α Self-irradiation Effects on Structural Properties of (U,Am)O_2±δMaterials

Abstract

Am isotopes are amongst the largest contributors to the radiotoxicity and heat load of spent nuclear fuels, especially in scenarios including plutonium recycling in MOX fuels. Transmutation in heterogeneous mode is a promising option to lower Am inventory in the ultimate nuclear waste, thus decreasing the ecological impact of deep-geological repository sites. It consists in the irradiation Ambearing blankets (AmBB, i.e., U 1-x Am x O 2±δ with x values close to 10-15 at.%) in the periphery of a fast neutron reactor core. In this context, studies are being performed to assess the feasibility and safety of AmBB transmutation, notably focusing on innovative fabrication processes [1][2][3] Am being a strong α emitter (activity of 130 MBq), U 1-x Am x O 2±δ behavior under α self-irradiation is also monitored [6][7][8].One of the main effects of α radiation in crystallized materials is a structural swelling. U 1-x Am x O 2±δ compounds with different Am/(U+Am) ratios (7.5 ≤ x ≤ 50 at.%) were monitored by X-ray diffraction (XRD) after a heat treatment during several months to observe their lattice parameter evolution with time. The results obtained first evidenced that the fluorite-type structure (Fm-3m) of the compounds remains stable all along the study. During the first days of storage, all samples underwent an oxidation, observed for the lowest Am/(U+Am) ratios (x ≤ 15 at.%) through a decrease of the lattice parameter or, for the other (30, 40 and 50 at.%), through an isostructural transition to a phase with a smaller lattice parameter (Fig. 1 left). After this phenomenon, an increase of the lattice parameter was observed for all the compositions, evidencing an asymptotic behavior leading to saturation after a few months. This evolution was fitted with the relation describing the structural swelling under α irradiation: ( − 0 ) 0 ⁄ = (1 − − ), with , the lattice parameter at a time , 0 the initial lattice parameter, the relative swelling at saturation and, , a kinetics constant. Satisfactory fitting results were obtained for all samples (with R 2 -factors superior to 99.5 %, see Fig 1 right). parameters obtained indicate that all compositions present lattice swellings at saturation ranging from 0.26 to 0.28 %, with a small decrease with the Am content. The swelling kinetics logically increases with the Am content, but similar kinetics are obtained for all samples after normalization by the sample α activity. Additional experimental data was acquired using X-ray absorption spectroscopy (XAS), which confirmed the stability of the fluorite structure and that the different interatomic distances (actinide-O and actinide-actinide) present an evolution similar to that of the lattice parameter.The effects of α self-irradiation in terms of structural disorder were also studied based on XRD and XAS data. Microstrain determined from XRD by the Williamson-Hall method did not exhibit any significant evolution with time (whereas it increases with the Am content, in line with previous results [5]). The Debye-Waller factors extracte...