Transcript • • • • • • • 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.

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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
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: 0.005±0.001, 68.9 a
 235: 0.720±0.001, 7.04E8 a
 238: 99.275±0.002, 4.5E9 a
RFSS: 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
1
U Fuel Cycle Chemistry Overview
• Uranium acid-leach
• Extraction and conversion
Understand fundamental chemistry of uranium and
its applications to the nuclear fuel cycle
2
Fuel Fabrication
Enriched UF6
Calcination, Reduction
Pellet Control
40-60°C
UO2
Tubes
Fuel Fabrication
Other species for fuel
nitrides, carbides
Other actinides: Pu, Th
3
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
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200 minerals contain uranium

Bulk are U(VI) minerals
 U(IV) as oxides, phosphates, silicates

Classification based on polymerization of
coordination polyhedra

Mineral deposits based on major anion
Pyrochlore

A1-2B2O6X0-1
 A=Na, Ca, Mn, Fe2+, Sr,Sb, Cs, Ba,
Ln, Bi, Th, U
 B= Ti, Nb, Ta
 U(V) may be present when
synthesized under reducing
conditions
* From XANES spectroscopy
4
* Goes to B site
Uraninite with oxidation
Uranium solution chemistry overview
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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
* UO22++4H++2e-U4++2H2O
• 5f electrons have strong influence on actinide chemistry

For uranyl, f-orbital overlap provide bonding
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Uranium chemical bonding: oxidation states
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Tri- and tetravalent U mainly related to
organometallic compounds

Cp3UCO and Cp3UCO+
 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
 VO2+, MoO22+, WO22+
* Oxygen atoms are cis to
maximize (pp)M(dp)
 Linear MO22+ known for
compounds of Tc, Re, Ru, Os
* Aquo structures unknown
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Short U=O bond distance of 1.75 Å
for hexavalent, longer for
pentavalent
 Smaller effective charge on
pentavalent U
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Multiple bond characteristics, 1 s
and 2 with p characteristics
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Uranium solution chemistry
Trivalent uranium
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Very few studies of U(III) in solution
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No structural information
 Comparisons with trivalent actinides and lanthanides
Tetravalent uranium
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Forms in very strong acid
 Requires >0.5 M acid to prevent hydrolysis
 Electrolysis of U(VI) solutions
* Complexation can drive oxidation
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Coordination studied by XAFS
 Coordination number 9±1
* Not well defined
 U-O distance 2.42 Å
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O exchange examined by NMR
Pentavalent uranium
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Extremely narrow range of existence

Prepared by reduction of UO22+ with Zn or H2 or dissolution of UCl5 in water
 U(V) is not stable but slowly oxidizes under suitable conditions
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No experimental information on structure

Quantum mechanical predictions
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Hexavalent Uranium
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Large number of compounds prepared
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Crystallization
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Hydrothermal
Determination of hydrolysis constants
from spectroscopic and titration
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Determine if polymeric species form
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Polynuclear species present except at
lowest concentration
Hexavalent uranium as uranyl in solution
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Uranyl chemical bonding
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Uranyl (UO22+) linear molecule
Bonding molecular orbitals
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sg2 su2 pg4 pu4
 Order of HOMO is unclear
* pg< pu< sg<< su proposed
 Gap for s based on 6p orbitals interactions
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5fd and 5ff LUMO
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Bonding orbitals O 2p characteristics
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Non bonding, antibonding 5f and 6d
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Isoelectronic with UN2
Pentavalent has electron in non-bonding orbital
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0.126 M UO
2+
2
0.2
8 M HNO
3
4 M HNO
0.15
3
1 M HNO
Absorbance
3
0.1 M HNO
3
0.1
0.05
0
350
400
450
Wavelength (nm)
500
10
550
Uranyl chemical bonding
• yl oxygens force formal charge on U below 6
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Net charge 2.43 for UO2(H2O)52+, 3.2 for fluoride
systems
 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
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Lower frequency for pentavalent U
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Weaker bond
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Uranium speciation
• Speciation variation with uranium concentration
 Hydrolysis as example
 Precipitation at higher concentration
 Change in polymeric uranium species concentration
CHESS Calculation
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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
leaching
 Removal of
carbonaceous or
sulfur compounds
 Destruction of
hydrated species
(clay minerals)
• Removal or uranium from
aqueous phase
 Ion exchange
 Solvent extraction
 Precipitation
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Acid solution leaching
* Sulfuric (pH 1.5)
 U(VI) soluble in sulfuric
 Anionic sulfate species
 Oxidizing conditions may be needed
 MnO2
 Precipitation of Fe at pH 3.8
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Carbonate leaching
 Formation of soluble anionic carbonate
species
* UO2(CO3)34 Precipitation of most metal ions in
alkali solutions
 Bicarbonate prevents precipitation of
Na2U2O7
* Formation of Na2U2O7 with further
NaOH addition
 Gypsum and limestone in the host
aquifers necessitates carbonate
leaching
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Recovery of uranium from solutions
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Ion exchange
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U(VI) anions in sulfate and carbonate solution
 UO2(CO3)34 UO2(SO4)34
Load onto anion exchange, elute with acid or NaCl
Solvent extraction
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Continuous process
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Not well suited for carbonate solutions
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Extraction with alkyl phosphoric acid, secondary and tertiary
alkylamines
 Chemistry similar to ion exchange conditions
Chemical precipitation
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Addition of base
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Peroxide
 Water wash, dissolve in nitric acid
 Ultimate formation of (NH4)2U2O7 (ammonium diuranate),
yellowcake
 heating to form U3O8 or UO3
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Uranium purification
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Tributyl phosphate (TBP) extraction
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Based on formation of nitrate species
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UO2(NO3)x2-x + (2-x)NO3- + 2TBP UO2(NO3)2(TBP)2
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Process example of pulse column below
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Uranium enrichment
• Once separated, uranium needs to be enriched for
nuclear fuel
 Natural U is 0.7 % 235U
• Different enrichment needs
 3.5 % 235U for light water reactors
 > 90 % 235U for submarine reactors
 235U enrichment below 10 % cannot be used for a
device
 Critical mass decreases with increased
enrichment
 20 % 235U critical mass for reflected device around
100 kg
 Low enriched/high enriched uranium
boundary
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Uranium enrichment
• Exploit different
nuclear properties
between U isotopes to
achieve enrichment
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Mass
Size
Shape
Nuclear magnetic
moment
Angular momentum
• Massed based
separations utilize
volatile UF6
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UF6 formed from
reaction of U
compounds with F2
at elevated
temperature
• Colorless, volatile solid at room
temperature
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Density is 5.1 g/mL
Sublimes at normal atmosphere
Vapor pressure of 100 torr
 One atmosphere at 56.5 ºC
• Oh point group

U-F bond distance of 2.00 Å
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Uranium Hexafluoride
• Very low viscosity
 7 mPoise
Water =8.9 mPoise
Useful property for enrichment
• Self diffusion of 1.9E-5 cm2/s
• Reacts with water
 UF6 + 2H2O UO2F2 + 4HF
• Also reactive with some metals
• Does not react with Ni, Cu and Al
 Material made from these elements need for
enrichment
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Uranium Enrichment: Electromagnetic
Separation
• 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
m cv
r
 Radius (r)
qB
m mass, v velocity, q ion charge, B
magnetic field
2Vq
• For V acceleration potential v 
m
r
c 2Vm
B
q
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Uranium Enrichment: Electromagnetic
Separation
• Radius of an ion is proportional to square root of
mass
c 2Vm
r
 Higher mass, larger radius
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
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Calutron
• 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
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Gaseous Diffusion
• 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
2
2
same energy
m352 v352
 m349 v349
* Ek=1/2 mv2
v349
m352
352


 1.00429
• For 235UF6 and 238UF6
v352
m349
349
 235UF6 and is 0.429 % faster on average
 why would UCl6 be much more complicated
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for enrichment?
Gaseous Diffusion
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235UF
6 impacts
barrier more often
Barrier properties
 Resistant to corrosion by UF6
 Ni and Al2O3
 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
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Gaseous Diffusion
• Barrier usually in tubes
 UF6 introduced
• Gas control
 Heater, cooler, compressor
• Gas seals
• Operate at temperature above 70 °C and pressures below
0.5 atmosphere
• R=relative isotopic abundance (N235/N238)
• Quantifying behavior of an enrichment cell
 q=Rproduct /Rtail
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Ideal barrier, Rproduct =Rtail(352/349)1/2; q= 1.00429
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Gaseous Diffusion
• Small enrichment in any given cell
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q=1.00429 is best condition
(qobserved 1)  e B (qideal 1)
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Real barrier efficiency (eB)
 eB can be used to determine total barrier area for a given
enrichment
 eB = 0.7 is an industry standard
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Can be influenced by conditions
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Pressure increase, mean free path decrease
 Increase in collision probability in pore
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Increase in temperature leads to increase velocity
 Increase UF6 reactivity
• Normal operation about 50 % of feed diffuses
• Gas compression releases heat that requires cooling
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Large source of energy consumption
• Optimization of cells within cascades influences behavior of 234U
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q=1.00573 (352/348)1/2
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Higher amounts of 234U, characteristic of feed
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Gaseous Diffusion
• 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
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Gaseous Diffusion
• 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
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Generally small levels of tails and product removed
• Separative work unit (SWU)
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Energy expended as a function of amount of U processed
and enriched degree per kg

3 % 235U
 3.8 SWU for 0.25 % tails
 5.0 SWU for 0.15 % tails
• Determination of SWU
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P product mass
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W waste mass
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F feedstock mass
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xW waste assay
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xP product assay
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xF feedstock assay
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Gas centrifuge
• Centrifuge pushes heavier 238UF6 against wall with center
having more 235UF6
 Heavier gas collected near top
• Density related to UF6 pressure
 Density minimum at center
p(r )
e
p(0)
mw 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 )
e
p(0)
mxw 2 r 2
2 RT
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Gas Centrifuge
• 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 UF6
 q=1.26
Gas distribution in centrifuge
qe
( m2  m1 )w 2 r 2
2 RT
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Gas Centrifuge
• 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,
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aramid, or carbon fibers
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Gas extracted from center post with 3 concentric
tubes
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Product removed by top scoop
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Tails removed by bottom scoop
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Feed introduced in center
Mass load limitations
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UF6 needs to be in the gas phase
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Low center pressure
 3.6E-4 atm for r = 10 cm
Superior stage enrichment when compared to gaseous
diffusion
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Less power need compared to gaseous
diffusion
 1000 MWe needs 120 K SWU/year
* Gas diffusion 9000 MJ/SWU
* centrifuge 180 MJ/SWU
Newer installations compare to diffusion
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Tend to have no non-natural U isotopes
Gas Centrifuge
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Laser Isotope Separation
• 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
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Laser Isotope Separation
• 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
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Laser isotope separation
• 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
 238U absorption peak
502.74 nm
 235U absorption peak
502.73 nm
• Deflection of ionized U by
electromagnetic field
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Laser Isotope Separation
• MLIS (LANL method) SILEX (Separation of
Isotopes by Laser Excitation) in Australia
 Absorption by UF6
 Initial IR excitation at 16 micron
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
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