Electrochemical reduction behavior of vitrified nuclear waste simulant in molten CaCl₂

Abstract

The electrochemical reduction of vitrified nuclear waste simulant glass produced by melting oxides of 33 different elements, including Si and four types of long-lived fission product elements (Cs, Zr, Pd, and Se), was investigated in molten CaCl₂ at 1103 K. Before the experiment, potential−pO²⁻ diagrams were constructed from the thermodynamic data to predict the behaviors of the constituent elements during the electrochemical reduction. In the first experiment, small crucible electrodes containing approximately 100 mg of the simulant were electrolyzed at 0.6–1.1 V vs. Ca²⁺/Ca. X-ray diffraction analysis confirmed that SiO₂ was reduced to Si. In addition, the formation of XB₆ (X = Ca, Si, or rare earth elements (REEs)) was confirmed. In the second experiment, a large crucible electrode containing approximately 10 g of the simulant was galvanostatically reduced at −2 A for 5 h. The distribution ratios of the constituent elements to the solid product and to the molten salt were calculated from the result of composition analysis using inductively coupled plasma atomic emission spectrometry and mass spectrometry. In particular, four types of long-lived fission product elements (Cs, Zr, Pd, and Se) were investigated. Consequently, alkali and alkaline earth elements, including Cs, were dissolved into the molten salt. Si mostly remained in the solid product as elemental silicon. Most of the other elements, including Zr and Pd, were also found in the solid product. Se and Zn were indicated to partially evaporate depending on the duration of electrolysis.