Electrochemical Activity Measurements of UCl₃ in Molten NaCl-MgCl₂-UCl₃

Open circuit potential (OCP) measurements were made to determine uranium chloride (UCl_x) species activity in NaCl–MgCl₂, which is a candidate salt for use in a molten salt reactor fuel. The operating temperature ranged from 500 to 650 °C, and the measurement system used a U-Zr working electrode (WE) and Ag/AgCl reference electrode (RE). The activity was calculated to range from $2.52×10−6$ to $2.99×10−8$ (with uncertainties of $±1.0×10−7$ and $±1.1×10−9$⁠, respectively), dependent upon the temperature and concentration of UCl₃ (ranging from 0.25 to 11.55 wt.%). Fit of the OCP data to the Nernst equation resulted in values of 3.1 to 4.8 electrons transferred per U atom. This indicates the possibility for codeposition of Mg²⁺ and U³⁺ under certain conditions. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photo-electron spectroscopy analyses of cathodic deposits confirm the presence of metallic magnesium.