Electrochemical and Microscopic Study of Thorium in a Molten Fluoride System

Abstract

The electrochemistry of thorium in molten LiF-CaF 2 was examined on inert (Mo) and reactive (Ni) electrodes at 1173 K using cyclic voltammetry and chronopotentiometry. A single-step reduction mechanism was observed on the Mo electrode. Voltammetric results show signs of irreversibility. The diffusion coefficient of Th(IV) ions was calculated from the chronopotentiometric data. Deposition of metallic thorium was verified from the electrolytic experiment and subsequent XRD and SEM analyses. The formation of Th-Ni alloys was confirmed on the Ni electrode using cyclic voltammetry, open-circuit chronopotentiometry and SEM + EDS analysis of the deposit after a short electrolysis.Molten salts (fluorides or chlorides) were initially considered for nuclear energy production because of their advantageous physical and chemical properties: good heat transfer capacity, radiation insensitivity, high boiling point, and wide range solubility for actinides. In addition, it was realized that molten salts could be used in numerous situations: high-temperature heat transfer, core coolants with solid fuels, liquid fuel in a molten-salt reactor, solvents for spent nuclear solid fuel in the case of pyro-reprocessing and coolant, among others. 1 Among other applications, the electrochemical separation of actinides from lanthanides, 2 and molten salt/liquid metal extractions 3 are considered to be suitable methods for the reprocessing of spent nuclear fuel in the fuel cycles of future types of nuclear reactors. 2,4 Pyrochemical processes based on molten-salt systems (chlorides or fluorides) have significant advantages over aqueous processes (higher radiation stability, wide temperature range of usability, low process volume, etc.). The knowledge of actinide and lanthanide electrochemistry is necessary to develop pyrochemical separation methods. This paper aims to describe the basic electrochemical behavior of thorium in a molten-fluoride system (LiF-CaF 2 ), characterize the solidified melts with SEM + EDS and evaluate possibilities of electrolytic deposition of Thorium based on the obtained results. Inert (Mo) and reactive (Ni) electrodes were used.Untill now, several papers were published on the electrochemistry of Th in both molten chlorides and fluorides. Two different mechanisms were proposed based on the obtained results in molten chlorides. Smirnov et al. 5 observed a two-step mechanism according to the reactions:The one-step reduction of Th(IV) ions in chloride media according to Equation 3 was described in papers. 6-11In fluoride systems, Clayton et al., 12 Baes 13 and Chamelot et al. 4,14 observed the one-step reduction (Eq. 3) with four exchanged electrons. In, 4 it is stated that the electrolysis of Th on an inert electrode is a suitable method to separate thorium from lanthanides. ExperimentalChemicals and materials.-LiF and CaF 2 of commercial origin (purity 99.50% and 99.96% respectively) were dried in a vacuum dryer at 250 • C. The ThF 4 purity was verified with XRD analysis. The impurities were under the detection level ...

Influence of Fluoride Ions on the Speciation and Electrochemical Behavior of Th(IV) in Molten LiCl-KCl with a Copper Electrode

“…1b) was observed at 0.88 V with the corresponding oxidation peak at 1.26 V. The difference of 0.38 V in potential between the oxidation and reduction peaks as well as the ratio of oxidation and reduction peak currents (1.35) indicate the irreversible character for the redox reactions involving Th(IV) and Th(0). 25,26 This is consistent with the conclusion obtained from the LiF-CaF 2 -ThF 4 system, 8 but different from other results where reversible or quasi-reversible mechanism was established based on the fact that the reduction potential was independent of the potential scanning rate. 7,15,19 In addition to the reduction peak arising from the Th(IV)/Th(0) couple, there is a weak reduction wave (R 2 in Fig.…”

“…The presence of oxide is common for freshly deposited metallic actinides as a result of oxidation. 8,22 The electrolysis result clearly demonstrates that the R 2 peak which shows up in Figs. 1b and 2 is not associated with the reduction of Th(IV) to Th(II), but due to a simple four-electron reduction to form metallic thorium.…”

“…Although formation of Th(II) has been claimed in the alkali chloride, bromide and iodide molten salts, 29 a single four-electron process was confirmed in the LiF-CaF 2 -ThF 4 and LiCl-KCl-ThCl 4 melts. 8,30 Therefore, the nature associated with peak R 2 needs to be further clarified. Origin of the R 2 peak in the voltammagrams of Th(IV) fluoride in molten LiCl-KCl.-The major difference in terms of whether Th(II) or Th(0) is formed from Th (IV) is that metallic thorium would be obtained if a potential between the R 1 and R 2 peaks were applied.…”

“…In fluoride melts, diffusion-controlled reversible or quasi-reversible reduction process was confirmed for the reduction of Th(IV) to Th(0) on molybdenum electrodes at low potential scanning rate, 6,7 while the same process was found to be irreversible in the LiF-CaF 2 -ThF 4 melt. 8 Besides these studies, investigations were also carried out on the separation and purification of thorium. Thorium and lanthanide separation has been achieved in molten LiF-CaF 2 with tantalum, molybdenum and nickel electrodes on the basis of their reduction potential differences.…”

“…9 It is also possible to recover thorium as a Th-Ni alloy from the LiF-CaF 2 -ThF 4 mixture. 8 Similar to the fluoride molten salt, chloride molten salt is a kind of electrolyte widely used for electrochemical studies as well. [10][11][12][13][14] It is therefore an alternative to dissolve fluorides in chloride molten salt for the purpose of thorium fuel reprocessing.…”