(389b) Development of Activity Coefficient Model for La(III) in LiCl-KCl Eutectic Salt

The pyroprocessing of spent nuclear fuel is a promising technology for managing nuclear waste that is currently under development by several nations.  While it has yet to be implemented on a commercial scale, the Republic of Korea (ROK) currently has plans to develop it for commercial scale treatment of the spent fuel from their commercial reactors. Presently, ROK and the United States are collaborating to evaluate the technological feasibility of the process.  One of the key steps in pyroprocessing is Rare Earth Drawdown.  In this process, rare earth elements are separated from the base LiCl-KCl electrolyte and disposed of as waste. This allows for the recycling of the LiCl-KCl salt to the electrorefiners, thus minimizing the total volume of waste generated.  One of the options to perform this separation would be electrowinning.  Such a process is similar to electrorefining and could probably be performed in-situ in the electrorefiner. The use of two relatively similar processes like electrowinning and electrorefining would have synergistic effects in terms of fundamental understanding of chemistry and industrial scale adaptation of pyroprocessing.

Electrowinning however has not been tested in complex salt mixtures like those found in spent fuel electrorefining systems. It has only been tested to date with simple ternary salt systems. For industrial adaptation to take place, it is important to be able to accurately model the electrochemical reduction behavior of the ions of interest in the molten salts.  One important parameter that needs to be known for the rare earths ions is the apparent standard reduction potential.  While these potentials have been extensively measured for simple ternary salt systems, little work has been done on effect of concentration and presences of other ions of similar electrochemical properties have on the reduction potential.  Most of the active metal ions in the salt have apparent standard reduction potentials within about 1 V of each other. The interaction of these different ions with each other will likely affect their reduction potentials.  Using the Nernst Equation, changes in apparent reduction potential can be directly related to changes in activity coefficients of the ions. While some data on activity coefficients of these ions have been reported in the literature, there is a scarcity of information on concentration or matrix effects.  Development of an activity coefficient model will help in the future to more accurately model both the electrowinning and electrorefining systems.  

In this study, the activity coefficient of the La³⁺ ions was calculated from the measurement of the electromotive force (emf) of the La|La³⁺ cell.  This was done by setting up a two electrode electrochemical cell with a La metal rod as the working electrode and a 5 mol % Ag/AgCl as the counter/reference electrode.  The open circuit potential of the cell was then measured at various concentrations within the 2 to 12 wt. % range.  These measurements were made at four temperatures of 500, 450, 425 and 400°C.  To examine potential matrix affects, the same series of La³⁺ tests was repeated with CsCl also dissolved in the salt.  La and Cs are key fission products that accumulate in the molten LiCl-KCl electrorefiner electrolyte.