16-1 Voltammetry Electrochemistry techniques based on current (i) measurement as function of voltage (E appl ) Working electrode §(microelectrode) place where redox occurs §surface area few mm 2 to limit current flow Reference electrode §constant potential reference (SCE) Counter electrode §inert material (Hg, Pt) §plays no part in redox but completes circuit Supporting electrolyte §alkali metal salt does not react with electrodes but has conductivity

16-2 Voltammetry Potentiostat (voltage source) drives cell §supplies whatever voltage needed between working and counter electrodes to maintain specific voltage between working and reference electrode àAlmost all current carried between working and counter electrodes àVoltage measured between working and reference electrodes àAnalyte dissolved in cell not at electrode surface

16-3 Method Excitation signal applied §Wave response based on method àLinear àDifferential pulse àSquare wave àCyclic §Developed current recorded

16-4 Signals

16-5 Electrodes

16-6 Potential ranges Number of useful elements for electrodes §Pt §Hg §C §Au Limits §Oxidation of water à2H 2 O->4H+ +O 2 (g) + 4e- §Reduction of water à2H 2 O+ 2e - ->H 2 + 2OH -

16-7 Overpotential Overpotential  always reduces theoretical cell potential when current is flowing   = E current - E equilibrium Overpotential due to electrode polarization: §concentration polarization - mass transport limited § adsorption/desorption polarization - rate of surface attach/detachment §charge-transfer polarization - rate of redox reaction §reaction polarization - rate of redox reaction of intermediate in redox reaction Overpotential means must apply greater potential before redox chemistry occurs

16-8 Voltammograms Current against applied voltage §Increase in current at potential at which analyte is reduced àReaction requires electrons *supplied by potentiostat Half wave potential (E 1/2 ) is close to E 0 for reduction reaction Limiting current proportional to analyte activity

16-9 Methods Current is just measure of rate at which species can be brought to electrode surface §Stirred - hydrodynamic voltammetry àNernst layer near electrode *Diffusion layer *Migration *convection

16-10 Methods Analyte (A) and product (P) In stirred solution convection dominates

16-11 Methods Current is a measure of how fast the analyte can go to electrode surface

16-12 Hydrodynamic Single voltammogram can quantitatively record many species §Requires sufficient separation of potentials Need to remove O 2

16-13 Hanging Hg electrode Polarography Differs from hydrodynamic §unstirred (diffusion dominates) §dropping Hg electrode (DME) is used as working electrode §current varies as drop grows then falls off

16-14 Linear Scan Advantages of DME §clean surface and constant mixing §constant current during drop growth §No H 2 formation Disadvantages of DME: §Hg easily oxidized §cumbersome to use

16-15

16-16

16-17 Cyclic Voltammetry Oxidation and reduction Variation of rates Peak potentials §Anode (bottom peak) §Cathode (top peak) àDifference /n Peak currents §Cathode (line to peak) §Anode (slope to bottom) àPeak currents equal and opposite sign Mechanisms and rates of redox

16-18 CV data

16-19 Molten Salt Processes 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 àLimited radiation effects àReduction by Li

16-20 Molten Salt Reactor Fluoride salt §BeF 2, 7 LiF, ThF 4, UF 4 used as working fluid àthorium blanket àfuel àreactor coolant àreprocessing solvent § 233 Pa extracted from salt by liquid Bi through Li based reduction §Removal of fission products by high 7 Li concentration §U removal by addition of HF or F 2

16-21 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

16-22 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

16-23 Electrorefining

16-24 Reduction of oxide fuel Input 445 kg oxide (from step 1) 135 kg Ca 1870 kg CaCl 2 Output 398 kg heavy metal (to step 3) To step 8 §2 kg Cs, Sr, Ba §189 kg CaO §1870 kg CaCl 2 1 kg Xe, Kr to offgas Step 2 Metal Operating Conditions T= 1125 K, 8 hours kg/1 PWR assembly

16-25 Anode Uranium Separation Input 398 kg heavy metal (from step 2) 385 kg U, 20 kg U 3+ (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 Step 3 Operating Conditions T= 1000 K, I= 500 A, 265 hours kg/1 PWR assembly

16-26 Anode Polishing Reduces TRU Discharge Input from Anode #3 5 kg Noble Metal, 2.8 kg U, 15 g TRU, 14 g RE, 1.1 kg U 3+, 18.8 kg NaCl-KCl Output Anode 5 kg Noble Metal, 0.15 g U, g TRU, 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 Step 4 Metal Operating Conditions T= 1000 K, I= 500 A, 2 hours 1 PWR assembly

16-27 Electrowinning Provide Feed for Fuel 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 Step 5 Metal Operating Conditions T= 1000 K, I= 500 A, 3.7 hours for U/TRU/RE, 6.2 hours for U 1 PWR assembly

16-28 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

16-29 Recycle Salt: Reduction of Oxide Input Chlorination §189 kg CaO, 1870 kg CaCl 2, 239 kg Cl 2 Electrowinning §2244 kg CaCl 2 Output Chlorination §2244 kg CaCl 2, 54 kg O 2 Electrowinning (to #2) §1870 kg CaCl 2, 135 kg Ca, (239 kg Cl 2 ) Step 8 Operating Conditions T= 1000 K, I= 2250 A, 80 hours

16-30 AnodeTRU U, TRU, and Fission Product Separation 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 Step 10 Operating Conditions T= 1000 K, I= 500 A, 6.7 hours

16-31 Electrowinning TRU for Salt Recycle Input from molten salt from # 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 Step 11 Metal Operating Conditions T= 1000 K, I= 500 A, 6.1hours for U/TRU/RE Salt from 10 electrorefining systems

16-32 Reduction to Remove Rare Earths 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 Step 13 Metal Operating Conditions T= 1000 K, 8 hours