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–BeF 2 (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 Eu 2+ 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 Eu 3+ and Be 2+ . 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.