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Effect of cathode material on the electrorefining of U in LiCl-KCl molten salts
Journal of Nuclear Materials 488 (2017) 210e214
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effect of cathode material on the electrorefining of U in LiCl-KCl molten salts Chang Hwa Lee a, *, Tack-Jin Kim a, Sungbin Park a, Sung-Jai Lee a, Seung-Woo Paek a, Do-Hee Ahn a, Sung-Ki Cho b a
Nuclear Fuel Cycle Process Development Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 34057, Republic of Korea Department of Chemical Engineering, Kumoh National Institute of Technology, 61 Daehakro, Gumi, Gyeongbuk 730-701, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2017 Received in revised form 15 March 2017 Accepted 15 March 2017 Available online 16 March 2017
The influence of cathode materials on the U electrorefining process is examined using electrochemical measurements and SEM-EDX observations. Stainless steel (STS), Mo, and W electrodes exhibit similar U reduction/oxidation behavior in 500 C LiCl-KCl-UCl3 molten salts, as revealed by the cyclic voltammograms. However, slight shifts are observed in the cathodic and anodic peak potentials at the STS electrode, which are related to the fast reduction/oxidation kinetics associated with this electrode. The U deposits on the Mo and W electrodes consist of uniform dendritic chains of U in rhomboidal-shaped crystals, whereas several U dendrites protruding from the surface are observed for the STS electrode. EDX mapping of the electrode surfaces reveals that simple scraping of the U dendrites from W electrodes pretreated in dilute HCl solutions to dissolve the residual salt, results in clear removal of the U deposits, whereas a thick U deposit layer strongly adheres to the STS electrode surface even after treatment. This result is expected to contribute to the development of an effective and continuous U recovery process using electrorefining. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Pyroprocessing is an important proliferation-resistant technology by which the extent of high-level wastes and geological repository space can be greatly reduced. In this method, U and transuranic (TRU) elements from used nuclear fuels (UNFs) can be electrochemically recovered and recycled, thereby maximizing material utilization [1,2]. The pyroprocess developed at Korea Atomic Energy Research Institute (KAERI) consists of a head-end process in which a feed material is produced from the UNFs and a back-end process in which U and U/TRU ingots are produced from the feed material using electro-reduction and subsequent electrorecovery processes [3]. The electro-recovery process includes electrorefining of U, which constitutes about 93 wt % of the UNFs, at a solid cathode and electrowinning of the U/TRU product at a liquid cathode by applying a voltage or current to the electrochemical cell. This process leaves behind fission products in the molten salt phase.
* Corresponding author. E-mail address: [email protected] (C.H. Lee). http://dx.doi.org/10.1016/j.jnucmat.2017.03.023 0022-3115/© 2017 Elsevier B.V. All rights reserved.
Metallic U is recovered on the solid cathode in the dendrite form during the U electrorefining step, while U is simultaneously dissolved from the UNFs in the anode basket owing to the following reaction that occurs in high temperature molten salts.
Anode : U/U3þ þ 3e
(1)
Cathode : U3þ þ 3e /U
(2)
The electrochemical behavior of U has been studied on various cathode materials [4e7]. However, it is difficult to directly compare the electrochemical parameters for the reduction/oxidation reactions of U (e.g., U(III)/U(0) couple) such as electromotive force (EMF), activity coefficient, and diffusion coefficient across different studies, because experimental conditions including electrolyte composition, operating temperature, electrode material, and cell geometry are different in different studies. Leseur reported that the diffusion coefficient of U(III) measured with a graphite electrode was lower than that obtained with a W electrode [4]. Poa et al. examined the effect of working electrode material on the electrochemical behavior of U and found that there was a larger residual
C.H. Lee et al. / Journal of Nuclear Materials 488 (2017) 210e214
current when Mo was used as the working electrode compared to steel, whereas there was a small cathodic peak preceding the reduction peak corresponding to U(III)/U(0) when a steel electrode was used for the electrochemical measurements. The latter might be attributed to the formation of an intermetallic compound with U [5]. In addition, Sakamura et al. reported that although the electrochemical behavior of U was similar on both the Mo and W working electrodes, the influence of the electrode material on the electrochemical behavior should be taken into account because the electrode material introduces complexities in the reaction such as underpotential deposition, alloy formation, and/or phase transition [7]. To utilize pyroprocessing for the large-scale treatment of UNFs, high throughput and high recovery yield of U are necessary. Therefore, it is important to effectively collect the U dendrites grown on the solid cathodes and ensure reusability of the cathode for enabling continuous operation. At Argonne National Laboratory (ANL) and Central Research Institute of Electric Power Industry (CRIEPI), several efforts have been made to increase the U recovery throughput at an engineering scale by increasing the electrode surface area as well as by improving the scraping system [8,9]. However, U dendrite residues strongly adhere to the surface of steel cathodes and the removal of the residues requires a further periodic stripping process. In a previous study, Kang et al. examined the superior selfscraping property of graphite cathodes in the context of U electrorefining, by comparing sticking coefficient calculations [10]. Although the graphite cathode can be used for continuous operation, the fact that the physical strength and durability of the graphite electrode are generally not as good as metal electrodes, may be problematic. In particular, lithium intercalation or corrosion caused by vaporized potassium metal might lead to deterioration in the properties of the graphite cathode in high temperature molten salts [11,12]. In addition, uranium-graphite intercalation compounds (U-GICs) [13] detached from the outer electrode surface could affect the purity of the recovered U owing to the formation of uranium carbide products (UCx) [14,15]. Therefore, in this study, we investigate the electrochemical behaviors of U on various electrode materials and the effect of the electrodes for U electrorefining on the morphological features and scraping properties of U dendrites. Specifically, we select stainless steel (STS), Mo, and W electrodes as alternatives for graphite. STS is the most commonly used electrode material for U deposition, which is Fe alloyed with Cr, Ni, Mn, and etc. On the other hand, Mo and W have larger lattice parameters (aMo,bcc ¼ 3.142, aW,bcc ¼ 3.155) compared to U (aU,orthorhombic ¼ 2.854) and Fe (aFe,bcc ¼ 2.856) or Cr (aCr,bcc ¼ 2.885), as a result of which larger interfacial stress may be present between the substrate and deposit, which may result in effective scraping properties. 2. Experimental An anhydrous LiCl-KCl eutectic (99.99 wt%, Sigma-Aldrich) was used for U electrorefining at 500 C and was prepared by adding UCl3 produced by the reaction of U metal with CdCl2 in a 600 C LiCl-KCl eutectic salt and subsequently separating from Cd. All the experiments were performed in an Ar-purged glove box, the oxygen and moisture levels of which were controlled within a few ppm. Electrochemical measurements and U electrorefining experiments were performed using a three-electrode electrochemical cell consisting of an STS basket filled with U metal as the anode and Ag/Agþ (made of Ag wire; 1Ø) as the reference electrode in LiClKCl-1 wt.% AgCl shrouded with a mullite tube. The working electrodes were prepared with various metallic materials including STS, Mo, and W rods in order to investigate the effect of electrode
211
material on the morphological features and scraping properties of electrorefined U. The STS was the Grade-304 type that consists of about 65e70 wt% of Fe, 18e20 wt% of Cr, 8e12 wt% Ni, ~2 wt% of Mn, and ~1 wt% of other impurities. The purities of Mo and W rods were 99.9 wt% and 99.95 wt%, respectively. The diameter and exposed length of the working electrodes were 3 mm and 30 mm, respectively. Cyclic voltammetry measurements were performed from the open circuit potential (~-1.0 V vs. Ag/Agþ) to 1.8 V for U ion reduction, followed by scanning to 0.8 V for U oxidation at a scan rate of 50 mV/s under quiescent conditions using a potentiostat/galvanostat (BioLogics SP-150). For analyzing the effect of substrate material on the U deposits, U electrorefining was conducted at a constant current of 150 mA (52 mA/cm2, calculated based on the initial electrode surface area) for 2 h. Field emission-scanning electron microscopy (FE-SEM, Hitachi SU8010) and energy dispersive X-ray spectroscopy (EDX) measurements were utilized to observe the morphological features and analyze the composition of the U dendrites as well as the electrode surface after removing the deposits by immersing in dilute HCl solutions for 30 min.
3. Results and discussion 3.1. Cyclic voltammetry Fig. 1 shows the cyclic voltammetric curves for U reduction/ oxidation on various electrode materials in 500 C LiCl-KCl-3 wt.% UCl3 molten salts. The onset potentials for U reduction on all the electrode materials are similar in the potential range of 1.39 to 1.37 V. However, the U reduction and oxidation peak potentials on the STS electrode are 1.57 and 1.09 V, respectively, which are more positive for the cathodic reaction and more negative for the anodic reaction compared to the potentials on the Mo and W electrodes. This deviation is a result of the faster U reduction/ oxidation kinetics on the STS surface, which might be associated with the interactions between U and Fe-Cr-Ni, which are the major elements of STS, during the nucleation and deposition of U on STS. The results of the cyclic voltammetry measurements at various scan rates are shown in Table 1. While peak shifts are evident for all the electrode materials, the peak shifts increase towards the negative direction with increase in the scan rate for the cathodic reaction, whereas it increases in the positive direction for the anodic reaction. This might be associated with quasi-reversible or irreversible electron transfer of U3þ/U, which can be seen from the wider separation between Ep,a and Ep,c compared to the theoretical value, for the reversible process. In the case of the Mo and W electrodes, the peak potential (Ep) and current (ip) could not be measured at high scan rates over 200 mV/s within the potential range considered, owing to the peak shifts. However, a nearly linear relationship between the peak current and square root of scan rate is evident for the STS electrode, which indicates that the reaction is diffusioncontrolled. From the relationship between the peak current and square root of scan rate, the diffusion coefficient for the reduction of U(III) ions in the 500 C LiCl-KCl salts can be calculated using Berzins-Delahay equation [16,17] as follows:
ip ¼ 0:4958
aF 3 RT
12
1
n2 AD20 C0* n2 3
1
(3)
Where F is the Faraday constant (96,480 C/mol), R is the molar gas constant (8.314 J/mol$K), n is the number of electrons, A is the surface area of the cathode (cm [2]), D0 is the diffusion coefficient of U ions (cm [2]/s), C0* is the bulk molar concentration of U ions (mol/ cm [3]), a is the charge transfer coefficient, which is assumed as 0.5,
212
C.H. Lee et al. / Journal of Nuclear Materials 488 (2017) 210e214
Fig. 1. (a) Cyclic voltammograms at various electrode materials in 500 C LiCl-KCl-UCl3 molten salts. The initial exposed surface area of the electrode was fixed at 2.9 cm2 and potential scan rate was 50 mV/s (b) A plot of ip vs. n1/2 for the reduction of U on the STS electrode. Table 1 Peak potentials and currents associated with U reduction and oxidation obtained at various scan rates in the cyclic voltammograms measured for various electrode materials. Electrode material
Scan rate (mV/s)
Ep,c (V)
Ep,a (V)
ip,c (mA)
ip,a (mA)
STS
50 100 200 400 50 100 200 400 50 100 200 400
1.57 1.64 1.71 1.80 1.61 1.73 e e 1.62 1.73 e e
1.10 1.01 0.94 0.87 1.03 0.943 0.888 0.850 1.02 0.929 0.874 0.841
226 336 449 588 199 324 e e 206 319 e e
362 490 601 705 345 425 483 521 360 446 510 546
Mo
W
and n is the scan rate (V/s). The calculated D0 value was found to be 8.78 106 cm2/s, which agrees reasonably well with values
reported in peer-reviewed literature [18e20]. To characterize the U dendrites and electrode surface after scraping, chronopotentiometric measurements were performed on the various electrode materials at a constant current of 150 mA for 2 h. The cathodic overpotential is constant at about 1.50 V during deposition for the Mo and W electrodes and at about 1.48 V for the STS electrode, which is in good agreement with the results of the I-V curves. After 2 h of electrorefining, each electrode was positioned and held in the middle of the heating zone out of the electrolyte for 1 h in order to reduce the amount of residual salt in the deposit. The first and second columns of Fig. 2 show photos of the electrode surfaces before and after U electrorefining, respectively. In the case of the STS electrode, needle-like long-branch shaped U is found to protrude from the surface of the U dendrite body, which might cause a short circuit problem for longer deposition times. On the other hand, U dendrites grown on the Mo and W electrodes are seen to be almost identical in terms of the morphological features. A purple-tinged surface indicates the presence of residual LiCl-KCl-UCl3 salt.
Fig. 2. Photographs of various electrode materials: (a) STS, (b) Mo, and (c) W, before (1st column) and after (2nd column) U electrorefining followed by U scraping (3rd column). A constant current of 150 mA was applied for 2 h between the anode and cathode. The U deposit was scraped by softly tapping the bottom after immersing the electrode for 30 min in dilute HCl solution.
C.H. Lee et al. / Journal of Nuclear Materials 488 (2017) 210e214
213
the STS electrode, residual particles are observed on the surface as indicated by the dark contrast, which might be associated with a rough surface of deposited U. However, the W electrode appears to have the shiniest surface among the electrodes.
3.2. Characterization of the U dendrites and electrode surfaces
Fig. 3. SEM images of electrorefined U dendrites grown on various electrode materials: (a) STS, (b) Mo, and (c) W rods. The samples were prepared from the electrode materials shown in Fig. 2 by rinsing in dilute HCl solution and drying in air.
In order to compare the scraping properties of various electrode materials, each electrode containing U dendrites was immersed in a dilute HCl solution for 30 min in order to dissolve the residual salt. Following this, the dendrites were dislodged from the electrode surface by gently hitting the electrode against the bottom of the HCl solution beaker. The U dendrites on the Mo and W electrodes could be easily detached without forced scraping, whereas the U dendrites on STS did not detach from the surface without forced scraping. Pictures of the electrode surfaces after the removal of the U dendrites are shown in the third column of Fig. 2. In the case of
For a more detailed observation of the U dendrites and electrode surfaces, SEM and EDX measurements were performed. The U dendrites grown on various electrodes were rinsed with deionized water, following which they were dried in air. Fig. 3 shows SEM images of the U deposits grown on various electrodes. Dendriteshaped chains of U are found to be composed of small rhomboidal crystals linked together for the deposits grown on both Mo and W. In the case of the STS electrode, thin and long-branched U crystals are apparent as identified in Fig. 2. To indirectly evaluate the scraping properties of U for various electrode materials, the surfaces of the electrodes were examined using SEM and EDX mapping, as shown in Fig. 4. As seen in the low magnification SEM images, there are substantial amounts of residues on the surface. The electrodes may be ordered as follows in terms of the amount of residues: STS > Mo > W. Compositional analysis using EDX mapping for various electrode materials reveals that the surface coverage of U and residual salts is high, with electrodes ranked in the order of STS > Mo > W. In particular, STS is almost fully covered with thick U layers in addition to the residual salt components. As a result, the Ka signals of Fe, which is a primary element of STS, from the substrate are convoluted with those of the residue. The strong adhesion between the U deposits and underlying STS is consistent with the interactions between U and STS in the CV curve. On the other hand, small U particles exist on the Mo electrode surface, although the Mo La signal from beneath the particles is clearly detected. In the case of the W electrode, the U Ma peak is found to be almost negligible and only residual salt crystals are observed on the surface, which confirms that W exhibits superior scraping properties for U dendrites compared to the STS and Mo electrodes. This observation might be considered to be associated with a bigger difference in the lattice parameters between W and U compared to that between Fe-Cr-Ni and U, which gives rise to a larger interfacial stress in the former case. In addition, W does not
Fig. 4. SEM and EDX mapping images of the surfaces of various electrode materials after U dendrite detachment; (a) stainless steel, (b) Mo, and (c) W.
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C.H. Lee et al. / Journal of Nuclear Materials 488 (2017) 210e214
form any binary alloy phases with U under the given operating conditions [21], unlike STS and Mo, which might facilitate the detachment of U from the W electrode surface. 4. Conclusions The effect of cathode materials on the U electrorefining process was investigated in 500 C LiCl-KCl-3 wt.% UCl3 molten salts. Although nearly similar electrochemical behavior of U is observed for the STS, Mo, and W electrodes based on the cyclic voltammograms, reduction and oxidation peak shifts are evident for the STS electrode, which might be associated with faster kinetics owing to the underpotential deposition of U on STS. Electrorefining of U at a constant current of 150 mA results in general U dendrite growth on the Mo and W electrode surfaces, whereas long-branch shaped U is observed on the STS electrode surface. Investigations on the scraping characteristics of U for various electrodes reveal that U dendrites are easily scraped off the electrodes. The electrodes may be ranked as follows in terms of the facility with which the U dendrites are scraped: W > Mo > STS. This is also verified by the residual U observation on the surface using SEM-EDX mapping analysis. The results of this study show the possibility of using W electrode as an alternative to graphite for effective U recovery. Acknowledgement This work was supported by the National Research Foundation of Korea grant funded by the Korean Ministry of Science, ICT and Future Planning (NRF-2012M2A8A5025699). References [1] Z. Tomczuk, J.P. Ackerman, R.D. Wolson, W.E. Miller, Uranium transport to solid electrodes in pyrochemical reprocessing of nuclear fuel, J. Electrochem. Soc. 139 (1992) 3523e3528. [2] J.J. Laidler, J.E. Battles, W.E. Miller, J.P. Ackerman, E.L. Carls, Development of pyroprocessing technology, Prog. Nucl. Energ 31 (1997) 131e140. [3] H. Lee, J.M. Hur, J.G. Kim, D.H. Ahn, Y.Z. Cho, S.W. Paek, Korean pyrochemical process R&D activities, Energy Proc. 7 (2011) 391e395.
[4] A. Leseur, Chronopotentiometry in Molten Chlorides, in Application to the Study of Electrochemical Properties of Uranium and Pluronium in the LiCl-KCl Eutectic, CEA R-3793, 1969. [5] D.S. Poa, Z. Tomczuk, R.K. Steunenberg, Voltammetry of uranium and plutonium in molten LiCl-NaCl-CaCl2-BaCl2, J. Electrochem. Soc. 135 (1988) 1161e1166. [6] Y. Sakamura, T. Hijikata, K. Kinoshita, T. Inoue, T.S. Storvick, C.L. Krueger, J.J. Roy, D.L. Grimmett, S.P. Fusselman, R.L. Gay, Measurement of standard potentials of actinides (U, Np, Pu, Am) in LiCl-KCl eutectic salt and separation of actinides from rare earths by electrorefining, J. Alloy Compd. 271 (1998) 271e273. [7] O. Shirai, T. Iwai, Y. Suzuki, Y. Sakamura, H. Tanaka, Electrochemical behaviour of actinide ions in LiCl-KCl eutectic melts, J. Alloy Compd. 271 (1998) 271e273. [8] D. Lewis, D. Graziano, J.F. Miller, G. Vandegrft, Chemical Engineering Division Annual Technical Report, ANL-04/06, Argonne National Laboratory, 2004. [9] K. Uozumi, High Throughput Electrorefining of Uranium in Pyro-reprocessing, CRIEPI Annual Research Report, 2004, pp. 38e39. [10] Y.H. Kang, J.H. Lee, S.C. Hwang, J.B. Shim, E.H. Kim, S.W. Park, Electrodeposition characteristics of uranium by using a graphite cathode, Carbon 44 (2006) 3113e3148. [11] R. Baboian, D.L. Hill, R.A. Bailey, Electrochemical studies on zirconium and hafnium in molten LiCl-KCl eutectic, J. Electrochem. Soc. 112 (1965) 1221e1224. [12] Z.X. Shu, R.S. McMilan, J.J. Murray, Electrochemical intercalation of lithium into graphite, J. Electrochem. Soc. 140 (1993) 922e927. [13] S. Danzenbacher, S.L. Molodtsov, J. Boysen, C. Laubschat, Behavior of uranium 5f states in a graphite intercalation compound, Phys. Rev. B 64 (2001) 035404. [14] J.W. Koger, C.E. Holcombe, J.G. Banker, Coatings on graphite crucibles used in melting uranium, Thin Solid Films 39 (1976) 297e303. [15] I.-Y. Song, Y.-S. Lee, E.-S. Kim, D.-B. Lee, C.-K. Kim, A study on the formation of uranium carbide in an induction furnace, Proc. Korean Nucl. Soc. Spring Meet. Jeju (2005) 429e430. [16] B. Reddy, S. Vandarkuzhali, T. Subramanian, P. Venkatesh, Electrochemical studies on the redox mechanism of uranium chloride in molten LiCl-KCl eutectic, Electrochim. Acta 49 (2004) 2471e2478. [17] R.S. Nicholson, I. Shain, Theory of stationary electrode polarography, Anal. Chem. 36 (1964) 706. [18] A.J. Bard, L.R. Faulkner, Electrochemical Methods Fundamentals and Applications, second ed., John Wiley & Sons Inc, 2001. [19] K. Serrano, P. Taxil, Electrochemical reduction of trivalent uranium ions in molten chloride, J. Appl. Electrochem 29 (1999) 497e503. [20] P. Masset, R.J.M. Konings, R. Malmbeck, J. Serp, J.-P. Glatz, Thermochemical properties of lanthanides (Ln¼La, Nd) and actinides (An¼U, Np, Pu, Am) in the molten LiCl-KCl eutectic, J. Nucl. Mater 344 (2005) 173e179. [21] H. Okamoto, Desk Handbook: Phase Diagrams for Binary Alloys, second ed., ASM international, 2010.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effect of cathode material on the electrorefining of U in LiCl-KCl molten salts Chang Hwa Lee a, *, Tack-Jin Kim a, Sungbin Park a, Sung-Jai Lee a, Seung-Woo Paek a, Do-Hee Ahn a, Sung-Ki Cho b a
Nuclear Fuel Cycle Process Development Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 34057, Republic of Korea Department of Chemical Engineering, Kumoh National Institute of Technology, 61 Daehakro, Gumi, Gyeongbuk 730-701, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2017 Received in revised form 15 March 2017 Accepted 15 March 2017 Available online 16 March 2017
The influence of cathode materials on the U electrorefining process is examined using electrochemical measurements and SEM-EDX observations. Stainless steel (STS), Mo, and W electrodes exhibit similar U reduction/oxidation behavior in 500 C LiCl-KCl-UCl3 molten salts, as revealed by the cyclic voltammograms. However, slight shifts are observed in the cathodic and anodic peak potentials at the STS electrode, which are related to the fast reduction/oxidation kinetics associated with this electrode. The U deposits on the Mo and W electrodes consist of uniform dendritic chains of U in rhomboidal-shaped crystals, whereas several U dendrites protruding from the surface are observed for the STS electrode. EDX mapping of the electrode surfaces reveals that simple scraping of the U dendrites from W electrodes pretreated in dilute HCl solutions to dissolve the residual salt, results in clear removal of the U deposits, whereas a thick U deposit layer strongly adheres to the STS electrode surface even after treatment. This result is expected to contribute to the development of an effective and continuous U recovery process using electrorefining. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Pyroprocessing is an important proliferation-resistant technology by which the extent of high-level wastes and geological repository space can be greatly reduced. In this method, U and transuranic (TRU) elements from used nuclear fuels (UNFs) can be electrochemically recovered and recycled, thereby maximizing material utilization [1,2]. The pyroprocess developed at Korea Atomic Energy Research Institute (KAERI) consists of a head-end process in which a feed material is produced from the UNFs and a back-end process in which U and U/TRU ingots are produced from the feed material using electro-reduction and subsequent electrorecovery processes [3]. The electro-recovery process includes electrorefining of U, which constitutes about 93 wt % of the UNFs, at a solid cathode and electrowinning of the U/TRU product at a liquid cathode by applying a voltage or current to the electrochemical cell. This process leaves behind fission products in the molten salt phase.
* Corresponding author. E-mail address: [email protected] (C.H. Lee). http://dx.doi.org/10.1016/j.jnucmat.2017.03.023 0022-3115/© 2017 Elsevier B.V. All rights reserved.
Metallic U is recovered on the solid cathode in the dendrite form during the U electrorefining step, while U is simultaneously dissolved from the UNFs in the anode basket owing to the following reaction that occurs in high temperature molten salts.
Anode : U/U3þ þ 3e
(1)
Cathode : U3þ þ 3e /U
(2)
The electrochemical behavior of U has been studied on various cathode materials [4e7]. However, it is difficult to directly compare the electrochemical parameters for the reduction/oxidation reactions of U (e.g., U(III)/U(0) couple) such as electromotive force (EMF), activity coefficient, and diffusion coefficient across different studies, because experimental conditions including electrolyte composition, operating temperature, electrode material, and cell geometry are different in different studies. Leseur reported that the diffusion coefficient of U(III) measured with a graphite electrode was lower than that obtained with a W electrode [4]. Poa et al. examined the effect of working electrode material on the electrochemical behavior of U and found that there was a larger residual
C.H. Lee et al. / Journal of Nuclear Materials 488 (2017) 210e214
current when Mo was used as the working electrode compared to steel, whereas there was a small cathodic peak preceding the reduction peak corresponding to U(III)/U(0) when a steel electrode was used for the electrochemical measurements. The latter might be attributed to the formation of an intermetallic compound with U [5]. In addition, Sakamura et al. reported that although the electrochemical behavior of U was similar on both the Mo and W working electrodes, the influence of the electrode material on the electrochemical behavior should be taken into account because the electrode material introduces complexities in the reaction such as underpotential deposition, alloy formation, and/or phase transition [7]. To utilize pyroprocessing for the large-scale treatment of UNFs, high throughput and high recovery yield of U are necessary. Therefore, it is important to effectively collect the U dendrites grown on the solid cathodes and ensure reusability of the cathode for enabling continuous operation. At Argonne National Laboratory (ANL) and Central Research Institute of Electric Power Industry (CRIEPI), several efforts have been made to increase the U recovery throughput at an engineering scale by increasing the electrode surface area as well as by improving the scraping system [8,9]. However, U dendrite residues strongly adhere to the surface of steel cathodes and the removal of the residues requires a further periodic stripping process. In a previous study, Kang et al. examined the superior selfscraping property of graphite cathodes in the context of U electrorefining, by comparing sticking coefficient calculations [10]. Although the graphite cathode can be used for continuous operation, the fact that the physical strength and durability of the graphite electrode are generally not as good as metal electrodes, may be problematic. In particular, lithium intercalation or corrosion caused by vaporized potassium metal might lead to deterioration in the properties of the graphite cathode in high temperature molten salts [11,12]. In addition, uranium-graphite intercalation compounds (U-GICs) [13] detached from the outer electrode surface could affect the purity of the recovered U owing to the formation of uranium carbide products (UCx) [14,15]. Therefore, in this study, we investigate the electrochemical behaviors of U on various electrode materials and the effect of the electrodes for U electrorefining on the morphological features and scraping properties of U dendrites. Specifically, we select stainless steel (STS), Mo, and W electrodes as alternatives for graphite. STS is the most commonly used electrode material for U deposition, which is Fe alloyed with Cr, Ni, Mn, and etc. On the other hand, Mo and W have larger lattice parameters (aMo,bcc ¼ 3.142, aW,bcc ¼ 3.155) compared to U (aU,orthorhombic ¼ 2.854) and Fe (aFe,bcc ¼ 2.856) or Cr (aCr,bcc ¼ 2.885), as a result of which larger interfacial stress may be present between the substrate and deposit, which may result in effective scraping properties. 2. Experimental An anhydrous LiCl-KCl eutectic (99.99 wt%, Sigma-Aldrich) was used for U electrorefining at 500 C and was prepared by adding UCl3 produced by the reaction of U metal with CdCl2 in a 600 C LiCl-KCl eutectic salt and subsequently separating from Cd. All the experiments were performed in an Ar-purged glove box, the oxygen and moisture levels of which were controlled within a few ppm. Electrochemical measurements and U electrorefining experiments were performed using a three-electrode electrochemical cell consisting of an STS basket filled with U metal as the anode and Ag/Agþ (made of Ag wire; 1Ø) as the reference electrode in LiClKCl-1 wt.% AgCl shrouded with a mullite tube. The working electrodes were prepared with various metallic materials including STS, Mo, and W rods in order to investigate the effect of electrode
211
material on the morphological features and scraping properties of electrorefined U. The STS was the Grade-304 type that consists of about 65e70 wt% of Fe, 18e20 wt% of Cr, 8e12 wt% Ni, ~2 wt% of Mn, and ~1 wt% of other impurities. The purities of Mo and W rods were 99.9 wt% and 99.95 wt%, respectively. The diameter and exposed length of the working electrodes were 3 mm and 30 mm, respectively. Cyclic voltammetry measurements were performed from the open circuit potential (~-1.0 V vs. Ag/Agþ) to 1.8 V for U ion reduction, followed by scanning to 0.8 V for U oxidation at a scan rate of 50 mV/s under quiescent conditions using a potentiostat/galvanostat (BioLogics SP-150). For analyzing the effect of substrate material on the U deposits, U electrorefining was conducted at a constant current of 150 mA (52 mA/cm2, calculated based on the initial electrode surface area) for 2 h. Field emission-scanning electron microscopy (FE-SEM, Hitachi SU8010) and energy dispersive X-ray spectroscopy (EDX) measurements were utilized to observe the morphological features and analyze the composition of the U dendrites as well as the electrode surface after removing the deposits by immersing in dilute HCl solutions for 30 min.
3. Results and discussion 3.1. Cyclic voltammetry Fig. 1 shows the cyclic voltammetric curves for U reduction/ oxidation on various electrode materials in 500 C LiCl-KCl-3 wt.% UCl3 molten salts. The onset potentials for U reduction on all the electrode materials are similar in the potential range of 1.39 to 1.37 V. However, the U reduction and oxidation peak potentials on the STS electrode are 1.57 and 1.09 V, respectively, which are more positive for the cathodic reaction and more negative for the anodic reaction compared to the potentials on the Mo and W electrodes. This deviation is a result of the faster U reduction/ oxidation kinetics on the STS surface, which might be associated with the interactions between U and Fe-Cr-Ni, which are the major elements of STS, during the nucleation and deposition of U on STS. The results of the cyclic voltammetry measurements at various scan rates are shown in Table 1. While peak shifts are evident for all the electrode materials, the peak shifts increase towards the negative direction with increase in the scan rate for the cathodic reaction, whereas it increases in the positive direction for the anodic reaction. This might be associated with quasi-reversible or irreversible electron transfer of U3þ/U, which can be seen from the wider separation between Ep,a and Ep,c compared to the theoretical value, for the reversible process. In the case of the Mo and W electrodes, the peak potential (Ep) and current (ip) could not be measured at high scan rates over 200 mV/s within the potential range considered, owing to the peak shifts. However, a nearly linear relationship between the peak current and square root of scan rate is evident for the STS electrode, which indicates that the reaction is diffusioncontrolled. From the relationship between the peak current and square root of scan rate, the diffusion coefficient for the reduction of U(III) ions in the 500 C LiCl-KCl salts can be calculated using Berzins-Delahay equation [16,17] as follows:
ip ¼ 0:4958
aF 3 RT
12
1
n2 AD20 C0* n2 3
1
(3)
Where F is the Faraday constant (96,480 C/mol), R is the molar gas constant (8.314 J/mol$K), n is the number of electrons, A is the surface area of the cathode (cm [2]), D0 is the diffusion coefficient of U ions (cm [2]/s), C0* is the bulk molar concentration of U ions (mol/ cm [3]), a is the charge transfer coefficient, which is assumed as 0.5,
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Fig. 1. (a) Cyclic voltammograms at various electrode materials in 500 C LiCl-KCl-UCl3 molten salts. The initial exposed surface area of the electrode was fixed at 2.9 cm2 and potential scan rate was 50 mV/s (b) A plot of ip vs. n1/2 for the reduction of U on the STS electrode. Table 1 Peak potentials and currents associated with U reduction and oxidation obtained at various scan rates in the cyclic voltammograms measured for various electrode materials. Electrode material
Scan rate (mV/s)
Ep,c (V)
Ep,a (V)
ip,c (mA)
ip,a (mA)
STS
50 100 200 400 50 100 200 400 50 100 200 400
1.57 1.64 1.71 1.80 1.61 1.73 e e 1.62 1.73 e e
1.10 1.01 0.94 0.87 1.03 0.943 0.888 0.850 1.02 0.929 0.874 0.841
226 336 449 588 199 324 e e 206 319 e e
362 490 601 705 345 425 483 521 360 446 510 546
Mo
W
and n is the scan rate (V/s). The calculated D0 value was found to be 8.78 106 cm2/s, which agrees reasonably well with values
reported in peer-reviewed literature [18e20]. To characterize the U dendrites and electrode surface after scraping, chronopotentiometric measurements were performed on the various electrode materials at a constant current of 150 mA for 2 h. The cathodic overpotential is constant at about 1.50 V during deposition for the Mo and W electrodes and at about 1.48 V for the STS electrode, which is in good agreement with the results of the I-V curves. After 2 h of electrorefining, each electrode was positioned and held in the middle of the heating zone out of the electrolyte for 1 h in order to reduce the amount of residual salt in the deposit. The first and second columns of Fig. 2 show photos of the electrode surfaces before and after U electrorefining, respectively. In the case of the STS electrode, needle-like long-branch shaped U is found to protrude from the surface of the U dendrite body, which might cause a short circuit problem for longer deposition times. On the other hand, U dendrites grown on the Mo and W electrodes are seen to be almost identical in terms of the morphological features. A purple-tinged surface indicates the presence of residual LiCl-KCl-UCl3 salt.
Fig. 2. Photographs of various electrode materials: (a) STS, (b) Mo, and (c) W, before (1st column) and after (2nd column) U electrorefining followed by U scraping (3rd column). A constant current of 150 mA was applied for 2 h between the anode and cathode. The U deposit was scraped by softly tapping the bottom after immersing the electrode for 30 min in dilute HCl solution.
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the STS electrode, residual particles are observed on the surface as indicated by the dark contrast, which might be associated with a rough surface of deposited U. However, the W electrode appears to have the shiniest surface among the electrodes.
3.2. Characterization of the U dendrites and electrode surfaces
Fig. 3. SEM images of electrorefined U dendrites grown on various electrode materials: (a) STS, (b) Mo, and (c) W rods. The samples were prepared from the electrode materials shown in Fig. 2 by rinsing in dilute HCl solution and drying in air.
In order to compare the scraping properties of various electrode materials, each electrode containing U dendrites was immersed in a dilute HCl solution for 30 min in order to dissolve the residual salt. Following this, the dendrites were dislodged from the electrode surface by gently hitting the electrode against the bottom of the HCl solution beaker. The U dendrites on the Mo and W electrodes could be easily detached without forced scraping, whereas the U dendrites on STS did not detach from the surface without forced scraping. Pictures of the electrode surfaces after the removal of the U dendrites are shown in the third column of Fig. 2. In the case of
For a more detailed observation of the U dendrites and electrode surfaces, SEM and EDX measurements were performed. The U dendrites grown on various electrodes were rinsed with deionized water, following which they were dried in air. Fig. 3 shows SEM images of the U deposits grown on various electrodes. Dendriteshaped chains of U are found to be composed of small rhomboidal crystals linked together for the deposits grown on both Mo and W. In the case of the STS electrode, thin and long-branched U crystals are apparent as identified in Fig. 2. To indirectly evaluate the scraping properties of U for various electrode materials, the surfaces of the electrodes were examined using SEM and EDX mapping, as shown in Fig. 4. As seen in the low magnification SEM images, there are substantial amounts of residues on the surface. The electrodes may be ordered as follows in terms of the amount of residues: STS > Mo > W. Compositional analysis using EDX mapping for various electrode materials reveals that the surface coverage of U and residual salts is high, with electrodes ranked in the order of STS > Mo > W. In particular, STS is almost fully covered with thick U layers in addition to the residual salt components. As a result, the Ka signals of Fe, which is a primary element of STS, from the substrate are convoluted with those of the residue. The strong adhesion between the U deposits and underlying STS is consistent with the interactions between U and STS in the CV curve. On the other hand, small U particles exist on the Mo electrode surface, although the Mo La signal from beneath the particles is clearly detected. In the case of the W electrode, the U Ma peak is found to be almost negligible and only residual salt crystals are observed on the surface, which confirms that W exhibits superior scraping properties for U dendrites compared to the STS and Mo electrodes. This observation might be considered to be associated with a bigger difference in the lattice parameters between W and U compared to that between Fe-Cr-Ni and U, which gives rise to a larger interfacial stress in the former case. In addition, W does not
Fig. 4. SEM and EDX mapping images of the surfaces of various electrode materials after U dendrite detachment; (a) stainless steel, (b) Mo, and (c) W.
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form any binary alloy phases with U under the given operating conditions [21], unlike STS and Mo, which might facilitate the detachment of U from the W electrode surface. 4. Conclusions The effect of cathode materials on the U electrorefining process was investigated in 500 C LiCl-KCl-3 wt.% UCl3 molten salts. Although nearly similar electrochemical behavior of U is observed for the STS, Mo, and W electrodes based on the cyclic voltammograms, reduction and oxidation peak shifts are evident for the STS electrode, which might be associated with faster kinetics owing to the underpotential deposition of U on STS. Electrorefining of U at a constant current of 150 mA results in general U dendrite growth on the Mo and W electrode surfaces, whereas long-branch shaped U is observed on the STS electrode surface. Investigations on the scraping characteristics of U for various electrodes reveal that U dendrites are easily scraped off the electrodes. The electrodes may be ranked as follows in terms of the facility with which the U dendrites are scraped: W > Mo > STS. This is also verified by the residual U observation on the surface using SEM-EDX mapping analysis. The results of this study show the possibility of using W electrode as an alternative to graphite for effective U recovery. Acknowledgement This work was supported by the National Research Foundation of Korea grant funded by the Korean Ministry of Science, ICT and Future Planning (NRF-2012M2A8A5025699). References [1] Z. Tomczuk, J.P. Ackerman, R.D. Wolson, W.E. Miller, Uranium transport to solid electrodes in pyrochemical reprocessing of nuclear fuel, J. Electrochem. Soc. 139 (1992) 3523e3528. [2] J.J. Laidler, J.E. Battles, W.E. Miller, J.P. Ackerman, E.L. Carls, Development of pyroprocessing technology, Prog. Nucl. Energ 31 (1997) 131e140. [3] H. Lee, J.M. Hur, J.G. Kim, D.H. Ahn, Y.Z. Cho, S.W. Paek, Korean pyrochemical process R&D activities, Energy Proc. 7 (2011) 391e395.
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