Electrochemistry of Zirconium in Molten Chloride for Processing Fuel Debris
Authors: Emori Tatsuya , Matsuura Haruaki 
Published: December 2024

INTRODUCTION: In order to develop processing nuclear fuel debris caused by nuclear accident of the Fukushima Daiichi Nuclear Power Plants with huge earthquake and tsunami 2011, we proposed selective fluorination and molten salt electrolysis to separate uranium from nuclear fuel debris. Process scheme is as follows; 1) selective fluorination; UF4 was fluorinated from nuclear fuel debris using HF[1], 2) selective dissolution; UF4 was dissolved in molten bath and 3) molten salt electrolysis; uranium ions were reduced in molten bath, and uranium metal or oxide was deposited on electrode. However coexisted zirconium can be fluorinated with uranium and zirconium oxide will be remained at the upstream process. Therefore, electro-reduction of uranium cannot be made because standard redox potential of zirconium is more positive than uranium. Zirconium electrochemistry due to the presence fluoride ions and oxide should be investigated to reduce zirconium concentration in molten bath.

EXPERIMENTS: All the electrochemical experiments using molten LiCl-KCl have been performed in an electric furnace which is built inside a glove box filled with argon atmosphere using an electrochemical analyzer. Electrochemical analysis has been performed according to both cyclic voltammetry and differential pulsed voltammetry by using the electrodes as follows: working electrode: tungsten, counter electrode: glassy carbon and reference electrode: silver wire dipped in molten LiCl-KCl + AgCl inside the borosilicate tube. Silica glass was used as a crucible. To observe the oxide addition effect, 0 and 10 times amount of Li2O to the concentration of zirconium (0.1 mol%) was added to the molten bath. Constant-voltage and constant-current electrolysis has been carried out during 2000 sec to obtain a sample of characterization for SEM/EDX and XRD by using the electrodes as follows: cathode: tungsten, anode and reference electrode: silver wire dipped in molten LiCl-KCl + AgCl inside the borosilicate tube. All measurements have been performed at 823.15 K. Sweep rate was 50 mV/s and sweep potential range was -0.2 to -1.7(V vs. Ag/AgCl).

RESULTS AND DISCUSSION: As shown in Fig. 1, cyclic voltammetry of cathodic sweeps and anodic sweeps was observed mainly zirconium reduction peaks and oxidation peaks[2]. Increasing current density from -0.8 (V vs. Ag/AgCl) and reduction potential of Zr4+/Zr2+ was observed at -1.4 to -1.0 (V vs. Ag/AgCl). Reduction potential of Zr2+/Zr0 was also observed at -1.6(V vs. Ag/AgCl). The potential of zirconium reduction to metal was not negatively shifted, while, the potential of zirconium to divalent was negatively shifted and the current of zirconium to divalent further decreasing by increasing measurement cycle. It was considered that structure of zirconium ions complex was changed by addition of oxide. Furthermore, this fact suggested that the substance with poor electrical conductivity such as ZrO might be deposited on the electrode. In the constant current electrolysis, it was confirmed that a difference in the voltage change was observed to compare containing Li2O sample and Li2O free sample. In the case of Li2O free sample, the current increases or decreases drastically. It can be expected that zirconium ions concentration in molten bath decreases and the behavior of deposition of zirconium metal changes according to formula of ZrCl4 + 2Li2O = ZrO2 + 4LiCl[3].

REFERENCES:

[1]T.Ono,N.Sato,A.Nezu,T.Uchiyama,H.Matuura,ECS,Trans.,75, 87,2016

[2] M.Iizuka, CRIEPI Report T98001,23,1998

[3] Y.Sakamura, T.Kato, M.Kurata, H.Moriyama, CRIEPI Report T96073,7,1997

Figure 1