Effect of Mg ²⁺ Ions on Transport Properties in Molten Chloride Fast Reactor Fuels

Molten chloride fast reactors (MCFRs) are emerging as a promising class of molten salt reactor (MSR) designs due to their ability to dissolve large amounts of major actinides while keeping low melting points and sustaining a hard neutron spectrum. In this context, understanding the transport properties of molten salts─such as viscosity and self-diffusivity─is essential for the design and optimization of MSRs with efficient heat transfer. However, an important gap remains in the literature regarding the transport properties in molten chlorides. In this work, we performed classical molecular dynamics (MD) simulations with the polarizable ion model (PIM) to evaluate the viscosities and self-diffusion coefficients of molten NaCl-MgCl₂-LaCl₃, which was used in our previous work as a simulant for the ARAMIS-A reactor fuel. To validate the accuracy of our approach, calculated transport properties for the NaCl-LaCl₃ binary system were benchmarked against capillary viscometry measurements from the literature and our own pulsed-field gradient nuclear magnetic resonance (PFG-NMR) experiments. Our study compares the roles of NaCl and MgCl₂ on the transport properties of MCFR fuels. Interestingly, we observe that MgCl₂ does not affect LaCl₃-poor and LaCl₃-rich melts in the same way due to the competition between multivalent ions for chloride ions in their first coordination shells and their ability to form cluster species.