Electrode Reaction of Pu3+/Pu Couple in LiCl-KCl Eutectic Melts: Comparison of the Electrode Reaction at the Surface of Liquid Bi with That at a Solid Mo Electrode

Abstract

Pyrochemical reprocessing of nuclear fuels using molten salt as a solvent shows promise for advanced nuclear engineering because of its compactness, economy, radiation resistance and nonproliferation. 1 In the electrorefining process, which is a key step in pyrochemical reprocessing, spent fuels are anodically dissolved in LiCl-KCl eutectic melt, and U, Pu and minor actinides (MAs; Np, Am, Cm, etc.) are recovered together at the cathode. 2,3 It has been proposed that U could be selectively recovered at the solid cathode due to the difference in the formation energies of chlorides, and Pu and MAs at the liquid metal cathode due to their small activity coefficients in the liquid metals such as Cd or Bi. 4,5 During the dissolution of spent fuel, metals or nitrides of alkali, alkaline earths and rare earths are also dissolved into the salt phase at the same time, while metal fission products such as Ru, Tc and Mo are not dissolved. In order to recover actinides effectively from the salt phase, electrolysis using a liquid metal cathode or extraction separation between the salt and the liquid metal has been proposed.2,3 Therefore, several studies were conducted on the distribution behaviors of actinides, alkali, alkaline earths and rare earths between LiCl-KCl eutectic melt and liquid metal, and the activities and separation factors of these elements in liquid metal phase were estimated. [4][5][6][7][8] From the standpoint of separation of actinides and rare earths, Bi is more convenient than Cd as the liquid metal. 4 However, there is little information on the electrode reaction of Pu and MAs at the liquid Bi electrode, and the mechanism has not been completely elucidated. For the liquid Cd electrode, the deviation of the redox potential of metal deposition was interpreted in terms of a lowering of activity of the deposited metal in the Cd phase, and the relation between the activity and the formation energy of intermetallic compounds such as PuCd6 was evaluated in the previous work. 9 The change of the redox potential on metal deposition at Hg electrode or amalgam electrode in aqueous solution has been interpreted in terms of a lowering of activity of the deposited metal in the Hg or amalgam phase. 10,11 The relation between the activity and the formation energy of intermetallic compounds between Hg and deposited metal, however, has not been analyzed in detail.In the present paper, the electrode reaction of Pu 3+ /Pu couple at the interface between LiCl-KCl eutectic melt and liquid Bi was investigated in comparison with a solid electrode. The difference between the redox potentials of Pu 3+ /Pu at the solid electrode and at liquid Bi electrode was thermodynamically interpreted based on the formation energy of PuBi2. The Gibbs free energies of formation of the intermetallic compound, PuBi2, were evaluated by the analysis of cyclic voltammograms in the temperature range from 723 to 823 K. ExperimentalThe electrochemical cell used for the voltammetric studies is shown in Fig. 1. A liquid Bi electrode, which was used a...

Electrochemical Behavior and Some Thermodynamic Properties of UCl[sub 4] and UCl[sub 3] Dissolved in a LiCl-KCl Eutectic Melt

“…Electrochemical behavior of U͑IV͒ was reported in several investigations. [1][2][3][4][5][6][7][8] Electroreduction of UCl 4 in LiCl-KCl eutectic melt proceeds in two steps involving transfer of one and three electrons, respectively. The reversibility of the reduction of U͑IV͒ to U͑III͒ was evidenced by the chronopotentiometry method 1 and confirmed by cyclic voltammetry measurements.…”

Electrochemical behavior of plutonium fluoride species in LiF-CaF2 eutectic melt

“…Among actinides, plutonium was chosen as element of study. In chloride media, electrochemical studies were carried out on PuCl 3 on inert electrode in LiCl-KCl by Serp et al [10] and O. Shirai et al [11,12]. They found that Pu(III) reduction proceeds via a one-step process with 3 electrons exchanged.…”

Vacuum distillation of a mixture of LiCl–KCl eutectic salts and RE oxidative precipitates and a dechlorination and oxidation of RE oxychlorides

“…This pyrochemical process shows promise for an advanced nuclear engineering design because of its compactness, economy, radiation resistance and nonproliferation [2]. Of all the unit operations of a pyrochemical process, an electrorefining is a key step [2,3].…”