Electrochemical and Spectroscopic Monitoring of Interactions of Oxide Ion with U (III) and Ln (III) (Ln = Nd, Ce, and La) in LiCl-KCl Melts

Abstract

Using cyclic voltammetry and UV-vis absorption spectroscopic methods, we studied the precipitation reactions of trivalent uranium and lanthanides (Nd, Ce, and La) with the oxide ion in LiCl-KCl molten salts, which are likely side reactions during the electrorefining process for nuclear fuel treatment. The in situ electrochemical and spectroscopic monitoring demonstrated that U 3+ consumed O 2− with the mole ratio of 2:1, whereas the lanthanide ions reacted with O 2− with the mole ratio of 1:1. Raman spectroscopy and X-ray diffraction were also employed to analyze the precipitates, confirming that U 3+ was precipitated and oxidized into UO 2 , while the trivalent lanthanide ions formed oxychlorides (LnOCl). As a preliminary study of actinide co-precipitation, we also electrochemically monitored the precipitation reactions in a mixed melt containing uranium and lanthanides, demonstrating a higher reactivity of the former with the oxide ion in LiCl-KCl. Understanding the chemical reactions in chloride-based molten salts at high temperatures is of fundamental importance for developing pyroprocessing technologies, which have been extensively examined over the last 30 years as a promising solution for accumulated spent nuclear fuel. [1][2][3][4] In particular, the electrochemical behaviors of actinides and lanthanides in LiCl-KCl melts have been widely studied to explain the fundamental electrochemical reactions in the electrorefining process, which partitions reusable metals and highly radioactive fission products.5-7 However, before the commercialization of this technology, it is necessary to obtain comprehensive insight of the molten salt chemistry of trace elements dissolved from the spent nuclear fuel, and the additives and/or impurities that enter during the process. 8,9 Li 2 O is a critical ingredient for the electrochemical reduction of spent oxide fuel. It affords O 2− ions that should diffuse to the anode at the beginning of the reduction process, and controls the O 2− concentration in the LiCl melt to prevent the Pt cathode from dissolving. 3 However, a trace amount of Li 2 O likely remains in the reduced fuel, even after distilling out the molten salt at the end of the oxide reduction process, and can enter the next process. During the electrorefining process, the oxide ion may react with trivalent actinide and lanthanide ions in the LiCl-KCl eutectic melt, affording insoluble oxides and oxychlorides 10,11 which contaminate the salt and reduce the purity and yields of recovered uranium. In addition, unreduced lanthanide oxides in the metal fuel feed play the role of oxo-donor in the LiClKCl melt.12 Therefore, knowledge of the reactions between the actinide/lanthanide ions and the oxide ion in the molten salt could lead to improvement in the electrorefining technology. Several research groups have studied such reactions by potentiometric titration of the oxide ion in the melt and product characterization, in order to estimate the thermodynamic properties for oxide reduction 10,12-15 and to partition lanthan...

Exploring the Complex Chemistry of Uranium within Molten Chloride Salts

“…While HCl­(g) purification was attempted with some salts, minor contaminants from the purification vessel interfered with the optical signals and chemical behavior. The literature suggests that upon the addition of UCl 4 to the molten salt containing oxygen, the UCl 4 is oxidized to a U­(V) species, , likely UO 2 Cl or UOCl 3 (eq ). The U­(V) species then disproportionates into U­(IV) and U­(VI) .…”

Reaction of Oxygen with Uranium (IV) Chloride in Fused Alkali Chlorides

“…8 Addition of oxide ions (as Li 2 O) to LiCl-KCl-UCl 3 melts led to precipitation of uranium dioxide, and uranium was completely removed from the melt at O 2-: U 3+ molar ratio of two. 11 This result was subsequently confirmed, although a small amount of metallic uranium was also detected in the precipitate in addition to UO 2 , constituting the bulk of the solid phase. 12 Treating solutions of uranium tetrachloride in fused alkali chlorides with oxygen or argon-oxygen mixtures resulted in partial precipitation of uranium as UO 2 ; most uranium was oxidized to uranyl species and remained in the soluble state in the melt.…”

Review—Concentration Measurements In Molten Chloride Salts Using Electrochemical Methods

“…Those interested in usable data from all the studies that were considered can consult the following list. 55,59,60,79,[90][91][92][93][94][105][106][107][108][109][110][111][112][114][115][116][117][122][123][124][125][126][127][128][129][130][131][132][133] In an effort to present the comparative effectiveness of each technique, criteria were applied to reviewed experiments when analyzing them. Only information presented clearly in the literature was able to be applied.…”

“…A variety of studies have been conducted in systems with multiple electroactive species. 59,60,94,98,[105][106][107][108][109][110][111][112][113][114][115][116][117] These studies typically find that in deposition reactions the accuracy of concentration measurement is diminished when reduction peaks overlap, and even when separated, the second deposition reaction is affected by the first. Standard reduction potentials indicate that corrosion of typical structural metals will impact the accuracy of the vital measurements for U and Pu in electrorefiners.…”