Chlorine Gas and Anion Radical Reactivity in Molten Salts and the Link to Chlorobasicity

April 1, 2025

The Science 

Reactive chlorine species formed in molten salts under radiation in nuclear systems play a critical role in corrosion and gas evolution, but their behavior depends strongly on the chemical environment. Researchers used first-principles molecular dynamics simulations to investigate how chlorine radicals evolve in different molten chloride salts. They found that the outcome of key reactions is governed by chlorobasicity—the ability of chloride ions to donate electron density. In ZnCl2, chlorine radicals stabilize as molecular Cl2, while in LiCl–KCl they preferentially form Cl3? species with distinct structural and transport behavior. These results show that chlorobasicity provides a unifying framework linking melt composition to the identity, stability, and dynamics of chlorine species. 

The Impact 

Understanding how chlorine species form and behave in molten salts is essential for interpreting measurements and managing chemical processes in reactor environments. This work shows that the speciation, mobility, and persistence of reactive chlorine species depend strongly on the chemical nature of the salt. These insights help explain how chlorine gas and related species form under irradiation and provide a foundation for predicting their behavior in different molten salt systems. 

Summary 

Ab initio molecular dynamics simulations were used to investigate the reactions and transport of chlorine radical species in molten ZnCl2 and LiCl–KCl. 

In ZnCl2, chlorine radicals combine to form neutral Cl2 molecules that remain structurally stable and diffuse through the melt via conventional molecular motion. In contrast, in LiCl–KCl, the same radicals form Cl3? ions, which exhibit dynamic bonding and transport through a combination of translational motion and bond exchange. This results in a Grotthuss-like mechanism, where atomic identities continuously rearrange during diffusion. 

These differences arise from the electronic structure of the melt. In ZnCl2, chloride ions are strongly coordinated to Zn²?, limiting their ability to donate electron density. In LiCl–KCl, chloride ions are more weakly bound and more chemically available, enabling the formation of Cl3? and dynamic exchange processes. 

Simulations also show that, likely due to its low charge density relative to other anions, Cl3? preferentially accumulates near the liquid–vapor interface, indicating that interfacial environments influence the localization of chlorine species. 

Overall, the results demonstrate that chlorobasicity governs chlorine speciation and transport, linking local electronic structure to observable chemical behavior, and that interfaces affect the localization of radiation-produced species. 

Contact 

Claudio J. Margulis, University of Iowa, claudio-margulis@uiowa.edu 
Vyacheslav S. Bryantsev, Oak Ridge National Laboratory, bryantsevv@ornl.gov 
Santanu Roy, Oak Ridge National Laboratory, roys@ornl.gov 

Funding 

This work was supported by the Molten Salts in Extreme Environments Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This research mainly used resources of the Oak Ridge Leadership Computing Facility, which is a DOE Office of Science User Facility, and, in part, the Compute and Data Environment for Science (CADES) at the Oak Ridge National Laboratory, both supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. We also acknowledge the University of Iowa High Performance Computing Facility. 

Citation 

Nguyen, H. H., L. D. Gibson, M. S. Emerson, B. Borah, S. Roy, V. S. Bryantsev, and C. J. Margulis. “Chlorine Gas and Anion Radical Reactivity in Molten Salts and the Link to Chlorobasicity.” Physical Chemistry Chemical Physics 27 (2025): 4290–4297. https://doi.org/10.1039/d4cp03285c. 

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