Electrochemical extraction of strontium from molten salts using reactive zinc and aluminum electrodes

Abstract

Herein, the electrochemical behaviors of Sr on inert W electrode and reactive Zn/Al electrodes were systematically investigated in LiCl–KCl–SrCl2 molten salts at 773 K using various electrochemical methods. The chemical reaction potentials of Li and Sr on reactive Zn/Al electrodes were determined. We observed that Sr could be extracted by decreasing the activity of the deposited metal Sr on the reactive electrode, although the standard reduction potential of Sr(II)/Sr was more negative than that of Li(I)/Li. The electrochemical extraction products of Sr on reactive Zn and Al electrodes were Zn13Sr and Al4Sr, respectively, with no codeposition of Li observed. Based on the density functional theory calculations, both Zn13Sr and Al4Sr were identified as stable intermetallic compounds with Zn-/Al-rich phases. In LiCl–KCl molten salt containing 3wt% SrCl2, the coulombic efficiency of Sr in the Zn electrode was ~54%. The depolarization values for Sr on Zn and Al electrodes were 0.864 and 0.485 V, respectively, exhibiting a stronger chemical interaction between Zn and Sr than between Al and Sr. This study suggests that using reactive electrodes can facilitate extraction of Sr accumulated while electrorefining molten salts, thereby enabling the purification and reuse of the salt and decreasing the volume of the nuclear waste.

1. Introduction

Nuclear energy represents a long-term source of energy to meet the demands of developed countries [1]. Advanced nuclear energy systems generate substantial quantities of spent nuclear fuel, which requires management via open or closed fuel cycles. Closed fuel cycles can considerably enhance the utilization efficiency of nuclear fuel but necessitate separation of the fission products [2].

Spent nuclear fuel comprises fission products, actinides, and unreacted uranium. Of the hundreds of fission product isotopes generated, most rapidly decay into nonradioactive isotopes. A notably important exception is 90Sr, one of the fission products posing considerable concern. The radiotoxicity of 90Sr, with a half-life of ~29 years, decays via the emission of highly energetic gamma rays. It represents a considerable proportion of the radioactivity in nuclear waste fission products, requiring ~300 years to diminish to the levels of natural uranium ore. Consequently, the separation of Sr from nuclear waste can substantially increase the spatial utilization efficiency of geological repository [3]. Meanwhile, pyroprocessing is currently one of the most promising technologies for treating commercially spent nuclear fuel and reducing the capacity required in nuclear waste storage facilities [4]. Electrorefining of metallic fuels is a key unit operation in pyroprocessing. As the electrorefining process proceeds, SrCl2 gradually accumulates in the molten salt, affecting the properties of the molten salt and thereby influencing the separation of lanthanides and actinides. Research on the separation and extraction of Sr has also contributed to the purification and reuse of molten salts in the electrorefining process [5].

Purification of molten salts following electrorefining enables the reuse of these salts, necessitating the extraction of accumulated fission products in the molten salt [6]. Fission products in the molten salts include rare earth elements, alkali metals, and alkaline earth metals. Selective separation of these elements can facilitate segregated storage, considerably reducing the volume of waste [7]. Accordingly, researchers have developed various methods, such as ion exchange [8–9], precipitation [10–13], and melt crystallization [14–16]. However, the efficiency of removing fission products via precipitation was relatively low. While ion exchange offers higher efficiency, it generates a considerable amount of ceramic waste and incurs higher costs. Melt crystallization provides higher efficacy in immobilizing Sr, although it requires further development for practical application.