One problematic issue with molten salt reactors (MSRs) is the tendency for the salt’s redox potential to increase with irradiation, causing greater corrosivity. U and Zr were identified as redox buffer candidates in a NaCl-CaCl2-UCl3 salt. Uranium and zirconium are attractive redox buffers since they should not reduce the redox potential below sustainable operating conditions for MSRs. Redox potentials—represented by open circuit potential (OCP)—were measured at a tungsten working electrode vs. a Ag/AgCl reference electrode housed in a mullite tube. Continuous OCP measurements were taken at 600°C, while NiCl2 was added to the molten salt to replicate irradiation effects on redox potential. OCP rose by 0.23 V with the addition of 0.1 wt% NiCl2. A zirconium rod was then submerged in the salt, causing an OCP drop of -0.63 V over a 100 min period. The potential drop for a U rod was -0.5 V over 160 min.

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Molten salt NaCl-CaCl2 is a viable candidate for the base fuel salt of a molten salt reactor (MSR). Molten salt mixtures NaCl-CaCl2-UCl3 (16 wt%) and NaCl-CaCl2-UCl3 (14.5 wt%)-CeCl3 (1.0 wt%) were analyzed using electrochemical methods to assess the feasibility of using these methods to measure the concentration of PuCl3 in MSR fuel in real time. CeCl3 was used as a surrogate for PuCl3 in this study. Different electrochemical test methods including electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and square wave voltammetry (SWV) were used at 923 K. Visible WE deposition is observed after running electrochemical tests, and a net 1 V underpotential deposition of Na from NaCl was observed. This indicates that there are serious challenges for application of electrochemical methods to quantitative analysis of actinides in MSR fuel containing NaCl.

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