Response of synthetic coffinite to energetic ion beam irradiation

Coffinite, USiO4, is one of the two most abundant and important naturally occurring U4+ phases (the other is UO2), and it is an alteration product of the UO2 in spent nuclear fuel when in contact with silica-rich groundwater under reducing conditions. Despite its ubiquity, there are very limited data on the response of coffinite to radiation. Here, we present the results of the first systematic investigation of energetic ion beam irradiation (1 MeV Kr2+) of ultra-fine, synthetic coffinite (20-50 nm). In situ transmission electron microscopy (TEM) showed that the crystalline-to-amorphous transformation occurs at a relatively low dose, ∼0.27 displacements per atom (dpa) at room temperature. The critical temperature, Tc, above which coffinite cannot be amorphized, is low (∼608 K). Synthetic coffinite is more stable as compared with isostructural zircon (ZrSiO4; Tc = 1000 K) and thorite (ThSiO4; Tc above 1100 K) upon ion beam irradiation at elevated temperature, suggesting enhanced defect annealing behavior in nano-sized synthetic coffinite. Irradiation was found to decrease the temperature required to induce phase decomposition process in coffinite upon thermal annealing. A good correlation among the critical amorphization temperature, Tc, phase decomposition temperature, Tf, and the temperature range of the two-phase (ZrO2 and SiO2) co-existed region was identified.