Tritium Accommodation and Diffusion in Li₈PbO₆ from First-Principles Simulations

Li₈PbO₆ has been proposed as an alternative candidate breeding blanket material for use in fusion reactors. As lithium is burned inside the blanket, tritium is produced within the ceramic matrix until it reaches the surface, from where it is recovered by isotope exchange reactions. To fully understand the tritium recovery process, it is essential to understand how tritium is accommodated in the fuel and subsequently migrates to the surface. Therefore, in this work, we employ density functional theory (DFT) to examine tritium accommodation in Li₈PbO₆. We then used the nudged elastic band (NEB) method to understand the mechanisms for the migration of tritium-accommodating defects in Li₈PbO₆. We have found tritium is more likely bind to an oxygen ion and form a hydroxyl than exist in the traditional interstitial sites. We predict the barriers for migration of tritium interstitials to be anisotropic, with barriers of 0.27 and 0.69 eV along the xy-plane and through the z-axis, respectively. The barrier for escape from a lithium vacancy trapping site we found to be in the range of 0.76-0.85 eV, and an activation energy range of 0.67-1.18 eV for the migration of the trapping site as a whole. Due to the low migration energies found, we predict that aging of the blanket will have a lower significance on tritium release compared to other leading candidate materials.