New Insight into Self-Irradiation Effects on Local and Long-Range Structure of Uranium?Americium Mixed Oxides (through XAS and XRD)

摘要

Uranium?americium mixed oxides could be used as transmutation targets to lower Am inventory in spent nuclear fuels. Due to ~(241)Am activity, these materials are subjected to α-selfirradiation which provokes crystallographic disorder. Previous studies on U?Am mixed oxides gave first insight into α-radiation tolerance of these compounds, but have never been carried out for more than a year, whereas these compounds might be stored up to a few years between fabrication and irradiation. In this work, we study effects of self-irradiation on the structure of U_(1?x)Am_xO_(2±δ) solid solutions (x = 0.15 and 0.20) aged 3 to 4 years. Especially, Xray diffraction and X-ray absorption spectroscopy are combined to observe these effects from both long-range and local perspectives. Results show that the fluorite-type structure of U?Am mixed oxides withstands ~(241)Am α-irradiation without major damage. Despite the increase of interatomic distances and crystallographic disorder observed during the first months of storage, the present results show that a steady state is then reached. Thus, no detrimental factors have been identified in this study in terms of structural damage for several-year storage of U_(1?x)Am_xO_(2±δ) pellets before irradiation. Furthermore, comparison between long-range and local evolution suggests that α-self-irradiationinduced defects are mainly located in low-ordered domains. Based on literature data and present results, the steady state appears related to the equilibrium between radioinduced defect formation and material self-healing.