In situ TEM study of heavy-ion irradiation-induced amorphisation and electron beam-induced recrystallisation in powellite (CaMoO4)

Molybdenum-rich radioactive waste streams in the UK will be vitrified into glass composite high-level waste (HLW) products containing powellite (CaMoO4) crystals. These materials will be exposed to self-irradiation in a geological disposal facility for thousands of years. Our heavy-ion and electron irradiation results, together with a comprehensive review of previous studies, reveal new details on the radiation tolerance of powellite. In this study, powellite crystals were exposed to 500 keV Ar2+ and 600 keV Xe2+ ion irradiations between -160 and -80°C and were analysed in situ via transmission electron microscopy (TEM). At low temperatures (between -160 and -105°C), the critical amorphisation dose (Dc) is higher for the Xe irradiation, most likely due to the significant defect recovery in the cascade core. Above -105°C, the Dc is higher for the Ar irradiation. The dynamic annealing of radiation-induced defects becomes more efficient as the temperature increases; our results suggest that point defects formed during Ar irradiation are more susceptible to dynamic annealing compared with defects in the cascades generated during Xe irradiation. We also present the first evidence of 300 keV electron beam-induced recrystallisation in amorphous powellite by studying its response from room temperature to cryogenic temperatures inside the TEM using a wide range of electron fluxes (0.6-5.2 × 1016 e/cm2/s). In contrast to previous research, our new data suggest that powellite is susceptible to amorphisation by alpha recoil nuclei, and that beta irradiation may cause defect recombination.