On the Use of Ion Beams to Simulate Alpha-Decay Damage in Crystalline Ceramic Phases

The principal source of radiation damage in ceramics for immobilization of actinide-containing nuclear waste is alpha-decay of the actinides, which occurs over geologic timescales due to the long half-lives of the actinides and their daughter products. Alpha-decay produces energetic alpha particles (4.5 to 5.5 MeV) and recoil nuclei (70 to 100 keV) that result in radiation damage and the accumulation of helium. Historically, the evolution of radiation damage due to alpha-decay has been studied using short-lived actinides; however, ion beam irradiations have become nearly the only approach currently employed to study radiation damage in relevant materials due to decreased costs, shorter irradiation timescales and the non-radioactive nature of the irradiated materials. While ion irradiations can be performed over a wide range of irradiation conditions, most studies are performed over a narrow range of irradiation conditions, which can complicate the interpretation and relevance. For example, ion irradiation studies on relevant materials may predict that a phase transformation (e.g., amorphization) occurs, but cannot predict the dose or temperature regime for the phase transformation under alpha-decay damage rates in a repository. In general, all ion irradiation experiments are conducted at relatively high dose rates compared to alpha-decay damage rates. Many experiments, such as in situ irradiation in an electron microscope or swift heavy ion irradiations, do not provide data suitable for predictive modeling, such as the dose, appropriate kinetics, and swelling associated with such phase transformations. Too often the dose and kinetics for phase transformation are dominated by processes related to the mass and energy of the ions used for irradiation (e.g., ratio of electronic to nuclear energy loss); in actuality, it is thermally dominated processes that will control the dose and kinetics of phase transformation over the timescales of radiation damage in actual waste forms. Fortunately, several collaborative studies over the years employing MeV heavy-ion irradiations have provided experimental data on a few relevant materials (pyrochlore, zirconolite, and silicate apatite) that are in good agreement with results from identical samples doped with short-lived actinides. Such benchmarking of ion irradiation results against alpha-decay damage results demonstrates that bulk-like irradiation with MeV heavy ions, such as Au, can provide the best approximation to damage accumulation from alpha decay, yielding swelling and phase transformation behavior that agree well with results from alpha-decay damage. However, radiation damage kinetics must be considered when predicting behavior for materials with low defect recovery and recrystallization temperatures, as in monazite and fluoroapatite, which amorphize under MeV heavy ion and swift heavy ion irradiations, but not from alpha-decay damage.