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Abstract: A leading candidate for the immobilisation of actinides, zirconolite’s suitability as a potential ceramic host for plutonium disposition, both in storage and geological disposal, has been the subject of much research. One key aim of this study is to understand the effects of radiation damage and noble gas accommodation within the zirconolite material. To this end, a series of ex situ irradiations have been performed on polycrystalline (Ca0.8Nd0.2)Zr(Ti1.8Al0.2)O7 zirconolite samples. Zirconolite samples, doped with Nd3+ (as a Pu surrogate) on the Ca-site and charge-balanced by substituting Al3+ onto the Ti-site, were irradiated with 36Kr+ (2MeV) ions at fluences of 1×1014 and 5×1015 cm−2 and 4He+ (200keV) ions at fluences of 1×1014, 5×1015 and 1×1017 cm−2 to simulate the impact of alpha decay on the microstructure. Microstructural analysis revealed no damage present at the lower Kr+ fluence, but that the higher 36Kr+ fluence rendered the zirconolite completely amorphous. Similarly, evidence of helium accumulation was only seen at the highest 4He+ fluence (1×1017 cm−2). Monte Carlo simulations using the TRIM code predict the highest concentration of helium accumulating at a depth of 720nm, in good agreement with the experimental observations. [Copyright &y& Elsevier]

Abstract: Understanding the effect of radiation damage and noble gas accommodation in potential ceramic hosts for plutonium disposition is necessary to evaluate their long-term behaviour during geological disposal. Polycrystalline samples of Nd-doped zirconolite and Nd-doped perovskite were irradiated ex situ with 2MeV Kr+ at a dose of 5×1015 ionscm−2 to simulate recoil of Pu nuclei during alpha decay. The feasibility of thin section preparation of both pristine and irradiated samples by Focused Ion Beam sectioning was demonstrated. After irradiation, the Nd-doped zirconolite revealed a well defined amorphous region separated from the pristine material by a thin (40–60nm) damaged interface. The zirconolite lattice was lost in the damaged interface, but the fluorite sublattice was retained. The Nd-doped perovskite contained a defined irradiated layer composed of an amorphous region surrounded by damaged but still crystalline layers. The structural evolution of the damaged regions is consistent with a change from orthorhombic to cubic symmetry. In addition in Nd-doped perovskite, the amorphisation dose depended on crystallographic orientation and possibly sample configuration (thin section or bulk). Electron Energy Loss Spectroscopy revealed Ti remained in the 4+ oxidation state but there was a change in Ti coordination in both Nd-doped perovskite and Nd-doped zirconolite associated with the crystalline to amorphous transition. [Copyright &y& Elsevier]