ELECTROCHEMICAL STUDY OF EUROPIUM TRICHLORIDE IN MOLTEN EUTECTIC LICL-KCL

Transcription

1 ELECTROCHEMICAL STUDY OF EUROPIUM TRICHLORIDE IN MOLTEN EUTECTIC LICL-KCL Cncha Caravaca, Guadalupe Córdba, María Jesús Tmás CIEMAT, Nuclear Fissin Department Avda. Cmplutense,, Madrid 8040, España Abstract The present wrk is part f a research prject that study the feasibility f the actinide separatin frm the rest f fissin prduct cntained in the nuclear spent fuel by pyrchemical prcesses with the aim f their transmutatin. In rder t design these prcesses it is necessary t determine basic thermdynamic data f the elements, especially actinides, in the mlten salt, such as standard redx ptential, standard free Gibbs energy f chlride frmatin, and activity cefficients f slvatin. The electrchemical study f eurpium trichlride in mlten eutectic LiCl-KCl has been perfrmed at slid cathdes, vitreus carbn and tungsten, in a temperature range f K in rder t btain these basic prperties. Here are presented sme preliminary results btained in this study, which indicate that Eu(III) is reduced t Eu(II) by a single step thrugh a quasi-reversible reactin. The average diffusin cefficient f Eu(III) ins is D cm s -1 at 73 K. 93

2 Intrductin Partitining and transmutatin (P&T) cncepts are being studied wrldwide as they intend t reduce the inventries f actinides and lng-lived fissin prducts in nuclear waste with the aim f reducing its lng-term raditxicity. At present, there is an increasing interest in the applicatin f pyrchemical separatin techniques in which the minr actinides are recycled and burned in dedicated reactrs. The fact that the fuels prpsed fr such reactrs will cntain significant amunts f actinides, and will reach high burn-ups, makes the pyrchemical separatin techniques quite attractive due t the high radiatin stability f the salt slvent (chlrides r flurides) and the resulting shrter cling times [1,]. Pyrchemical methds in mlten chlrides have been tested in labratry and ht pilt scale in rder t separate uranium, plutnium and minr actinides frm fissin prducts. Amng all the pyrchemical prcesses prpsed, t achieve the separatin f actinides, the electrrefining f U int a slid cathde and Pu and minr actinides int a liquid Cd cathde in a mlten salt electrlyte, applied t metal fuels frm the EBR-II reactr (IFR prcess), is ne f the mst develped [3,4]. Later n ther prcesses such as reductive extractin using a metallic phase such as Cd r Bi and a salt phase, r the electrdepsitin f TRU int mlten cathdes (Cd r Bi) [5, 6, 7], and mre recently the cdepsitin f actinides n slid cathdes (Al) [8] have been envisaged. The aim f this research is t increase the actinide separatin efficiency and t minimise the lanthanide cntent in the prduct. Therefre, it is imprtant t have an accurate thermdynamic and electrchemical data base fr all the actinides and lanthanides [9, 10, 11]. In this wrk several electrchemical techniques have been used in rder t calculate sme thermdynamic prperties, the mechanism f reductin, and the diffusin cefficient f Eu in the mlten eutectic LiCl-KCl. Experimental The electrchemical experiments were carried ut under argn atmsphere f high quality (<0.5 ppm water and < 0.1 ppm xygen). The electrlyte was cnstituted by the eutectic LiCl-KCl (59-41%ml). Befre each experiment the melt was purified by bubbling HCl(g) fr at least 1 hur, then Ar (g) was bubbled (45-60 min) in rder t remve the HCl disslved int the mlten bath. The mixture was placed in a vitreus carbn crucible that is intrduced in a quartz cell which in turn is placed in a well type ven. Temperature cntrl was carried ut by a K type thermcuple shielded in an Al O 3 tube. The electrlytic bath cnsisting f Eu 3+ disslved in LiCl-KCl eutectic was prepared by disslving anhydrus EuCl 3 (Aldrich 99.9%) in the melt with HCl(g) r Cl (g)+c (s). The use f Cl +C was bserved t be necessary in rder t reach the cmplete disslutin f the EuCl 3 ; accrding t the literature reviewed this is due t the frmatin f the Eu xychlride, EuOCl [1]. Wrking electrdes used in the electrchemical study were a tungsten wire (1mm) and a vitreus carbn rd (3mm). The surface area f the electrdes was determined after each experiment by measuring the immersin depth f the electrde. As cunter electrde a graphite rd (3mm) was used. The reference electrde was an Ag/LiCl-KCl-AgCl (0.75 ml kg -1 ) prepared in a PYREX glass tube. The electrchemical techniques used, i.e. cyclic vltammetry and chrnptentimetry, were carried ut in an electrchemical cell having three electrde set-up and a PAR 73A ptentistatgalvanstat with a EG&E M70 electrchemical sftware. 94

3 Cncentratin f Eu was determined by ICP-MS analysis. Samples were taken frm the mlten salt, disslved and diluted in nitric acid. Results and discussin Reversibility study The cyclic vltammgrams recrded at different scan rates ( V s -1 ) at a vitreus carbn and tungsten electrde, in a slutin f EuCl 3 ( ml kg -1 ) in LiCl-KCl are shwn in Figure 1 and Figure respectively. A single cathdic peak I c is assciated with an andic peak I a. The shape f the vltammgrams is characteristic f a sluble prduct frmatin. The number f electrns exchanged in the reductin reactin was determined frm the width f the peak at half height f the square wave vltammgrams [ 1 ]. The number f electrns calculated that way is ne, therefre, the reductin reactin prduced is: 1 EuCl3 + 1 e EuCl + Cl (g) (1) Figure 1. Cyclic vltammgrams f Eu 3+ in LiCl-KCl at 450 C I a Current /A I c Ptential /V vs Ag/AgCl Scan rate: 0.08 V/s 0.1 V/s 0.3 V/s 0.5 V/s 0.7 V/s 0.9 V/s [Eu 3+ ]: ml kg -1, vitreus carbn wrking electrde, S= 0.7 cm. 95

4 Figure. Cyclic vltammgrams f Eu 3+ in LiCl-KCl at 450 C I a Current /A Scan rate: V/s 0.08 V/s V/s 0.3 V/s I c 0.5 V/s V/s 0.8 V/s Ptential /V vs Ag/AgCl [Eu 3+ ]= ml kg -1, W wrking electrde, S= 0.3 cm The analysis f these vltammgrams indicates that the current f the cathdic peak (Ic) is directly prprtinal t the square rt f the scan rate, v [V s -1 ], as it is shwn in Figure 3, and that up t scan rates f 0. V s -1 the ptential peak, Ep, is cnstant and independent f the scan rate. Accrding t the thery f the linear sweep vltammetry [14], fr scan rates lwer than 0.V s -1, the electrde prcess is reversible and cntrlled by the diffusin f the electractive especies. Fr scan rates higher than 0. Vs -1, the cathdic peak ptential shifts negatively, this indicate that the system becmes quasi-reversible [13,15]. This behaviur has als been bserved by S.A. Kuznetsv [16] in the equimlar NaCl-KCl melt in the temperature range K. 96

5 Figure 3. Variatin f the current peak with the ptential scan rate Current /ma v 1/ / V 1/ s -1/ [Eu 3+ ] = ml kg -1, W wrking electrde, S=0.3cm Diffusin cefficient The diffusin cefficient f Eu (III) species was calculated by using tw techniques, cnvlutin and chrnptentimetry. Accrding t the cvlutin principle [13,17,18] the cyclic vltammgram data were transfrmed int a resembling steady-state vltammgram curves. The typical semi-integral curves at lw scan rates at a W electrde are indicated in Figure 4, in which it is bserved that the curves at different scan rates are cincident and that the limiting value f the current, m*, is cnstant fr scan rates lwer than 0.1 V s

6 Figure 4. Semi-integrals f cyclic vltammgrams at different scan rates m* Scan rate: 0.07 V/s 0.08 V/s 0.09 V/s Ptential /V vs Ag/AgCl EuCl 3 cncentratin: ml kg -1, W wrking electrde, temperature 73K. The diffusin cefficient f Eu (III) can be determined accrding t the cnvlutin thery using the fllwing equatin [17,18]: m* = nfscd 1/ () where n is the number f electrns exchanged, S the surface area f the wrking electrde (cm ), m* is the limiting current value f the semi-integral curve (A), F the Faraday Cnstant (C ml -1 ), C the Eu(III) cncentratin (ml cm -3 ), and D the diffusin cefficient (cm s -1 ). The results btained fr bth wrking electrdes at the temperatures tested are shwn in Table 1. Table 1. Diffusin cefficient f Eu(III) in LiCl-KCl by cnvlutin Temperature /K D 10 5 / cm s -1 (Tungsten) D 10 5 / cm s -1 (V. Carbn) ΔH /kj ml

7 Diffusin cefficients were als calculated by chrnptentimetry, at different impsed current densities. The prduct iτ ½ is independent f bth i, as can be bserved in Figure 5 the plt f iτ ½ versus i gives a straight line, fr tw different cncentratins; based n that, the reductin f Eu(III) can be cnsidered as a diffusin cntrlled prcess [13,19]. Figure 5. Plt f the functin f iτ ½ vs. i ml cm ml cm i τ 1/ /A cm - s 1/ i /A cm - Temperature: 73K, W wrking electrde. Reference electrde Ag/AgCl(0.75 ml kg -1 ). The diffusin cefficient can be, therefre, calculated using the Sand equatin [13,0]: i nfc πd τ = (3) i being the current density (A cm - ). Frm the slpe f the plt f i versus τ -½ (Figure 6), the diffusin cefficient was calculated. 99

8 Figure 6. Dependence f current density versus τ 1/ i / A cm τ -1/ / s -1/ EuCl 3 cncentratin: ml kg -1. Temperature: 73K, W wrking electrde (S= 0.78 cm ) In Table are indicated the average values btained fr different temperatures and EuCl 3 cncentratins Table. Diffusin cefficient f Eu(III) in LiCl-KCl by chrnptentimetry Temperature /K D 10 6 / cm s -1 Tungsten D 10 6 / cm s -1 Vitreus Carbn ΔH /kj ml The cmparisn f results btained frm bth techniques, Table 1 and, indicates that there is a great difference between them, being quite higher thse btained by cnvlutin. The extraplatin f the results btained by chrnptentimetry at 673K wuld be clse t the value btained by G.J. Janz [1] at this temperature ( cm s -1 ). Diffusin cefficients variatin with temperature fllws the Arrhenius law. Frm the slpe f the plt f lg D versus 1/T the activatin energy fr diffusin has been determined accrding the equatin [13]: ΔH lg D = cte (4). 3 RT The values btained fr the activatin energy using bth techniques (Table 1 and Table ) indicate that values btained by cnvlutin are clse t thse btained by S.A. Kuznetsv [16] in NaCl-KCl in the temperature range K by different electrchemical techniques. 300

9 Frmal standard redx ptential In the eutectic LiCl-KCl nly the xidatin states f Eu(III) and Eu(II) has been bserved, the ptential f the Eu(II)/Eu(0) system is mre negative than.7 V versus AgCl/Ag electrde []. The apparent standard ptential f the redx cuple Eu(III)/Eu(II) has been determined using the fllwing equatin [16]: c a ( ) ( ) 1 / Ep + Ep /. 3 RT nf lg D D E' + Eu( III ) / Eu( II ) = x / red (5) Values f D Eu(II) were estimated frm the current intensity peaks f the cyclic vltammgrams at 0.1 Vs -1 using the Randles-Sevcik equatin [13,0]: ip = ( nf) 3 / ( RT ) 1/ SCD 1/ v 1/ (6) Table 3 shws the results btained at different temperatures fr the tw electrde tested, in mlal scale, versus Cl /Cl - reference. Table 3. Frmal standard redx ptential E Eu(III)/Eu(II) versus Cl /Cl - electrde. Tungsten Vitreus carbn T /K E Eu(III)/Eu(II) γ 10 3 (EuCl 3 ) E Eu(III)/Eu(II) γ 10 3 (EuCl 3 ) The ptential value btained at 73K with the vitreus carbn electrde is gd agreement with the ne btained by K.E. Jhnsn et al. [3] in this melt at the same temperature at Pt electrde. The standard free energy f frmatin, ΔG 3 = ΔG EuCl3 ' EuCl nfe Eu ( III ) / Eu( II ), is calculated accrding t: (7) The plt f the ΔG EuCl3 by the fllwing expressin: as a functin f temperature shws a linear dependence, and can be expressed ΔG = ΔH T ΔS EuCl3 EuCl3 EuCl 3 (8) frm which the values f enthalpies ( ) and entrpies ( ) f frmatin are btained fr bth electrdes. ΔH EuCl3 ΔS EuCl3 Δ / 1 GEuCl 3 = T kj ml Δ / 1 GEuCl 3 = T kj ml fr vitreus carbn electrde. fr tungsten electrde 301

10 Activity cefficients f EuCl 3 in the LiCl-KCl, γ EuCl 3, have been calculated frm the difference between the Gibbs free energy f frmatin btained frm the electrchemical measurements and the Gibbs free energy f frmatin fr pure cmpunds at the liquid state btained frm literature [4] RT lnγ EuCl = ΔG Δ 3 EuCl G 3 EuCl3 (9) The activity cefficient values btained are shwn in Table 3, it is bserved fr bth electrdes an increase f the values as the temperature increases which indicate a decrease f the strength f the chlride ins cmplex frmed by the EuCl 3 with the temperature. Cnclusin The electrchemical behaviur f EuCl 3 has been investigated in mlten LiCl-KCl in the temperature range K. It has been bserved that Eu(III) is reduce t Eu(II) by a single step with exchange f ne electrn, presenting the reductin reactin a reversible behaviur fr lw sweep ptential rate and a quasi-reversible behaviur fr higher scan rates. The average diffusin cefficient f EuCl 3 calculated is clse t cm s -1. Apparent standard ptential has been calculated by the cyclic vltammetry technique, the value btained at 73K with the vitreus carbn electrde is in gd agreement with the data fund in the literature. Activity cefficients f EuCl 3 indicate that this in frm cmplexes with the chlride ins, EuCl 6 3-, f the mlten bath, and that the strength f these cmplexes decreases with the increase f the temperature. REFERENCES [1] Actinide and Fissin Prduct Partitining and Transmutatin (1999). Status and Assessment Reprt. OECD. [] U.S. DOE, Office f Nuclear Energy, Science and Technlgy (003). Reprt t Cngress n Advanced Fuel Cycle Initiative. The Future Path fr Advanced Spent Fuel Treatment and Transmutatin Research. [3] J. Laidler, J. Battler, W. Miller, J. Ackerman and E. Carls (1997). Prg. Nucl. Energy, 31, (1/), 131. [4] T. Inue (00). Actinide recycling by pyr-prcess with metal fuel FBR fr future nuclear fuel cycle system, Prg. Nucl. Energy, 40, (3/4), 547. [5] O. Shirai, K. Uzumi, T. Iwai, and Y. Arai (001). Analytical Sciences, 17, [6] O. Shirai, M. Iizuka, T. Iwai, Y. Arai (001). Analytical Sciences, 17, 51. [7] O. Shirai, K. Uzumi, T. Iwai, Y. Arai (004). J. Applied Electrchem., 34,

11 [8] J. Serp, R. Malmbeck, E. Yakub, J.P. Glatz (003). Electrdepsitin f actinides n slid aluminium in LiCl-KCl eutectic, Prc. f Glbal 003, New Orleans, Lusiana, USA, 16-0 Nvember, [9] J.J. Ry, I.F. Grantham, D.L. Grimmett (1996). J. Electrchem Sc. 143, 487. [10] Y. Sakamura Y., T. Hijikata, K. Kinshita, T. Inue, T.S. Strvick, et al. (1998). J. f Allys &Cmpunds, 71-73, 59. [11] S.P. Fusselman, J.J. Ry, D.L. Grimmett, L.F. Grantham, et al. (1999). J. Electrchem. Sc., 146, 573. [1] A. Nvselva, V. Khkhlv and V. Shishkin (00). Int. Jmat Thnsatad Sympsium. Haarberg, Trndheim, Nrway [13] A. J. Bard and L.R. Faulkner (1980). Electrchemical Methds. Fundamentals and Applicatins, Jhn Wiley & sns, New Yrk. [14] R.S. Nichlsn and I. Sahain (1964). Anal. Chem. 45, 043. [15] R. Greef, R. Peat, L.M. Peter, D. Pletcher, (Suthamptn Electrchemistry Grup) (1990). Instrumental Methds in Electrchemistry, Ellis Hrwd Ltd. [16] S.A.Kuznetsv, M. Gaune-Escard (001). Electrchim. Acta, 46, [17] M. Gt, B. Oldham (1973). Anal. Chem., 45, 043. [18] M. Grennes, B. Oldham (197). Anal. Chem. 44, 111. [19] R.K. Jain, H.C. Gaur and B.J. Welch (1977). J. Electranal. Chem., 79, 11. [0] P. Delahay (1954). New Instrumental Methds in Electrchemistry. Interscience Inc. New Yrk. [1] G.J. Janz, N.P. Bansal (198). J. f Phys. Chem. reference data, 11 (3), 505. [] J.A. Plambeck, Encyclpedia f Electrchemistry f the Elements, Fused salt systems. Vl. X., [3] K.E. Jhnsn, J.R. Mackenzie (1969).. J. Electrchem. Sc., 116, [4] I. Barin, O. Knacke (1977). Thermchemical Prperties f Inrganic Substances. Springer- Verlag, Berlin. [ 1 ] A. J. Bard and L.R. Faulkner (1980). Electrchemical Methds. Fundamentals and Applicatins, Jhn Wiley & sns, New Yrk. 303