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Electrochemical Properties of Zirconium, Plutonium and Lanthanides in Fluoride M Elts (1) R. Zakirov, V. Ignatiev, (2) V. Subbo

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Electrochemical Properties of Zirconium, Plutonium and Lanthanides in Fluoride M Elts (1) R. Zakirov, V. Ignatiev, (2) V. Subbo O22-09 Electrochemical Properties of Zirconium, Plutonium and Lanthanides in Fluoride M elts (1) R. Zakirov, V. Ignatiev, (2) V. Subbotin, A. Toropov (1) RRC —Kurchatov Institute“, Moscow, 123182, RF (2) Institute of Technical Physics, Snezhinsk, 456770, RF zakirov@ imp.kiae.ru Abstract - Today the evaluation of pyrochemical treatment capability request more experimental data and demonstrations then what has been done up to now. Much more calculated and experimental results are available on a thermodynamic of electrochemical processes in chloride melts compared to fluoride ones. New experimental data on electrochemical properties of 60LiF-40NaF and 15LiF- 58NaF-27BeF2 (mol. %) eutectic melts containing ZrF4, PuF3 and LnF3 (NdF3, LaF3, CeF3) are presented. deposition potentials on solid Mo electrode on INTRODUCTION the basis of experimental results obtained. Molten salt processes for irradiated EXPERIM ENTAL nuclear fuel have been studied internationally for several decades. Potential overall Preparation of major solvent constituents applications of pyrochemical processing include: Powdered lithium, sodium and beryllium • Fuel processing for recycle [1]; fluorides were used as initial components for • Fuel and fuel residues conditioning for preparation of LiF-NaF (60:40 mol.%) and disposal [2]; LiF-NaF-BeF2 (15:58:27 mol.%) mixtures. • Advanced waste processing, which could The content of metal (Ni, Fe, Cr, Cu, Mo, Ln) potentially include waste management options compounds impurities in powdered lithium, such as separation of heat generating or long- sodium and beryllium fluorides was less than lived wastes [3]; 0.01 % wt. (analysis by the ICP-AES method). • Fission product clean up for molten salt The powders of LiF, NaF and BeF2 were reactors (MSR), particularly as applied to mixed in prescribed proportion and dried at single stream Li,Na,Be/F transmuter system 500? for 3-4 h. Then the following order of without U-Th support [4]. operations was made in a separate test section Today the evaluation of pyrochemical for the chosen molten salt mixture preparation: treatment capability request more experimental • Heating of the starting powder mixture in -2 data and demonstrations, then what has been a glassy carbon crucible under vacuum (10 - -3 done up to now. It is not an easy task because 10 mm Hg) at gradual temperature increase the material to be treated is not completely from 300? up to 1100K during 8-10 h; defined and because many options for reducing • Double melting of the prepared fusion -2 -3 the long-lived radionuclide inventory are still cake at 1000? under vacuum (10 -10 mm under examination. Hg) for 4-5 h and mechanical removal of Note, that much less calculated and impurities from the frozen melt surface; experimental results are available on a • Transfer of glassy carbon crucible with thermodynamic of electrochemical processes fusion cake to electrochemical cell and pre- in fluoride melts compared to chloride ones. electrolysis of the melt (C=900?, vacuum 10-2- In this paper the experimental data on 10-3 mm Hg) using molybdenum cathode and electrochemical properties of 60LiF-40NaF graphite anode. and 15LiF-58NaF-27BeF2 (in mol. %) eutectic In the process of the potentiostatic pre- melts, containing zirconium, plutonium and electrolysis the cathode potential was kept at lanthanides fluorides are presented. the level of (œ1.2 V) versus molybdenum quasi Particularly, consideration includes the reference electrode. The pre-electrolysis plutonium trifluoride behavior in the Be- duration was approximately 3 h. The final containing solvent system: determination of value of the pre-electrolysis current was ∼ 2 the difference between plutonium and mA, surface area of the electrodes ∼5 cm2. beryllium deposition potentials on solid The fusion cake prepared in such a molybdenum electrode and evaluation of the manner with the mass about 75 g was used as difference between plutonium and neodymium solvent system in our experiments. ATALANTE 2004 Nîmes (France) June 21-25, 2004 1 O22-09 The cell electrodes pass through the upper Preparation of ZrF4, PuF3 and LnF3 flange inside the cell through Teflon insulators. The cell electrodes were manufactured of a Zirconium tetrafluoride, plutonium and molybdenum (∅1mm wire), glassy carbon lanthanides trifluorides used in the experiments (∅2mm rod, SU-2000) and graphite. The cell were synthesized by a method based on direct electrodes were cleaned mechanically and fluorinating of metals or oxides by anhydrous degreased by a solvent before experiments. hydrogen fluoride. The content of the basic The three-electrode scheme was used in substance in the specified metal fluorides was the electrochemical experiments with the no less than 99 wt.%. quasi-reference Mo electrode, the working Mo ZrF4, LaF3 and CeF3 were added to the electrode (cathode) and the auxiliary glassy molten salt mixture as a powder. In case of carbon (graphite) electrode (anode). The PuF3 and NdF3, its pressed pellet was placed to immersion depth of the working electrode was electrochemical cell on surface of the solvent equal to 5-7 mm, of the quasi reference fusion cake. Then the fusion cake was heated electrode œ 10 mm and of the auxiliary up to the operating temperatures (870-900K) at electrode œ 15mm. continuous flowing of high purity argon through the cell. For full dissolution of PuF3 M easurements procedure and NdF3 tablets the molten salt mixture was bubbled by high purity argon. To determine the electrochemical All operations with plutonium trifluoride properties of the fluoride melts constituents were carried out in a shielded glove box. following two methods were used [5,6]: • Cyclic voltammetry with linear potential Electrochemical cell design sweep, • Measurement (versus time) of the open A schematic drawing of the circuit potential (OCP) of cathode (versus Mo electrochemical cell used is shown in Fig. 1. quasi reference electrode) after transitory polarization of the electrochemical cell (hereafter OCP transient curves method). PI-50-1.1 potentiostate with a PR-8 programmator was used in our measurements. 8 XY- recorder N-307 was used for recording 8 the voltammograms and OCP transient curves. 5 A resistance furnace with thermoregulator was used for the cell heating. The salt melt 7 temperature was measured during experiment by a chromel-alumel thermocouple (its 3 9 junction was in the thermostat close to the nickel crucible). The precision of the 6 temperature control inside the cell during the 1 measurements was ±1 ?. 2 A position of current peaks of some metal 4 ions present in the melt on potential axis was Fig. 1. Schematic drawing of the cell for determined from the voltammograms. A value electrochemical studies in fluoride melts. of the deposition potential difference between 1- nickel crucible; 2- auxiliary electrode individual metals was qualitatively determined (graphite, glassy carbon); 3-working electrode from a position of current peaks on potential (Mo); 4-molten Li,Na/F or Li,Na,Be/F axis. More precisely (±20 mV) the deposition mixture; 5- quasi-reference electrode (Mo); 6- potential difference between the metals was thermocouple; 7-stainless steel vessel; 8-argon determined from the OCP transient curves (on input and output; 9- tube used for gas bubbling their horizontal plateaus). and tablets addition to the melt. RESULTS AND DISCUSSION The cell is installed in outer gas-tight vessel with flanges made of stainless steel. On LiF-NaF-ZrF4-LnF3 systems its bottom the massive copper thermostat is placed on alumina ring insulator. The nickel The main reason for experiments with crucible with the melt under investigation is LiF-NaF eutectic melt was to determine the placed inside of the thermostat. The volume of sodium fluoride effect on zirconium and the nickel crucible is equal to 80 cm3. lanthanides electrodeposition from melts. ATALANTE 2004 Nîmes (France) June 21-25, 2004 2 O22-09 Cyclic voltammograms (CV) of LiF-NaF (60:40 mol %) eutectic melt are presented in Fig. 2. One can see, that the preliminary procedures, including; melting under vacuum, mechanical removal of impurities and pre- electrolysis, resulted in high solvent purity. Fig.4. OCP transient curve of Mo working electrode after electrolysis (current-15 mA, time-10 s) in LiF-NaF-ZrF4 melt. Electrodes: Mo (∅1mm)- cathode, Mo (∅1mm)-quasi Fig.2. Cyclic voltammogram of LiF-NaF reference, graphite- anode. C=1000?, argon eutectic melt. Electrodes: Mo ( 1mm)- ∅ working (0.25cm2), Mo (∅1mm)-quasi ref., -2 Cyclic voltammogram and OCP transient graphite œ auxiliary. C=1000?, P∼10 mm Hg. curve of a molten LiF-NaF mixture, containing 1 mol. % of NdF3 are shown in Figs. 5-6. Cyclic voltammogram of LiF-NaFeutectic melt containing 0.5 mol. % of ZrF4 is shown on Fig. 3. The clearly defined current peak corresponding to the reduction of zirconium ions on a molybdenum working electrode is observed on a cathode part of voltammogram at potential about (-1.10 V) versus Mo quasi reference electrode. Anodic current peak at potential (-0.90 V) correspond to this cathodic peak in the anodic area of the voltammogram and determined by Zr dissolution. Fig.5. Cyclic voltammogram of a LiF-NaF + NdF3 (1 mol.%) melt. Electrodes: Mo - working, Mo -quasi reference, graphite œ auxiliary. C=1000?, P∼ 10-2 mm Hg. Fig. 3. Cyclic voltammogram of a LiF-NaF + ZrF4 (0.5 mol.%) melt. Electrodes: Mo (∅1mm)- working, Mo (∅1mm)-quasi ref., graphiteœauxiliary. C=1000?, argon. To confirm this conclusion OCP transient curve obtained after preliminary electrolysis of the melt is presented in Fig. 4. Fig. 6. OCP transient curve of a Mo working One can see, that at potential E = (-0.8 V) electrode after electrolysis (current-100 mA, there is a clearly defined extended plateau time-30 s) in a LiF-NaF-NdF3 melt. determined by Zr deposited on working Electrodes: Mo (∅1mm)- cathode, Mo electrode at electrolysis. (∅1mm)-quasi ref., graphite- anode. -2 C=1000?, P∼ 10 mm Hg. ATALANTE 2004 Nîmes (France) June 21-25, 2004 3 O22-09 As can seen, there are well enough formed anodic area of voltammogram.
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