Helium Migration in Zirconolite: a Density Functional Theory Study

Nuclear energy, while emitting far less CO₂ than fossil fuels, still presents challenges in waste disposal. Zirconolite is proposed as a solution for the safe and effective long-term immobilization and disposal of high-level radioactive waste in nuclear waste management. Here, we investigate helium behaviour in zirconolite using density functional theory (DFT) calculations and the Ab initio random structure searching (AIRSS) method. Our findings reveal the migration pathways and diffusion coefficients of helium, both without and in the presence of vacancies. The most stable helium interstitial site in bulk zirconolite lies within the [010] channel. This pathway exhibits a lower migration barrier (1.42 eV) compared to the [110] pathway (2.00 eV). Helium atoms are found to prefer various interstitial positions within the zirconolite bulk without a tendency to aggregate, though the presence of vacancies can promote helium aggregation. The formation of these vacancy structures is highly dependent on their charge states, with O defects notably influenced by the surrounding chemical environment. Vacancies facilitate helium migration by lowering migration barriers and distances, while also stabilizing helium atoms within the crystal lattice. Additionally, we determined the diffusion coefficients of helium in zirconolite along different orientations: D[010] = 3.87×10^⁻7exp(-137.00 kJ/mol/RT) m²/s and D[110] = 4.58×10^⁻7exp(-192.96 kJ/mol/RT) m²/s. Helium diffusion in zirconolite is significantly anisotropic, with marked differences in diffusion rates across different crystallographic directions, although as temperatures rise, helium diffusion becomes increasingly isotropic. Furthermore, helium diffusion is significantly pressure-dependent, particularly in the [010] direction; increasing pressure up to 12 GPa significantly decreases helium diffusion rates by raising activation energies. These findings contribute to understanding helium behaviour in zirconolite, essential for assessing waste form performance in nuclear waste disposal.