Fuel Cycle Chemistry
• Chemistry in the fuel cycle

Uranium
 Separation
 Fluorination and enrichment
• Chemistry in fuel

speciation
• Fundamental of fission products and actinides

Production

Solution chemistry

Speciation

Spectroscopy
• Focus on chemistry in the fuel cycle

Speciation (chemical form)

Oxidation state

Ionic radius and molecular size
11-1
Reactor basics
• 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
induced fission cross section for 235U and 238U as
function of the neutron energy.
11-2
Nuclear properties
• 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
 234: 0.005±0.001
 235: 0.720±0.001
 238: 99.275±0.002
• 233U from 232Th
11-3
Uranium chemistry
• Separation and enrichment of U
• Uranium separation from ore
 Solvent extraction
 Ion exchange
• Separation of uranium isotopes
 Gas centrifuge
 Laser
11-4
Natural U chemistry
• Natural uranium consists of 3 isotopes
 234U, 235U and 238U
• Members of the natural decay series
 Earth’s crust contains 3 - 4 ppm U
 As abundant as As or B
• U is also chemically toxic
 Precautions should be taken against inhaling
uranium dust
 Threshold limit is 0.20 mg/m3 air
 About the same as for lead
• U is found in large granitic rock bodies formed by
slow cooling of the magma about 1.7 - 2.5 E 9 years
ago
11-5
Natural U chemistry
•
•
•
•
•
•
U is also found in younger rocks at higher concentrations called “ore
bodies”

Ore bodies are located downstream from mountain ranges
 Atmosphere became oxidizing about 1E9 years ago
 Rain penetrated into rock fractures, oxidizing the uranium to
U(VI)
 Dissolving it as an anionic carbonate or sulfate complexes
 Water and the dissolved U migrated downstream, reducing
material was encountered forming ore bodies
* Reduction to insoluble U(IV) (U4+) compounds
Most important mineral is uraninite (UO2+x, x = 0.01 to 0.25)
Inorganic (pyrite) or organic (humic) matter
Uranium concentration is 50 - 90%
Carnotite (a K + U vanadate) 54% U
U is often found in lower concentrations, of the order of 0.01 - 0.03% in
association with other valuable minerals such as apatite (phosphate rock),
shale, or peat
11-6
Uranium minerals
URANINITE
UO2
uranium oxide
CARNOTITE
K2(UO2)2(VO4)2• 1-3 H2O
hydrated potassium uranyl vanadate
AUTUNITE
Ca(UO2)2(PO4)2•10 H2O11-7
hydrated calcium uranyl phosphate.
Uranium solution chemistry
• Uranyl(VI) most stable 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
• yl oxygens have slow exchange
 Half life 5E4 hr in 1 M HClO4
 Rate of exchange catalyzed by UV light
• yl forms from f orbitals in U
11-8
Aqueous solution complexes
• 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)
• Uranium coordination with ligand can change protonation
behavior

HOCH2COO- pKa=17, 3.6 upon complexation of UO2
 Inductive effect
* Electron redistribution of coordinated ligand
* Exploited in synthetic chemistry
• U(III) and U(V)

No data in solution
 Base information on lanthanide or pentavalent actinides
11-9
Uranyl chemical bonding
•
•
Bonding molecular orbitals

sg2 su2 pg4 pu4
 Order of HOMO is unclear
* pg< pu< sg<< su proposed
 Gap for s based on 6p orbitals interactions

5fd and 5ff LUMO

Bonding orbitals O 2p characteristics

Non bonding, antibonding 5f and 6d

Isoelectronic with UN2
Pentavalent has electron in non-bonding orbital
11-10
11-11
11-12
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
 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
11-13
11-14
Acid-Leach Process for U Milling
U ore
Water
H2SO4 40-60°C
Steam
NaClO3
Crushing & Grinding
Slurry
Acid Leaching
Separation
Tailings
Solvent Extraction
Recovery, Precipitation
Drying (U3O8)
Organic Solvent
NH4+
11-15
In situ mining
Acidic solution (around pH 2.5)
11-16
Uranium purification
• TBP extraction
 Based on formation of nitrate species
 UO2(NO3)x2-x + (2-x)NO3- + 2TBP UO2(NO3)2(TBP)2
11-17
Solvent Extraction
•
•
•
•
•
Two phase system for separation

Sample dissolved in aqueous phase
 Normally acidic phase
Aqueous phase contacted with organic
containing ligand

Formation of neutral metal-ligand
species drives solubility in organic
phase
Organic phase contains target
radionuclide

May have other metal ions, further
separation needed
 Variation of redox state,
contact with different aqueous
phase
Back extraction of target radionuclide
into aqueous phase
Distribution between organic and
aqueous phase measured to evaluate
chemical behavior
11-18
Solvent extraction
•
•
Distribution coefficient

[M]org/[M]aq=Kd

Used to determine separation
factors for a given metal ion
 Ratio of Kd for different metal
ions
Distribution can be used to evaluate
stoichiometry

Plot log Kd versus log [X], slope is
stoichiometry
11-19
U Fluorination
HNO3
U ore concentrates
Solvent extraction purification
Conversion to UO3
H2 Reduction
UO2
HF
UF4
Mg
U metal
F2
UF6
MgF2
11-20
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
11-21
U enrichment
• Utilizes gas phase UF6
 Gaseous diffusion
 lighter molecules have a higher velocity at same
energy
* Ek=1/2 mv2
 For 235UF6 and 238UF6
• 235UF6 impacts barrier more often
11-22
Gas centrifuge
• Centrifuge pushed heavier 238UF6 against
wall with center having more 235UF6
 Heavier gas collected near top
• Enriched UF6 converted into UO2
 UF6(g) + 2H2OUO2F2 + 4HF
 Tc follows light U fraction if present
• Ammonium hydroxide is added to the
uranyl fluoride solution to precipitate
ammonium diuranate
 2UO2F2 + 6NH4OH  (NH4)2U2O7
+ NH4F + 3 H2O
• Calcined in air to produce U3O8 and
heated with hydrogen to make UO2
Final Product
11-23
•
•
•
•
•
Laser Enrichment
Based on photoexcitation

Atomic Vapor Laser Isotope
Separation (AVLIS)

Molecular Laser Isotope
Separation (MLIS)

Separation of Isotopes by Laser
Excitation (SILEX).

All use laser systems, optical
systems, and separation module
system

AVLIS used a uranium-iron (UFe) metal alloy
 Three excitation
wavelengths used

SILEX and MLIS use UF6
238U absorption peak 502.74 nm, 235U is
502.73 nm
Use of tunable lasers so only 235U is
excited
Then excited to ion state
Charge separation by electrostatic
11-24
Radiochemistry in reactor
• Speciation in irradiated fuel
• Utilization of resulting isotopics
• Fuel confined in reactor to fuel region
 Potential for interaction with cladding
material
Initiate stress corrosion cracking
 Chemical knowledge useful in events where
fuel is outside of cladding
• Some radionuclides generated in structural
material
11-25
Radionuclides in fresh fuel
• Actual Pu isotopics in MOX fuel may vary



Activity dominated by other Pu isotopes
Ingrowth of 241Am
MOX fuel fabrication in glove boxes
11-26
Fission process
•
•
•
Recoil length about 10 microns, diameter of 6 nm

About size of UO2 crystal

95 % of energy into stopping power
 Remainder into lattice defects
* Radiation induced creep

High local temperature from fission
 3300 K in 10 nm diameter
Delayed neutron fission products

0.75 % of total neutrons
 137-139I and 87-90Br as examples
Some neutron capture of fission products
eff    sf
11-27
Fuel variation during irradiation
•
•
•
•
Chemical composition
Radionuclide inventory
Pellet structure
Higher concentrations of Ru,
Rh, and Pd in Pu fuel
• Total activity of fuel effected
by saturation
 Tends to reach
maximum
• Radionuclide fuel
distribution studied
 Fission gas release
 Axial distribution by
gamma scanning
 Radial distribution to
evaluate flux
11-28
Perovskite phase (A2+B4+O3)
• Most fission products
homogeneously distributed in UO2
matrix
• With increasing fission product
concentration formation of
secondary phases possible
 Exceed solubility limits in UO2
• Perovskite identified oxide phase
 U, Pu, Ba, Sr, Cs, Zr, Mo, and
Lanthanides
 Mono- and divalent elements
at A
• Mechanism of formation
 Sr and Zr form phases
 Lanthanides added at high
burnup
11-29
Epsilon phase
• Metallic phase of fission
products in fuel
 Mo (24-43 wt %)
 Tc (8-16 wt %)
 Ru (27-52 wt %)
 Rh (4-10 wt %)
 Pd (4-10 wt %)
• Grain sizes around 1
micron
• Concentration nearly
linear with fuel burnup
 5 g/kg at 10MWd/kg
U
 15 g/kg at 40
MWd/kg U
11-30
Epsilon Phase
• Formation of metallic phase
promoted by higher linear
heat
 high Pd concentrations
(20 wt %) indicate a
relatively low fuel
temperature
 Mo behavior controlled
by oxygen potential
 High metallic Mo
indicates O:M of 2
 O:M above 2, more
Mo in UO2 lattice
11-31of the
Relative partial molar Gibbs free energy of oxygen
fission product oxides and UO2
Properties of fission products in oxide fuel
11-32
Burnup
•
•
•
Measure of extracted energy

Fraction of fuel atoms that underwent fission
 %FIMA (fissions per initial metal atom)

Actual energy released per mass of initial fuel
 Gigawatt-days/metric ton heavy metal (GWd/MTHM)
 Megawatt-days/kg heavy metal (MWd/kgHM)
Burnup relationship

Plant thermal power times days of dividing by the mass of the initial fuel loading

Converting between percent and energy/mass by using energy released per fission
event.
 typical value is 200 MeV/fission
 100 % burnup around 1000 GWd/MTHM
Determine burnup

Find residual concentrations of fissile nuclides after irradiation
 Burnup from difference between final and initial values
 Need to account for neutron capture on fissile nuclides

Find fission product concentration in fuel
 Need suitable half-life
 Need knowledge of nuclear data
* cumulative fission yield, neutron capture cross section
 Simple analytical procedure
 137Cs(some migration issues) 142Nd(stable isotope), 152Eu are suitable fission
products

Neutron detection also used
11-33
 Need to minimize 244Cm
Fuel variation during irradiation
11-34
Radionuclide Inventories
• Fission Products
 generally short lived (except 135Cs, 129I)
 ß, emitters
 geochemical behavior varies
• Activation Products
 Formed by neutron capture (60Co)
 ß, emitters
 Lighter than fission products
 can include some environmentally important
elements (C,N)
• Actinides
 alpha emitters, long lived
11-35
Plutonium
• Isotopes from 228≤A≤247
• Important isotopes
238Pu

 237Np(n,)238Np
* 238Pu from beta decay of 238Np
* Separated from unreacted Np by ion exchange
 Decay of 242Cm
 0.57 W/g
 Power source for space exploration
* 83.5 % 238Pu, chemical form as dioxide
* Enriched 16O to limit neutron emission
 6000 n s-1g-1
 0.418 W/g PuO2
 150 g PuO2 in Ir-0.3 % W container
11-36
Pu nuclear properties
•
239Pu
 2.2E-3 W/g
 Basis of formation of higher Pu isotopes
 244-246Pu first from nuclear test
• Higher isotopes available
 Longer half lives suitable for experiments
11-37
11-38
Questions
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?
11-39
Pop Quiz
• What are the influences of 5f electrons on the
chemistry of the actinides?
11-40