Radiation effects in Sm-monazite – a potential waste matrix for plutonium and minor actinides

During the last decades, various ceramic materials have been proposed as potential waste forms for the immobilization of special nuclear waste streams, such as separated plutonium from civilian sources, excess weapons plutonium, or separated minor actinides (MA: Np, Am, Cm). Among them, monazites (monoclinic LnPO4) are seen as promising candidates due to their specific properties including high structural flexibility for actinide incorporation, high chemical durability, and high radiation resistance. One of the key issues for crystalline ceramic waste forms in general concerns potential property changes in response to radiation-induced phase transformations, due to the alpha decay of the incorporated actinides. Radiation damage due to self-irradiation in ceramic waste forms is caused mainly by the recoiling nuclei and to a lesser extent by the emitted alpha particles. This study focuses on radiation-induced effects in Sm-monazite with respect to its long-term performance in the repository environment. Heavy ion bombardment experiments were performed on SmPO4 to simulate ballistic effects similar to those resulting from recoiling nuclei during alpha-decay. Therefore, the Sm-monazite samples were irradiated with Au ions with energies of 1, 3.5, and 7 MeV up to fluencesof 5.1·1014 ions·cm-2. Besides the irradiation experiments, computational simulations using SRIM/TRIM were performed for interpretation and support of the experimental findings. Since experimental data on threshold displacement energies (Ed) are lacking for monazites so far, the respective Ed-values for all atoms in the Sm-monazite structure were derived from intensive molecular dynamics simulations using the LAMMPS code. Irradiation with fluences up to ~2·1013 ions·cm-2 produced only minor structural changes in the SmPO4 lamellae, whereas vast amorphisation was observed for the maximum fluence. However, even in this case the material was not completely amorphised due to local annealing effects leading to the formation of recrystallised domains. Studying the Raman band broadening suggests that the amorphisation of monazite is mainly due to the breaking of Ln-O bonds while the PO4 tetrahedra behave as a rigid unit. Annealing effects that can be observed already during irradiation experiments at ambient temperature, the fact that dose rates occurring in an actinide bearing waste matrix would be significantly lower compared to ion beam irradiation as well as the potential additional annealing effect of alpha-radiation emphasise the high potential of Sm-monazite as a suitable actinide waste form capable of withstanding amorphisation