• 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
 Metal
• Focus on chemistry in the fuel cycle
 Speciation (chemical form)
 Oxidation state
 Ionic radius and molecular size
RFSS: Lecture 11 Uranium
Chemistry and the Fuel Cycle
• 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
 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
• Fission properties of uranium
 Defined importance of


 element and future investigations
Identified by Hahn in 1937
200 MeV/fission
2.5 neutrons
1

• Uranium acid-leach
• Extraction and conversion
Understand fundamental chemistry of uranium and its applications to the nuclear fuel cycle 2

Enriched UF
6
Calcination, Reduction UO
2
Pellet Control
40-60°C
Tubes
Fuel Fabrication
Other species for fuel nitrides, carbides
Other actinides: Pu, Th
3

• 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
 U(IV) as oxides, phosphates, silicates

 coordination polyhedra
• Pyrochlore

Mineral deposits based on major anion
A
1-2
B
2
O
6
X
0-1
 A=Na, Ca, Mn, Fe 2+ , Sr,Sb, Cs, Ba,
 B= Ti, Nb, Ta
 U(V) may be present when
*
*
From XANES spectroscopy
Goes to B site
Uraninite with oxidation

• Strong Lewis acid, Hard electron acceptor
 F >>Cl >Br  I -
 Same trend for O and N group
 based on electrostatic force as dominant factor
• Hydrolysis behavior
 U(IV)>U(VI)>>>U(III)>U(V)
• U(III) and U(V)
 No data in solution
 Base information on lanthanide or pentavalent actinides
• Uranyl(VI) most stable oxidation state in solution
 Uranyl(V) and U(IV) can also be in solution
 U(V) prone to disproportionation
 Stability based on pH and ligands
 Redox rate is limited by change in species
 Making or breaking yl oxygens
* UO
2
2+ +4H + +2e  U 4+ +2H
2
O
• 5f electrons have strong influence on actinide chemistry
 For uranyl, f-orbital overlap provide bonding
5

• Tri- and tetravalent U mainly related to organometallic compounds
 Cp
3
UCO and Cp
3
UCO +
 Cp=cyclopentadiene
* 5f CO p backbonding

Metal electrons to p of ligands
*
Decreases upon oxidation to U(IV)
• Uranyl(V) and (VI) compounds
 yl ions in aqueous systems unique for actinides
 VO
2
+ , MoO
2
2+ , WO maximize (p p
2
2+
2
2+
* Oxygen atoms are cis to
) )  M(d p

Linear MO known for compounds of Tc, Re, Ru, Os
*
Aquo structures unknown
 Short U=O bond distance of 1.75 Å for hexavalent, longer for pentavalent

Smaller effective charge on pentavalent U

Multiple bond characteristics, 1 and 2 with characteristics s
6

• Trivalent uranium


 Comparisons with trivalent actinides and lanthanides
• Tetravalent uranium

Very few studies of U(III) in solution
No structural information

Forms in very strong acid
 Requires >0.5 M acid to prevent hydrolysis
 Electrolysis of U(VI) solutions
* Complexation can drive oxidation
Coordination studied by XAFS
 Coordination number 9±1
* Not well defined
 U-O distance 2.42 Å
 O exchange examined by NMR
• Pentavalent uranium




Extremely narrow range of existence
Prepared by reduction of UO
2
2+ with Zn or H
No experimental information on structure
Quantum mechanical predictions
2 or dissolution of UCl
5
 U(V) is not stable but slowly oxidizes under suitable conditions in water
7

• 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
• Hexavalent uranium as uranyl in solution
8

• Uranyl (UO
2
2+ g
2 s u
2 p
) linear molecule
• Bonding molecular orbitals
 s g
4 p u
4

Order of HOMO is unclear
* p g
< p u
< s g
<< s

5f d and 5f f
LUMO u proposed

Gap for s based on 6p orbitals interactions

Bonding orbitals O 2p characteristics

Non bonding, antibonding 5f and 6d

Isoelectronic with UN
2
•
Pentavalent has electron in non-bonding orbital
9

0.126 M UO
2
2+
0.2
0.15
0.1
0.05
0
350 400 450
Wavelength (nm)
8 M HNO
3
4 M HNO
3
1 M HNO
3
0.1 M HNO
3
550

• yl oxygens force formal charge on U below 6
 Net charge 2.43 for UO
2 systems
(H
2
O)
5
2+ , 3.2 for fluoride
 Net negative 0.43 on oxygens
 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

• Speciation variation with uranium concentration
 Hydrolysis as example
 Precipitation at higher concentration
 Change in polymeric uranium species concentration
12
CHESS Calculation

• Preconcentration of ore
 Based on density of ore
• Leaching to extract uranium into aqueous phase
 Calcination prior to leaching
 Removal of carbonaceous or sulfur compounds
 Destruction of hydrated species
(clay minerals)
• Removal or uranium from aqueous phase
 Ion exchange
 Solvent extraction
 Precipitation


Acid solution leaching
* Sulfuric (pH 1.5)
 U(VI) soluble in sulfuric
 Anionic sulfate species
 Oxidizing conditions may be needed
 MnO
2
 Precipitation of Fe at pH 3.8
Carbonate leaching
 Formation of soluble anionic carbonate species
* UO
2
(CO
3
)
3
4-
 Precipitation of most metal ions in alkali solutions
 Bicarbonate prevents precipitation of
Na
2
U
2
O
7
* Formation of Na
NaOH addition
2
U
2
O
7 with further
 Gypsum and limestone in the host aquifers necessitates carbonate leaching
13

• Ion exchange
 U(VI) anions in sulfate and carbonate solution
 UO
2
(CO
 UO
2
(SO
4
3
)
)
3
4-
3
4-
 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 alkylamines
 Chemistry similar to ion exchange conditions
• Chemical precipitation
 Addition of base
 Peroxide
 Water wash, dissolve in nitric acid
 Ultimate formation of (NH
4 yellowcake
)
2
U
2
O
7
(ammonium diuranate),
 heating to form U
3
O
8 or UO
3
14

• Tributyl phosphate (TBP) extraction



Based on formation of nitrate species
UO
2
(NO
3
) x
2-x + (2-x)NO
3
+ 2TBP 
Process example of pulse column below
UO
2
(NO
3
)
2
(TBP)
2
15

• Once separated, uranium needs to be enriched for nuclear fuel
 Natural U is 0.7 % 235 U
• Different enrichment needs
 3.5 % 235 U for light water reactors
 > 90 % 235 U for submarine reactors
 235 U enrichment below 10 % cannot be used for a device
 Critical mass decreases with increased enrichment
 20 % 235 U critical mass for reflected device around
100 kg
 Low enriched/high enriched uranium boundary
16

• Exploit different nuclear properties between U isotopes to achieve enrichment
 Mass
 Size
 Shape
 Nuclear magnetic moment
 Angular momentum
• Massed based separations utilize volatile UF
 UF
6
6 formed from reaction of U compounds with F at elevated temperature
2
• Colorless, volatile solid at room temperature
 Density is 5.1 g/mL
 Sublimes at normal atmosphere
 Vapor pressure of 100 torr
 One atmosphere at 56.5 ºC
• O h point group
 U-F bond distance of 2.00 Å 17

• Very low viscosity
 7 mPoise
 Water =8.9 mPoise
 Useful property for enrichment
• Self diffusion of 1.9E-5 cm 2 /s
• Reacts with water
 UF
6
+ 2H
2
O  UO
2
F
2
+ 4HF
• Also reactive with some metals
• Does not react with Ni, Cu and Al
 Material made from these elements need for enrichment
18

• Volatile U gas ionized
 Atomic ions with charge +1 produced
• Ions accelerated in potential of kV
 Provides equal kinetic energies
 Overcomes large distribution based on thermal energies
• Ion in a magnetic field has circular path
 Radius ( r )
•
 m mass, v velocity, q ion charge, B magnetic field
For V acceleration potential v

2 Vq m r  c 2 Vm
B q r  mcv
19 qB

• Radius of an ion is proportional to square root of mass
 Higher mass, larger radius r  c 2 Vm
B q
• Requirements for electromagnetic separation process
 Low beam intensities
 High intensities have beam spreading
* Around 0.5 cm for 50 cm radius
 Limits rate of production
 Low ion efficiency
 Loss of material
• Caltrons used during Manhattan project
20

• Developed by Ernest Lawrence
 Cal. U-tron
• High energy use
 Iraqi Calutrons required about
1.5 MW each
 90 total
• Manhattan Project
 Alpha
 4.67 m magnet
 15% enrichment
 Some issues with heat from beams
 Shimming of magnetic fields to increase yield
 Beta
 Use alpha output as feed
* High recovery
21

• High proportion of world’s enriched U
 95 % in 1978
 40 % in 2003
• Separation based on thermal equilibrium
 All molecules in a gas mixture have same average kinetic energy
 lighter molecules have a higher velocity at same energy m 2
352 v
352
 m 2
349 v
349
• For 235 UF
*
6
 235 UF
6
E k
=1/2 mv and 238 UF
6
2 v
349 v
352
 m
352
 m
349 and is 0.429 % faster on average
352
349

1 .
00429
 why would UCl
6 for enrichment?
be much more complicated
22

• 235 UF
6 impacts barrier more often
• Barrier properties
 Resistant to corrosion by UF
6
 Ni and Al
2
O
3
 Hole diameter smaller than mean free path
 Prevent gas collision within barrier
 Permit permeability at low gas pressure
 Thin material
• Film type barrier
 Pores created in non-porous membrane
 Dissolution or etching
• Aggregate barrier
 Pores are voids formed between particles in sintered barrier
• Composite barrier from film and aggregate
23

• Barrier usually in tubes
 UF
6 introduced
• Gas control
 Heater, cooler, compressor
• Gas seals
• Operate at temperature above 70 °C and pressures below
0.5 atmosphere
• R=relative isotopic abundance (N
235
/N
238
)
• Quantifying behavior of an enrichment cell
 q=R product
/R tail
 Ideal barrier, R product
=R tail
(352/349) 1/2 ; q= 1.00429
24

• Small enrichment in any given cell
 q=1.00429 is best condition
 Real barrier efficiency ( e
 e
B
B
)
( q 1 ) e
( q can be used to determine total barrier area for a given enrichment
 e
B
= 0.7 is an industry standard
 Can be influenced by conditions observed
 
B ideal

1 )
 Pressure increase, mean free path decrease
 Increase in collision probability in pore
 Increase in temperature leads to increase velocity
 Increase UF
6 reactivity
• Normal operation about 50 % of feed diffuses
• Gas compression releases heat that requires cooling
 Large source of energy consumption
• Optimization of cells within cascades influences behavior of 234 U
 q=1.00573 (352/348) 1/2
 Higher amounts of 234 U, characteristic of feed
25

• Simple cascade
 Wasteful process
 High enrichment at end discarded
• Countercurrent
 Equal atoms condition, product enrichment equal to tails depletion
• Asymmetric countercurrent
 Introduction of tails or product into nonconsecutive stage
 Bundle cells into stages, decrease cells at higher enrichment
26

• Number of cells in each stage and balance of tails and product need to be considered
• Stages can be added to achieve changes in tailing depletion
 Generally small levels of tails and product removed
• Separative work unit (SWU)
 Energy expended as a function of amount of U processed and enriched degree per kg
 3 % 235 U
 3.8 SWU for 0.25 % tails
 5.0 SWU for 0.15 % tails
• Determination of SWU
 P product mass
 W waste mass
 F feedstock mass
 x
W
 x
P
 x
F waste assay product assay feedstock assay
27

• Centrifuge pushes heavier 238 UF
6 having more 235 UF
6 against wall with center
 Heavier gas collected near top
• Density related to UF
6 pressure
 Density minimum at center p ( r ) p ( 0 )
 e m w
2 r
2
2 RT
 m molecular mass, r radius and w angular velocity
• With different masses for the isotopes, p can be solved for each isotope p x
( r ) p ( 0 )
 e m x w
2 r
2
2 RT
28

• Total pressure is from partial pressure of each isotope
 Partial pressure related to mass
• Single stage separation
(q)
 Increase with mass difference, angular velocity, and radius
• For 10 cm r and 1000
Hz, for UF
 q=1.26
6
Gas distribution in centrifuge q
 e
( m
2
 m
1
) w
2 r
2
2 RT
29

• More complicated setup than diffusion
 Acceleration pressures, 4E5 atmosphere from previous example
 High speed requires balance
 Limit resonance frequencies
 High speed induces stress on materials
 Need high tensile strength
* alloys of aluminum or titanium
* maraging steel
 Heat treated martensitic steel
* composites reinforced by certain glass, aramid, or carbon fibers 30

• Gas extracted from center post with 3 concentric tubes
 Product removed by top scoop


Tails removed by bottom scoop
Feed introduced in center
• Mass load limitations


UF
6 needs to be in the gas phase
Low center pressure

 3.6E-4 atm for r = 10 cm
• Superior stage enrichment when compared to gaseous diffusion
Less power need compared to gaseous diffusion
 1000 MW e needs 120 K SWU/year
* Gas diffusion 9000 MJ/SWU
* centrifuge 180 MJ/SWU
• Newer installations compare to diffusion
 Tend to have no non-natural U isotopes
31

• Isotopic effect in atomic spectroscopy
 Mass, shape, nuclear spin
• Observed in visible part of spectra
• Mass difference in IR region
• Effect is small compared to transition energies
 1 in 1E5 for U species
• Use laser to tune to exact transition specie
 Produces molecule in excited state
• Doppler limitations with method
 Movement of molecules during excitation
• Signature from 234/238 ratio, both depleted
32

• 3 classes of laser isotope separations
 Photochemical
 Reaction of excited state molecule
 Atomic photoionization
 Ionization of excited state molecule
 Photodissociation
 Dissociation of excited state molecule
• AVLIS
 Atomic vapor laser isotope separation
• MLIS
 Molecular laser isotope separation
33

• AVLIS
 U metal vapor
 High reactivity, high temperature
 Uses electron beam to produce vapor from metal sample
• Ionization potential 6.2 eV
• Multiple step ionization
 238 U absorption peak
502.74 nm
 235 U absorption peak
502.73 nm
• Deflection of ionized U by electromagnetic field
34

• MLIS (LANL method) SILEX (Separation of
Isotopes by Laser Excitation) in Australia
 Absorption by UF
6
 Initial IR excitation at 16 micron
 235 UF
6 in excited state
 Selective excitation of 235 UF
6
 Ionization to 235 UF
5
 Formation of solid UF
5
(laser snow)
 Solid enriched and use as feed to another excitation
35