Local Structure and Ionic Diffusion in LiF-BeF₂-ThF₄ Molten Salts: Insights from Ab Initio Molecular Dynamics

Area of Science:

• Materials Science
• Nuclear Engineering
• Computational Chemistry

Background:

• Understanding LiF-BeF2-ThF4 (FLiBeTh) molten salts is crucial for fourth-generation nuclear reactors.
• Experimental studies on these radioactive and toxic salts are challenging.
• Computational simulations offer a safe and efficient alternative for material analysis.

Purpose of the Study:

• To investigate the impact of ThF4 concentration on the local structure and ionic diffusion in FLiBeTh molten salts.
• To provide insights into the behavior of FLiBeTh for advanced nuclear reactor fuel applications.

Main Methods:

• Ab initio molecular dynamics (AIMD) simulations were performed on 2LiF-BeF2 systems with varying ThF4 concentrations (0-18.18 mol %) at 973 K.
• Analysis included radial distribution function (RDF), angular distribution function (ADF), coordination number (CN), mean square displacement (MSD), and anion residence ratio r(t).

Main Results:

• Increasing ThF4 concentration had minimal effect on cation first coordination shells but indicated short-range order.
• Coordination numbers revealed Li+ (tetra-/penta-), Be2+ (tetra-), and Th4+ (octa-) coordination.
• Ionic diffusion rates followed Li+ > F- > Be2+ > Th4+; Be-F and Th-F coordination shells were stable, Li-F was dynamic. Higher ThF4 decreased overall MSD.

Conclusions:

• AIMD simulations accurately characterized the local structure and ionic diffusion in FLiBeTh molten salts.
• The study confirms that increasing ThF4 concentration reduces ionic diffusion, impacting molten salt performance.
• Findings support the development of FLiBeTh as a fuel salt for fourth-generation nuclear reactors.