A systematic study of Light and Heavy Ion Beams Irradiation Effects of Three Zirconolite Polytypes

Zirconolite ceramics are emerging as the promising immobilization matrix for high-level radioactive waste (HLW) in deep geological repositories. The radiation stability is a critical factor in assessing their long-term safety. In this work, three zirconolite polytypes of zirconolite-2M, zirconolite-3O and zirconolite-4M were irradiated by 1.5 MeV Xe 10+ (heavy) and 400 keV Ne 4+ (light) ions at fluences of 1 × 10 14 to 2 × 10 15 ions/cm 2 and 1 × 10 15 to 1 × 10 16 ions/cm 2 , respectively. The irradiation damage was systematically studied using GIXRD, Raman and XPS. Results showed that zirconolite-4M had marginally higher radiation resistance than zirconolite-2M, which slightly outperformed zirconolite-3O under heavy ion irradiation. For light ion irradiation, zirconolite-3O demonstrated comparable or slightly better stability than zirconolite-2M. However, Direct comparisons of radiation tolerance were complicated by variations in displacement per atom (dpa) at equivalent fluences. In addition, the phase evolution of zirconolite to cubic defect-fluorite derivative structures (pyrochlore or defect-fluorite) was observed during the amorphization process for the three zirconolite polytypes, irrespective of the ion type (heavy or light). Zirconolite-2M and -3O reached near-complete amorphization at approximately 4 dpa, while zirconolite-4M predominantly retained its crystalline structure even at higher radiation doses (2.5 dpa for light ions and 3.78 dpa for heavy ions). These findings provide critical insights into the design of HLW matrices and the evaluation of their radiation stability, contributing to the development of safer and more durable waste forms for long-term geological storage.