Chlorine gas and anion radical reactivity in molten salts and the link to chlorobasicity

Abstract

Next-generation nuclear power plants may include exciting novel designs in which molten salts are the coolant or a combination of the coolant and fuel. Whereas it is straightforward to see why having a low volatility coolant can be advantageous for safety, much is not understood about the production of volatile halogen gases as a result of radiation and even less is known about the distribution of these species at and away from interfaces. Using first principles molecular dynamics simulations, we investigate the product of the disproportionation reaction between chlorine anion radicals (nominally Cl₂˙⁻) in the bulk and slab configurations. We find that the product depends on the chlorobasicity of the medium. For example, in ZnCl₂, Cl₂ forms, but in a eutectic mixture of LiCl and KCl, Cl₃⁻ is formed as a product. We also find that Cl₃⁻ prefers to form at the vapor interface and this may have implications for corrosion and reactivity. Furthermore, the mechanisms of the mobility of Cl₂ and Cl₃⁻ are radically different, the first one being vehicular and the second Grotthus-like. Chlorobasicity is linked to the electronic structure of the host melt; ZnCl₂ forms extended networks along which metal ions and anionic counterions have significant electronic orbital overlap forming long, linear, molecular-like constructs; the opposite is true for the alkali metal eutectic salt.