydrated UO 3 compounds UO 3 .
2H 2 O Anhydrous UO 25-70 ºC 3 exposed to water from
Heating resulting compound in air to 100 ºC forms
a
-UO 3 .
0.8 H 2 O
a
-UO 2 (OH) 2 [
a
UO
3 .
b
H 2 O] forms in hydrothermal experiments -UO 3 .
H 2 O also forms
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• • •
Uranium-oxygen single crystals
UO
2 from the melt of UO 2 Arc melter used
Vapor deposition 2.0 ≤ U/O ≤ 2.375
Fluorite structure powder Uranium oxides show range of structures
Some variation due to existence of UO 2 2+ in structure
Some layer structures
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• • •
UO
2
Heat Capacity
Room temperature to 1000 K
Increase in heat capacity due to harmonic lattice vibrations
Small contribution to thermal excitation of U 4+ localized electrons in crystal field 1000-1500 K
Thermal expansion induces anharmonic lattice vibration 1500-2670 K
Lattice and electronic defects
7
Vaporization of UO
2
• •
Above and below the melting point Number of gaseous species observed
U, UO, UO 2 , UO 3 , O, and O 2
Use of mass spectrometer to determine partial pressure for each species For hypostiochiometric UO 2 , partial pressure
levels comparable to UO 2 O increases dramatically at O/U above 2
8
•
Uranium oxide chemical properties
Oxides dissolve in strong mineral acids
Valence does not change in HCl, H 2 SO 4 , and H 3 PO 4
Sintered pellets dissolve slowly in HNO 3
Rate increases with addition of NH
*
4 F, H 2 O 2 , or carbonates H 2 O 2 reaction
UO 2 + at surface oxidized to UO 2 2+
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Solid solutions with UO
2
• • • •
Solid solution
crystal structure unchanged by addition of another compound
mixture remains as single phase
ThO 2 -UO 2 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 UO 2 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 M y U 1-y O 2+x
x is positive and negative
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Solid solutions with UO
2
•
Lattice parameter change in solid solution
Changes nearly linearly with increase in y and x
M y U 1-y O 2+x
Evaluate by change of lattice parameter with change in y
*
δa/δy
a is lattice parameter in
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
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•
Tetravalent M y U 1-y O 2+x
Zr solid solutions
Large range of systems
Solid solutions of UO
2 y=0.35 highest value Metastable at lower temperature
•
Tri and tetravalent M y U 1-y O 2+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)O 2 and (Ce,U)O 2-x x<-0.04, y=0.1 to x<-0.24, y=0.7
0≤x≤0.18, solid solution y<0.5
Th solid solution
Continuous solid solutions for 0≤y≤1 and x=0
•
Air oxidized hyperstoichiometric
*
y 0.56 to 1 at 1100 ºC
*
y 0.26-1.0 1550 ºC
For x>0, upper limit on solubility
*
y=0.45 at 1100 ºC to y=0.36 at 1500 ºC Tri and divalent
Reducing atmosphere
x is negative
fcc structure Maximum values vary with metal ion
Also has variation with O 2 partial pressure
*
At 0.2 atm., y=0.383 at 700 ºC to y=0.068 at 1500 ºC
Oxidizing atmosphere
Solid solution can prevent formation of U 3 O 8
Some systematics in trends
*
For Nd, when y is between 0.3 and 0.5, x = 0.5-y
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U-Zr oxide system
13
•
Solid solution UO
2 Oxygen potential
Zr solid solution
Lower than the UO 2+x system
*
x=0.05, y=0.3
-270 kJ/mol for solid solution
-210 kJ/mol for UO 2+x Th solid solution
Increase in
D
G with increasing y Compared to UO 2
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
D
G for CeO compared to PuO 2-x 2-x
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• • •
Metallic Uranium
Three different phase
a, b, g
phases
Dominate at different temperatures Uranium is strongly electropositive
Cannot be prepared through H 2 reduction Metallic uranium preparation
UF 4 or UCl 4 with Ca or Mg
UO 2 with Ca Electrodeposition from molten salt baths
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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
a ‐phase U-U distances in layer (2.80±0.05) Å and between layers
Gamma phase soft, difficult fabrication
3.26 Å
Beta phase brittle and hard Paramagnetic Temperature dependence of resistivity Alloyed with Mo, Nb, Nb-Zr, and Ti
16 b -phase
•
Intermetallic compounds
Wide range of intermetallic compounds and solid solutions in alpha and beta uranium
Hard and brittle transition metal compounds
U 6 X, X=Mn, Fe, Co, Ni
Noble metal compounds
Ru, Rh, Pd
*
Of interests for reprocessing
Solid solutions with:
Mo, Ti, Zr, Nb, and Pu
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Uranium-Aluminum Phase Diagram Uranium-Titanium Phase Diagram
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Chemical properties of uranium metal and alloys
• • • • •
Reacts with most elements on periodic table
Corrosion by O 2 , air, 2 Dissolves in HCl
Also forms hydrated UO 2 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
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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
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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 UO 2 2+ What else could be used instead of 235 U as the fissile isotope in a reactor?
Describe two processes for enriching uranium. Why does uranium need to be enriched?
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Questions
• •
Respond to PDF Quiz 11 Post comments on the blog
http://rfssunlv.blogspot.com/
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