Molten Salt Processes and Room Temperature Ionic Liquids

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Molten Salt Processes and Room Temperature Ionic
Liquids

• Inorganic phase solvent
• High temperature needed to form liquid phase
• Different inorganic salts can be used as solvents
• Separations based on precipitation
• Reduction to metal state
• Precipitation
• Two types of processes in nuclear technology
• Fluoride salt fluid
• Chloride eutectic
• Limited radiation effects
• Reduction by Li

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Molten Salt Reactor

• Fluoride salt
• BeF2, 7LiF, ThF4, UF4 used as working fluid
• thorium blanket
• fuel
• reactor coolant
• reprocessing solvent
• 233Pa extracted from salt by liquid Bi through
  Li based reduction
• Removal of fission products by high 7Li
  concentration
• U removal by addition of HF or F2

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Pyroprocesses

• Electrorefining
• Reduction of metal ions to metallic state
• Differences in free energy between metal ions and
  salt
• Avoids problems associated with aqueous chemistry
• Hydrolysis and chemical instability
• Thermodynamic data at hand or easy to obtain
• Sequential oxidation/reduction
• Cations transported through salt and deposited on
  cathode
• Deposition of ions depends upon redox potential

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Electrochemical Separations

• Selection of redox potential allows separations
• Can use variety of electrodes for separation
• Developed for IFR and proposed for ATW
• Dissolution of fuel and deposition of U onto
  cathode
• High temperature, thermodynamic dominate
• Cs and Sr remain in salt, separated later
• Free energies
• noble metals
• iron to zirconium
• actinides and rare earths
• Group 1 and 2
• Solubility of chlorides in cadmium

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Electrolyte Salt and CdCl2 Oxidant
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Electrorefining
Electrorefining
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Electrorefining
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Spent Fuel Decladding Feed Material

• Step 1
• Support hardware remove from assembly
• Pins chopped
• Existing methods
• Oxide fuel separated from cladding
• Oxide fuel sent to reduction process
• Cladding use as Zr source for ATW fuel
• Offgas released in decladding collected and sent
  to storage/disposal

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Reduction of oxide fuel
Step 2

• Input
• 445 kg oxide (from step 1)
• 135 kg Ca
• 1870 kg CaCl2
• Output
• 398 kg heavy metal (to step 3)
• To step 8
• 2 kg Cs, Sr, Ba
• 189 kg CaO
• 1870 kg CaCl2
• 1 kg Xe, Kr to offgas

Metal
Operating Conditions T 1125 K, 8 hours 4 100
kg/1 PWR assembly
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Uranium Separation
Step 3

• Input
• 398 kg heavy metal (from step 2)
• 385 kg U, 20 kg U3(enriched, 6)
• 3.98 kg TRU, 3.98 kg RE
• 188 kg NaCl-KCl
• Output
• 392 kg U on cathode
• To step 4 (anode)
• 15 g TRU, 14 g RE, 2.8 kg U, 5 kg Noble Metal
• Molten Salt to step 5
• 10 kg U, 3.9 kg TRU,
• 3.9 kg RE, 188 kg NaCl-KCl

Operating Conditions T 1000 K, I 500 A, 265
hours 4 100 kg/1 PWR assembly
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Polishing Reduces TRU Discharge
Step 4

• Input from Anode 3
• 5 kg Noble Metal, 2.8 kg U, 15 g TRU, 14 g RE,
  1.1 kg U3, 18.8 kg NaCl-KCl
• Output
• Anode
• 5 kg Noble Metal, 0.15 g U, 0.045 g TRU, 0.129 g
  RE
• Cathode
• 1.5 g Noble Metal, 2.9 kg U
• Molten Salt (to 3)
• 28 g Noble Metal, 1 kg U, 15 g TRU, 14 g RE, 18.8
  kg NaCl-KCl

Metal
Operating Conditions T 1000 K, I 500 A, 2
hours 1 PWR assembly
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Electrowinning Provide Feed for Fuel
Step 5

• Input from molten salt from 3
• 10 kg U, 4 kg TRU, 4 kg RE, 4.3 kg Na as alloy,
  188 kg NaCl-KCl
• Output
• Cathode
• U extraction 9.2 kg
• U/TRU/RE extraction, 1 kg U, 4 kg TRU, 0.5 kg RE
• Molten Salt (to 7)
• 3.5 kg RE, 192 kg NaCl-KCl

Metal
Operating Conditions T 1000 K, I 500 A, 3.7
hours for U/TRU/RE, 6.2 hours for U 1 PWR assembly
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ATW Fuel Fabrication
Step 6

• Input
• From 5
• 1 kg U, 4 kg TRU, 0.5 kg RE
• From 1
• 14.7 kg Zr
• Output
• 20 kg alloy fuel
• Fuel Preparation
• Rods machined to proper diameter
• Rods cut into pellets for use in fuel pins

Vacuum Casting Furnace
Metal
Operating Conditions Vacuum Casting T 1900 K,
moderate vacuum
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Reduction of Rare Earths
Step 7

• Input
• Molten Salt from 5
• 3.4 kg RE
• 1.7 kg Na as alloy
• 188 kg NaCl-KCl
• Output
• Molten Salt (to step 3)
• 189 kg NaCl-KCl
• Metal Phase
• 3.4 kg RE

Metal
Operating Conditions T 1000 K, 8 hours
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Recycle Salt Reduction of Oxide
Step 8

• Input
• Chlorination
• 189 kg CaO, 1870 kg CaCl2, 239 kg Cl2
• Electrowinning
• 2244 kg CaCl2
• Output
• Chlorination
• 2244 kg CaCl2, 54 kg O2
• Electrowinning (to 2)
• 1870 kg CaCl2, 135 kg
• Ca, (239 kg Cl2)

Operating Conditions T 1000 K, I 2250 A, 80
hours
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Electrorefining
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ATW Assembly for Feed Material

• Step 9
• ATW assembly is used to produce feed material for
  electrorefining process
• Hardware removed from assembly
• ATW fuel chopped into small sections
• Cladding is sent to storage
• Offgas is collected and stored

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U, TRU, and Fission Product Separation
Step 10

• Input
• 45 kg from Step 9 (includes Zr)
• Includes 9.5 kg TRU, 0.5 kg RE
• Output
• Anode
• 33 kg NM, 2 kg U
• Molten Salt (to 11)
• Small amounts of U, TRU, RE
• Cathode (to 12)
• Most TRU, RE

Operating Conditions T 1000 K, I 500 A, 6.7
hours
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Electrowinning TRU for Salt Recycle
Step 11

• Input from molten salt from 10
• 1.7 kg U, 7.4 kg TRU, 0.5 kg RE, 2.8 kg Na as
  alloy, 188 kg NaCl-KCl
• Output
• Cathode (to 12)
• U/TRU/RE extraction, 1.7 kg U, 7.4 kg TRU, 0.1 kg
  RE
• Molten Salt (to 13)
• 0.4 kg RE, 191 kg NaCl-KCl

Metal
Operating Conditions T 1000 K, I 500 A,
6.1hours for U/TRU/RE Salt from 10
electrorefining systems
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ATW Fuel Fabrication
Step 12

• Input
• From 10 and 11
• 1.7 kg U, 17 kg TRU, 0.5 kg RE,
• From 1
• 52 kg Zr
• Output
• 71 kg alloy fuel
• Fuel Preparation
• Rods machined to proper diameter
• Rods cut into pellets for use in fuel pins

Vacuum Casting Furnace
Metal
Operating Conditions Vacuum Casting T 1900 K,
moderate vacuum Four Batches required to prepare
fuel alloy
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Reduction to Remove Rare Earths
Step 13

• Input
• 0.4 kg RE (from 11), 188 kg NaCl-KCl, 0.2 kg Na
  as alloy
• Output
• Molten Salt
• 188 kg NaCl-KCl
• Metal Phase
• 0.4 kg RE

Metal
Operating Conditions T 1000 K, 8 hours
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Treatment Scheme

• To treat 70000 metric tons of spent fuel
• 2 MT/day in each plant
• 2 chemical plants required to treat LWR and ATW
  waste
• 300 day/year at 24 hours/day
• Need 60 years
• For ATW waste
• 360 kg/day/plant

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DORDirect Oxide Reduction
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ATW Waste
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Project TRU Waste to Repository

• Results based on simulations
• LWR, 12 ppm TRU
• ATW spent fuel, 10 ppm TRU
• Should expect high amounts due to engineering
  scale
• Total TRU to repository
• In 60 years, lt 300 kg TRU in approximately 900 MT

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Segregated Waste Streams

• Uranium
• Low activity of waste
• Metals
• Spent fuel clad and assembly to repository
• Transition metals and lanthanides
• Oxides to repository
• Active Metals into engineered containers
• No separation of fissile metals

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Reprocessing Overview

• The oxide fuel is dispersed in a molten (800 C)
  CaCl2 /CaF2 salt along with calcium metal and
  reduced to a metal.
• The reduced metals are dissolved in a molten Cu -
  40 Mg - Ca receiver alloy.
• Uranium exceeds the solubility limits in this
  receiver alloy and precipitates out as a solid
  metal.
• Pu, other actinides, rare-earths, and noble metal
  fission products accumulate in the receiver
  alloy.
• The the alkali metals (Rb and Cs), alkali-earths
  (Sr and Ba),and remaining iodine and bromine
  accumulate in the CaCl2/CaF2 salt.
• The salt contains CaO from the reduction process.
  The CaO is electrolytically reduced to metal for
  reuse.

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Overview

• The actinides are separated from the acceptor
  alloys by distilling the Cd-Mg alloy.
• The electrorefining process described above is
  then used to purify the final metal uranium and
  actinide product.
• Because there is no water to enhance criticality,
  containers typically can have 20 or 30 kg of
  fissile material

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Overview

• Introduction to Room Temperature Ionic Liquids
• Physical Properties
• Coordination Chemistry
• Metal Deposition
• From Lecture of Dave Costa, LANL

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Room Temperature Molten Salts as Alternatives to
Traditional Actinide Recovery Processes

• Project Goal Develop a room temperature ionic
  liquid flow sheet for the electrochemical
  recovery and purification of uranium and
  plutonium from spent nuclear feed stocks.
• Proliferation resistant recovery of
  uranium/plutonium
• Uranium/Plutonium metal production
• Zero effluent discharge operations
• Room temperature operation
• Greater criticality safely margin

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Current Pu Processing
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Plutonium
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Criticality calculations for Pu metal - solution
systems
Metal-Water Mix
Metal-AlCl3 Mix
Metal-BF4 Mix
1.0E04
1.0E03
1.0E02
Critical Mass (kg)
1.0E01
1.0E00
1.0E-01
1.0E00
1.0E01
1.0E02
1.0E03
1.0E04
1.0E05
Pu Concentration (g/liter)
Harmon, C.D. Smith, W.H. Costa, D.A. Rad. Phy
Chem. 60, 157, (2001). Criticality calculations
for plutonium metal-room temperature ionic liquid
solutions
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Ionic Liquid Cations
Bonhote Inorg. Chem. 1996, 35, 1168
N
N
N
N
N
N
N
N
N
N
mp 150 C
ambient temperature liquids...
mp 56 C
O
MacFarlane J. Phys. Chem. 1999, 103, 4164
N
N
O
N
N
N
O
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Ionic Liquids Quaternary Ammonium Cations
MacFarlane J. Phys. Chem. B. 1998, 102, 8860
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Physical Properties
Density (g/mL)
1.52 1.45 1.39 1.38 1.35
1.32 1.30
Reference 0.1M KCl 14mS/cm
Reference H2O 1.002 cP C6H6 0.64 cP Olive
Oil 81
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Electrochemical Windows of Ionic Liquids
The electrochemical window of an imidazolium NTf2
salt is compared with a typical ammonium ionic
liquid. The CV trace is referenced to Ag/AgOTf
and confirmed with ferrocene.
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Potential Ionic Liquid Anions
0.1 M NaCl/H2O
H3PO4
0.1 M Bu4N -B(C6F5)4/CH2Cl2
cyclohexanol
ethylene glycol
Bonhote et. al. Inorg. Chem. 1996, 35, 1168.
Dupont et. al. Organometallics 1998, 17, 815.
-N(SO2CF3)2 abbreviated as -NTf2
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Structural Characterization of a Room Temperature
Ionic Liquid
P21/n a 12.225(3) Å b 8.547(2) c
34.322(8) b 92.749(4) R 6.8
Top view
3dx2-y2
S(1)N(3) 1.571(4) Å S(2)N(3)
1.580(4) SOaverage 1.425 S(1)N(3)S(2) 126
Side view
NS in H3NSO3 1.75 Å NS in HN(SO2CF3)2
1.644 Å
3dz2
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Coordination Modes of N(SO2CF3)2
See Chem. Commun., 2005, 1438-1440
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Coordination Chemistry of NTf2 Synthesis of
FpNTf2
2071, 2029 cm-1
n(CO) 2005, 1945 cm-1
2020, 1960 cm-1
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Coordination Chemistry of NTf2 Synthesis of
FpNTf2
2071, 2029 cm-1
n(CO) 2005, 1945 cm-1
2020, 1960 cm-1
n(CO) BF4 2072, 1994 cm-1 SbF6 2074,
2039 ClO4 2071, 2009 OSO2CF3 2068, 2017
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Coordination Chemistry of NTf2 Synthesis of
FpNTf2
2071, 2029 cm-1
n(CO) 2005, 1945 cm-1
2020, 1960 cm-1
Fe(1)N(1) 2.084(4) Å N(1)S(1)
1.630(4) N(1)S(2) 1.643(4) SOave
1.421 S(1)N(1)-S(2) 117.1(2)
n(CO) BF4 2072, 1994 cm-1 SbF6 2074,
2039 ClO4 2071, 2009 OSO2CF3 2068, 2017
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Synthesis of Cp2Ti(NTf2)2 Novel MetalOxygen
Binding Mode
Ti(1)O(1) 2.050(3) Å N(1)S(1)
1.523(5) S(1)O(1) 1.467(4) N(1)S(2)
1.613(5) S(1)O(2) 1.416(4) S(1)N(1)S(2)
126.1
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Influence of NTf2 Coordination on E1/2 Values
Reference Ag/AgOTf/EMINTf2 Working electrode
platinum Scan rate 50 mV/s
Cp2Ti(NTf2)2 E1/2 -0.103 V
Cp2TiCl2 E1/2 -1.031 V
?E1/2 0.928 V
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Cyclic Voltammetry of UCl62- Salts
U(V), U(IV), and U(III) are all stable species
for UCl6-n (n1, 2, 3)
5/4
4/3
Reference Ag/AgOTf/EMINTf2 Working electrode
platinum Scan rate 50 mV/s
Reversible 5/4 E1/2 0.27 V Reversible 4/3
E1/2 -1.98 V
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Bulk Electrolysis of UCl6EMI2 in EMINTf2
Stirred Solution Voltammograms 1.5 mm GC disc, 3
mV/s
Pale Blue
Pale Blue
Yellow
U(IV)
U(V)
U(IV)
Eapp during bulk was set 300 mV positive of
E1/2 for U(IV)/U(V) couple
U(V)Cl6- is stable in EMINTf2 on the bulk
electrolysis time scale
Coulometry was 94 efficient for a 1-electron
oxidation process
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Electroplating of Sodium and Potassium
Standard reduction potential (aq) -2.714 V
Standard reduction potential (aq) -2.924 V
Comparison to the actinide elements demonstrates
electro-refining feasibility Thorium
-1.90 Neptunium -1.86 Americium -2.32 Uranium
-1.80 Plutonium -2.07
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Synthesis and Characterization of U(NTf2)4 RTIL
Solutions
UV/vis Characterization indicates that U(IV)
solutions are formed
UCl62-
U(NTf2)4
Reversible uranium 4/3 E1/2 -0.24 V
Josh Smith
The 4/3 couple of U(NTf2)x shifts 1.74 V
more positive compared to UCl62-
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Summary and Future Directions
RTILs are promising solvents for
electrochemical applications enabling
high-quality data acquisition
Exemplified with electrochemical results on
several uranium and titanium metal complexes
Electrochemical plating and stripping
demonstrated for mono- and multi-valent
electropositive metals
Future Work
Electroplating Analysis of metal
precipitate on electrode surface with
microscopy Quantitative electrochemical
analysis Oxidative electrodissolution of
metals into RTIL Further studies on the
electroplating of actinide metals
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Acknowledgements
Uranium Disposition Team Brad Schake, Minnie
Martinez, Jim Rocha, Coleman Smith, Phil Banks
RTIL Working Group David Costa NMT-15 Warren
Oldham C-INC Bridgett Williams NMT-15 Rene
Chavarria NMT-15 Mike Stoll NMT-15 Wayne
Smith MST-11
ARIES Chris James NMT-DO Dave Kolman
NMT-15 Doug Wedman NMT-15
Plutonium Review G.T. Seaborg Institute for
Transactinium Science David Clark
NMT-DO Web Keogh NMT-DO
Los Alamos Primer Carol Hogsett LANL College
Recruiting Coordinator ARIES Development Project