Photon Factory Activity Report 2016 #34(2017) B

BL27B/2015G026

Electrochemical deposition of uranium dioxide in highly concentrated electrolyte atroom temperature
Akihiro UEHARA1,*, Toshiyuki FUJII2, and Yoshihiro OKAMOTO3

Division of Nuclear Engineering Science, Research Reactor Institute, Kyoto University,2-1010, Asashiro Nishi, Kumatori, Sennan, Osaka, 590-0494, Japan
2
Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering,Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
3
Material Science Research Center, Japan Atomic Energy Agency,
2-4, Shirakatashirane, Tokai, Ibaraki 319-1195, Japan
1. Introduction
The calcium chloride hexahydrate, CaCl2·6H2O, andsimilar highly concentrated electrolytes (i.e., 14 M (moldm-3) LiCl) are of considerable interest as solvents withproperties intermediate between those of aqueous solution
and molten salts. Activity coefficients, viscosity andstructural analyses all indicate that the water molecule inthe melts strongly coordinates to the calcium ion showingthat this melt is a liquid with the properties of an ionicmelt composed of bulky hydrated calcium cations andchloride anions. Namely, this material has advantages
such as inorganic-based melt and low melting
temperature. Uranium forms various oxidation states inaqueous solution (trivalent to hexavalent), and it isimportant to estimate the chemical state of uranium ionsin the fields of nuclear fuel cycles and environmentalscience. In the present study, redox reactions of uranium
UO22+ in 14 M LiCl were investigated by electrochemical
methods and X-ray absorption spectroscopy. The
chemical behavior of uranium ion in the hydrate melt wascompared with that in the other solvents such as ionicliquids[2].
2. Experimental
Sample preparation
Uranyl chloride hydrate was dissolved into 14 M LiCl.
The uranium concentration used in the EXAFS
measurements was 0.05 M. Other chemicals were usedwithout further purification.
Electrochemical cellThree electrodes consist of reference, working, andcounter electrodes, respectively Reference electrode is
Ag|AgCl containing 1 M LiCl, working electrode isglassy carbon fiber to enlarge the current density[1,3].
Counter electrode is Ag wire containing 1M LiCl
separated from sample solution by porous film to avoidgas evolution. The cell was stored in containment vessel.
IVIUM compact stat was used for electrochemicalmeasurements.
EXAFS data acquisition
EXAFS measurements were carried out at BL27B
beam-line of the Photon Factory in High Energy
Accelerator Research Organization (KEK), Tsukuba,
Japan. EXAFS measurements based on the U LIIIabsorption edge were performed. All the solutions weremeasured in fluorescence mode using a Ge solid state
detector.
3. Results
In-situ XAFS was performed by applying a constantpotential or current. The 10 scans were collected andaveraged, as shown in Fig. 1. The edge jump measured
before the electrolysis slightly shifted lower energy
depending on the electrolysis (curves 1 to 4). Curve 5,
which was measured after the electrolysis, was similar tothe spectrum with that of UO2 solid standard. Also, thesolution showed black color afterwards, indicating typicalUO2 deposit. This reaction is not normally observedaqueous acidic solution at room temperature but hightemperature ionic liquids.

before1Normalized XANES

1

17.13

2
3
4
5

17.18
17.23
E (keV)

Fig. 1: LIII-edge XANES spectra of uranium ions in 14 M
LiCl before and after the electrolysis. Ten scans wereaveraged.
Reference[1] A. Uehara, et al., J. Appl. Electrochem. 42 (2012) 455.
[2] N. Asanuma, et al., J. Nucl. Sci. Technol. 44 (2007)
368.
[3] A. Uehara et al., Radiochim. Acta, 104 (2016) 1.
*auehara@rri.kyoto-u.ac.jp