Electrochemical transient techniques for study of the electrochemistry and thermodynamics of nuclear materials in molten salts
Introduction
The pyrometallurgical process is now considered as one of the most promising options in an innovative nuclear fuel cycle [1]. The lanthanide (Ln) and actinide (An) elements present in spent fuel from fast nuclear reactors can be converted into molten salts by anodic dissolution [1]. The actinides (An) are selectively deposited at the cathode due to the differences among the redox potentials of the elements while fission products remain in the anode and in the electrolyte [2], [3]. Due to their similar chemical properties the lanthanide elements are the most difficult fission products to separate from actinides. Until now, the various elements present in nuclear wastes are separated by hydrometallurgical processes. Accordingly, many investigations focus at the present time on alternative technologies based on pyrochemical processes for a more compact fuel cycle, allowing the reduction of all wastes. In such techniques, molten alkali halide salt baths are widely used.
Electrochemical transient techniques (ETT) is a powerful method for studying kinetics especially for the highly radioactive isotopes of Am and Cm, because the experiment should be carried out in limited time. In these pyrochemical processes, actinide and lanthanide chlorides are usually diluted in a molten salt medium. In this case, as we already showed [4], [5], the electrochemical transient techniques is also an effective method for the determination of thermodynamic properties.
Nevertheless, before publication [4] there was a certain discussion about the correctness of non-stationary electrochemical methods for thermodynamic determinations in molten salts. Up to now, a certain opinion exists [6] that ETT can be used for rough marks, for example, of formal standard potentials of lanthanides, actinides in melts, and only the EMF method or potentiometry provide reliable data.
The aim of this study to describe our approach and advantages of electrochemical transient techniques in comparison with the EMF method and potentiometry for determining the Ln and An thermodynamic properties in molten salts.
Section snippets
Chemicals
Synthesis of lanthanide chlorides was described in detail earlier [7], [8], [9]. Tetrachloride of uranium was synthesized from U3O8 (Merck) by hydrogen reduction to UO2 and then by chlorination with CCl4 vapors [10]. Trichloride of uranium was prepared by the oxidation of U with CdCl2 in LiCl–KCl melt at 773 K [11]. Polagraphic-grade alkali chlorides (NaCl, KCl, CsCl and NaF) were purchased from Prolabo (99.5 wt% min.) and they were recrystallized twice in a platinum crucible. LiCl–KCl eutectic
Interaction of oxide materials with LiCl-KCl-UCl4 and LiCl-KCl-UCl3 melts
Transformation of voltammetric curves after the addition of oxide ions (Li2O) to the melt LiCl–KCl–UCl4 is shown in Fig. 2. After Li2O introduction to the uranium-containing melt the heights of the waves (R1, R2), corresponding to the electroreduction of uranium chloride complexes [13], [14], [15]:decreased and two new waves (R3, R4) appeared at more positive potentials (Fig. 2(b)). Finally, at the ratio O2−/U(IV) = 1.3–1.5 only these two waves (R3, R4) remained on
Conclusions
Advantages and disadvantages of electrochemical transient techniques and potentiometry for the determination of formal standard potentials were discussed. It was shown that electrochemical transient techniques offer several advantages over potentiometry. The main convenience of electrochemical transient techniques is the possibility of formal standard potentials determination for irreversible process and redox process accompanied by reaction of disproportionation.
Acknowledgement
SAK wishes to thank the Ecole Polytechnique de Marseille for hospitality and support during this work.
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