(385d) Thermodynamic Analysis of Minor Components in Molten Salt Mixtures

This study focuses on the various models that can be applied to predict the thermodynamic behavior of minor components in molten salt mixtures, defined here as any component with a mole fraction of approximately 0.02 or less. Defined in this way, minor components include corrosion products in molten salt mixtures in general, and fission products in liquid-fueled molten salt reactors, making them important for corrosion control and for separations processes used in nuclear reprocessing. Various factors that affect their equilibrium behavior, especially concentration and base salt composition, were examined using published experimental data from both older and recent studies.

A common reference state for activity in current molten salt literature is that of the pure component at the given temperature in either a solid or liquid state. The liquid has been particularly recommended as being closer to the physical state of the component since the molten salt mixture is itself a liquid. A minor component in a molten salt mixture interacts predominately with the base salt components rather than with itself. This means that use of a pure component reference state requires activity coefficients to make large corrections in order to accurately represent interactions with a given salt. The activity coefficient is also the only way to quantify the influence of different base salts. The present study examines an alternative model that encapsulates the important interactions between minor components and the base salt mixture in a single number. An advantage of this method is that it requires only slight activity coefficient adjustments over the concentration range of interest when estimating the reduction potential of a minor component. This can result in significantly less error than would otherwise be the case—0.02 V or less instead of 0.1 V or more. Use of the method to improve the accuracy of Pourbaix diagram-style corrosion analysis of molten salt mixtures is demonstrated.