Atomic-Scale Europium Solute Landscape Governs the Local Structure and Transport Properties of the Molten FLiBe Salt

Computational prediction of structural and physicochemical properties of molten salts plays a vital role in advancing spent nuclear fuel reprocessing, as it circumvents the experimental challenges imposed by extreme operation conditions. Here, first-principles molecular dynamics simulation is employed to investigate the influence of the fission product europium (Eu) on the local structure and transport properties of the LiF-BeF₂ (FLiBe) molten salt. It is discovered that the spatial distribution of solute Eu significantly modulates ionic self-diffusivity, introducing a striking 60% uncertainty in the diffusivity of Eu²⁺ as it transitions from an isolated to a clustered configuration. A clear correlation emerges between the system free energy and the Eu-Eu distance, wherein cluster formation confers lower energy and stronger stability. A comparison of Eu oxidation states reveals that Eu(III) markedly suppresses both ionic self-diffusion and total conductivity relative to Eu(II) due to compromised charge transport efficiency. Structural analysis further indicates strengthened Be-F and Eu-Be interactions in the Eu(III) system, rationalizing the depressed mobility of Eu³⁺ and Be²⁺. Together, Bader charge and phonon calculations establish connections between the ion migration behavior and charge transfer, polarization strength, and vibrational characteristics. These findings provide fundamental atomic-scale insights into ionic conduction mechanisms, advance the predictive modeling of fission product behavior, and establish a quantitative basis for assessing their separation rates.