Transcript PowerPoint
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
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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
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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
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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
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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
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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) + 2H2OUO2F2 + 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
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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
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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
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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
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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
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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
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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