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
9-1
Reduction by Li
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
9-2
 U removal by addition of HF or F2
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
9-3
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
9-4
9-5
9-6
9-7
Electrolyte Salt and CdCl2 Oxidant
9-8
Electrorefining
Electrorefining
9-9
Electrorefining
9-10
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
9-11
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
Metal
 2 kg Cs, Sr, Ba
 189 kg CaO
Operating Conditions
 1870 kg CaCl2
T= 1125 K, 8 hours
• 1 kg Xe, Kr to offgas
4 100 kg/1 PWR assembly9-12
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
Anode
• Molten Salt to step 5
 10 kg U, 3.9 kg TRU,
Operating Conditions
3.9 kg RE, 188 kg NaCl-KCl
9-13 hours
T= 1000 K, I= 500 A, 265
4 100 kg/1 PWR assembly
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
Metal
Molten Salt (to #3)
Anode
• 28 g Noble Metal, 1 kg U, 15 g TRU, 14 g
RE, 18.8 kg NaCl-KCl
Operating Conditions
9-14
T= 1000 K, I= 500 A, 2 hours
1 PWR assembly
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
Metal
TRU, 0.5 kg RE
Molten Salt (to #7)
• 3.5 kg RE, 192 kg NaCl-KCl
Operating Conditions
9-15 U
T= 1000 K, I= 500 A, 3.7 hours for U/TRU/RE, 6.2 hours for
1 PWR assembly
ATW Fuel Fabrication
Step 6
Input
Vacuum Casting Furnace
• From #5
 1 kg U, 4 kg TRU, 0.5 kg RE
• From #1
metal
 14.7 kg Zr
Output
20 kg alloy fuel
Fuel Preparation
Metal
• Rods machined to proper diameter
• Rods cut into pellets for use in fuel pins
Operating Conditions: Vacuum Casting
T= 1900 K, moderate vacuum
9-16
Reduction of Rare Earths
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
Step 7
Metal
Operating Conditions
T= 1000 K, 8 hours
9-17
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)
Operating Conditions
 1870 kg CaCl2, 135 kg
T= 1000 K, I= 2250 A, 80 9-18
hours
Ca, (239 kg Cl2)
Electrorefining
9-19
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
9-20
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)
Anode
TRU
 Small amounts of U, TRU,
RE
Operating Conditions
• Cathode (to #12)
9-21
T= 1000 K, I= 500 A, 6.7 hours
 Most TRU, RE
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
9-22
ATW Fuel Fabrication
Step 12
Input
Vacuum Casting Furnace
• From #10 and #11
 1.7 kg U, 17 kg TRU, 0.5 kg RE,
• From #1
metal
 52 kg Zr
Output
71 kg alloy fuel
Fuel Preparation
Metal
• Rods machined to proper diameter
• Rods cut into pellets for use in fuel pins
Operating Conditions: Vacuum Casting
T= 1900 K, moderate vacuum
Four Batches required to prepare fuel alloy
9-23
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
9-24
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
9-25
9-26
DOR=Direct Oxide Reduction
ATW Waste
9-27
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, < 300 kg TRU in approximately
900 MT
9-28
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
9-29
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.
9-30
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
9-31
Overview
• Introduction to Room Temperature Ionic Liquids
 Physical Properties
 Coordination Chemistry
 Metal Deposition
• From Lecture of Dave Costa, LANL
9-32
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
9-33
Current Pu Processing
9-34
Plutonium
9-35
Criticality calculations for Pu metal solution systems
Metal-Water Mix
Metal-AlCl3 Mix
Metal-BF4 Mix
1.0E+04
Critical Mass (kg)
1.0E+03
1.0E+02
1.0E+01
1.0E+00
1.0E-01
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
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
9-36
Ionic Liquid Cations
N
N
N
mp = 150 °C
N
Bonhote Inorg. Chem. 1996, 35, 1168
N
N
N
mp = 56 °C
N
N
N
ambient temperature liquids...
O
N
O
N
MacFarlane J. Phys.
Chem. 1999, 103, 4164
N
N
N
O
9-37
Ionic Liquids: Quaternary
Ammonium Cations
N
viscosity = 167 cP
viscosity = 595 cP
N
NTf2
NTf2
MacFarlane J. Phys. Chem. B. 1998, 102, 8860
+ alkylhalide
N
CH3 CN
N
R
X
N
R
+ LiNTf2
H2 O
N
X
N
mp = 150 °C
R
+ LiX
NTf2
N
mp = 56 °C
N
N
N
N
ambient temperature liquids...
9-38
Physical Properties
Density (g/mL)
1.52
F3C
1.35
1.32
1.30
10
140
8
120
6
100
80
4
60
2
40
O
N
S
S
O
1.38
160
viscosity (cP @30 °C)
O
1.39
O
20
CF3
Conductance (mS/cm @30 °C)
Reference:
H2O = 1.002 cP
C6H6 = 0.64 cP
Olive Oil = 81
1.45
Reference:
0.1M KCl = 14mS/cm
0
EMI MOMP BMP Propyl Butyl Pentyl Hexyl
O
NTf2
N
N
N
NTf2
N
NTf2
N R
NTf2
9-39
Electrochemical Windows of Ionic
Liquids
The electrochemical window of an imidazolium NTf salt is compared with a typical ammonium
2
ionic liquid. The CV trace is referenced to Ag/AgOTf and confirmed with ferrocene.
Cp2Fe


N
N + N
NTf2
NTf2
4000
3000
2000
1000
0
-1000
-2000
-3000
-4000
Potential (mV)
9-40
Potential Ionic Liquid Anions
350
12
300
ethylene glycol
viscosity (cP)
cyclohexanol
250
8
200
150
6
N + N
4
100
0
0
0.1 M Bu4N+
-B(C F ) /CH Cl
6 5 4
2 2
PF 6
BF4
3
CH CO
2
C F CO
3 7
2
2
3 2
-N(SO
SO 3CF 3
3
2
CF CO
2
N(SO CF )
50
conductivity (mS/cm)
H3PO4
10
N + N
0.1 M NaCl/H2O
• Bonhote et. al. Inorg. Chem. 1996, 35, 1168.
• Dupont et. al. Organometallics 1998, 17, 815.
2CF3)2
abbreviated as -NTf2
9-41
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%
F 3C
O
S
N
O
S
O
F 3C
O
O
CF3
A
S
S
N
O
S
O
O
CF3
B
Top view
N
N
CF3
O
S
3dx2-y2
CF3
S(1)—N(3) = 1.571(4) Å
S(2)—N(3) = 1.580(4)
S—Oaverage = 1.425
S(1)–N(3)–S(2) = 126°
O
O
O
Side view
Nlp
S
Nlp
O
O
N—S in H3N—SO3–
= 1.75 Å
N—S in HN(SO2CF3)2 = 1.644 Å
S
3dz2
CF3
CF3
O
O
9-42
Coordination Modes of N(SO2CF3)2
O O
S CF 3
M N
S O
F3C O
O
S O
O
S
1-O
O
CF 3
N S
O
O
S
O
O CF
3
See Chem. Commun., 2005, 1438-1440
N
O CF 3
1-N
M
CF 3
M
2-N,O
O
CF 3
S
M
N
O
S
O
CF 3
2-O,O
9-43
Coordination Chemistry of NTf2: Synthesis of
Fp–NTf2
OC
OC
HNTf2
Fe
Me
n(CO): 2005, 1945 cm-1
-CH4
OC
Fe
OC
N
SO2 CF3
SO2 CF3
2071, 2029 cm-1
AgNTf2
-AgI
OC
Fe
OC
I
2020, 1960 cm-1
9-44
Coordination Chemistry of NTf2: Synthesis of
Fp–NTf2
OC
OC
HNTf2
Fe
Me
n(CO): 2005, 1945 cm-1
BF4
SbF6
ClO4
OSO2CF3
-CH4
OC
Fe
OC
N
SO2 CF3
SO2 CF3
2071, 2029 cm-1
AgNTf2
-AgI
OC
Fe
OC
I
2020, 1960 cm-1
n(CO)
2072, 1994 cm-1
2074, 2039
2071, 2009
2068, 2017
9-45
Coordination Chemistry of NTf2:
Synthesis of Fp–NTf2
HNTf2
Fe
OC
-CH4
Me
OC
n(CO): 2005, 1945 cm-1
BF4
SbF6
ClO4
OSO2CF3
n(CO)
2072, 1994 cm-1
2074, 2039
2071, 2009
2068, 2017
F 3C
O
S
N
O
S
Fe
N
OC
SO2 CF3
AgNTf2
-AgI
SO2 CF3
2071, 2029 cm-1
OC
Fe
OC
I
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)
S–Oave
1.421
S(1)–N(1)-S(2) 117.1(2)°
O
F 3C
O
O
CF3
A
OC
S
N
O
S
O
O
CF3
B
9-46
Synthesis of Cp2Ti(NTf2)2: Novel Metal–Oxygen
Binding Mode
O
O
F 3C S
N
Cl
Ti
2 AgNTf 2
-2 AgCl
Cl
O
O
Ti
O
O
S
S
F 3C
F 3C
O
S
N
O
S
A
O
N(1)—S(1) = 1.523(5)
N(1)—S(2) = 1.613(5)
S(1)–N(1)–S(2) = 126.1°
O
F 3C
O
O
CF3
CF3
N
O
Ti(1)—O(1) = 2.050(3) Å
S(1)—O(1) = 1.467(4)
S(1)—O(2) = 1.416(4)
CF3
S
S
N
O
S
O
O
CF3
B
9-47
Influence of –NTf2 Coordination on E1/2
Values
Reference: Ag/AgOTf/EMINTf2
Working electrode: platinum
Scan rate: 50 mV/s
CF3
O
S
O
N
CF3
S
Current
O
Ti
O S
N
O
S
O
O
CF3
O
F3C
0.5
0.0
-0.5
+
Potential (V vs Fc/Fc )
Cp2Ti(NTf2)2
E1/2 = -0.103 V
-1.0
-1.5
Cp2TiCl2
E1/2 = -1.031 V
∆E1/2 = 0.928 V
9-48
Cyclic Voltammetry of [UCl6]2- Salts
U(V), U(IV), and U(III) are all stable species for [UCl6]-n (n=1, 2, 3)
Cl
Cl
Cl
Cl
U Cl
Cl
+
e-
- e-
Cl
Cl
Cl
Cl 2
U Cl
Cl
+
e-
-
e-
Cl
Cl
Cl
Cl 3
U
Cl
Cl
5+/4+
Reference: Ag/AgOTf/EMINTf2
Working electrode: platinum
Scan rate: 50 mV/s
4+/3+

Reversible 5+/4+ E1/2 = 0.27 V
Reversible 4+/3+ E1/2 = -1.98 V
1000
500
0
-500
-1000
-1500
-2000
Potential (mV)
9-49
-2500
Bulk Electrolysis of [UCl6][EMI]2 in
[EMI][NTf2]
1.2
Anodic Current
1.0
0.8
Potential (V)
U(IV)
0.6
0.4
Yellow
1.2
1.0
Cathodic Current
0.8
0.6
0.4
Pale
Blue
Current (µA)
Pale
Blue
Current (µA)
Current (µA)
Stirred Solution Voltammograms: 1.5 mm GC disc, 3 mV/s
0.2
Anodic Current
1.2
1.0
Potential (V)
U(V)
0.8
0.6
0.4
Potential (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 [EMI][NTf2] on the bulk electrolysis time scale
• Coulometry was 94% efficient for a 1-electron oxidation process
9-50
0.2
Electroplating of Sodium and
MH + HNTf
MNTf + H
(1)
Potassium
2
2
2
M0 + NTf2-
MNTf 2 + e-
(2)
5 µA
5 µA
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
Potential (V)
Standard reduction potential (aq): -2.714 V
-3.0
0
-1
-2
-3
Potential (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
9-51
Synthesis and Characterization of “U(NTf2)4”
RTIL Solutions
RTIL
UCl4 + 4 AgNTf2
-AgCl
"U(NTf2)4"
– 4 e-
Uranium Anode
UV/vis Characterization indicates that
U(IV) solutions are formed
[UCl6]2-
U(NTf2)4
Reversible uranium 4+/3+ E1/2 = -0.24 V
2000
1000
0
-1000
-2000
-3000
The 4+/3+ couple of “U(NTf2)x” shifts
1.74 V more positive compared to [UCl6]2-
Josh Smith
Potential (mV)
9-52
O
O
O
F3C
F3C
S
O
S
N
F3C
S
S
S
O
N
CF3
S
N
O
S
O
CF3
S
O
O
U
N
F3C
O
O
O
O
CF3
O
CF3
O
[UCl6]2–
U(NTf2)4
2000
1000
0
-1000
-2000
-3000
Potential (mV)
9-53
Summary and Future Directions
• RTIL’s are promising solvents for electrochemical applications enabling highquality 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
9-54
Acknowledgements
RTIL Working Group
David Costa
NMT-15
Warren Oldham
C-INC
Uranium Disposition Team
Brad Schake, Minnie Martinez,
Jim Rocha, Coleman Smith, Phil Banks
Bridgett Williams NMT-15
ARIES
Rene Chavarria
NMT-15
Chris James
NMT-DO
Mike Stoll
NMT-15
Dave Kolman
NMT-15
Wayne Smith
MST-11
Doug Wedman
NMT-15
Plutonium Review
Los Alamos Primer
Carol Hogsett: LANL College
Recruiting Coordinator
ARIES Development Project
G.T. Seaborg Institute for
Transactinium Science
David Clark
NMT-DO
Web Keogh
NMT-DO
9-55