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

Abstract

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-self-irradiation 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(x)O(2±δ) solid solutions (x = 0.15 and 0.20) aged 3 to 4 years. Especially, X-ray 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(x)O(2±δ) pellets before irradiation. Furthermore, comparison between long-range and local evolution suggests that α-self-irradiation-induced 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.

Melting behaviour of americium-doped uranium dioxide

“…The (Me + Me) bond lengths are about (3.86 ± 0.01)10 À1 nm for both Am and U, indicating a random distribution in the cationic sub-lattice. One can then observe that Am and U local environments are similar to those previously reported [17,18,[45][46][47]. At the U L II edge, the Debye-Waller factors are slightly larger to those of the as-synthesized materials [17,18,20], which suggests a higher structural disorder and agrees with the slight hyper-stoichiometry.…”

“…However, because a delay of few weeks (for XRD) to a couple of months (for XAS) was inevitable between the laser heating/cooling treatments and the post-melting material characterisation, the samples underwent some self-irradiation damage, notably those with the highest Am content. This could be viewed as a partial explanation for the complex phase splitting and higher O/M ratio observed in the U 0.8 Am 0.2 O 2+d sample as several studies have shown that a-selfirradiation-induced structural effects on (U, Am)O 2 compounds are limited to lattice swelling reaching up to 0.8 vol.% and a small structural disorder increase [45,46,[52][53][54].…”

Extreme multi-valence states in mixed actinide oxides

“…For the refinement, the structural parameters obtained by XRD-reported in Table 1 (corrected for the self-irradiation effects due to the 241 Am α-decay 15,21 )-were adopted as starting values. The fits were then performed by optimising the interatomic distances R, the number of atoms N in the neighbour shells and the Debye-Waller factors σ 2 of the models.…”

Raman microspectroscopic studies of unirradiated homogeneous (U0.76Pu0.24)O2+x: the effects of Pu content, non‐stoichiometry, self‐radiation damage and secondary phases

“…Because the MOX samples have various Pu isotopic compositions, they consider effective decay constant ( λ ′) to calculate the saturation level . Lebreton has reported that after the saturation is reached for (U 0.85 Am 0.15 )O 2±x solid solution, there is an equilibrium between the defect formation from alpha self‐irradiation and self‐healing of the defects . He has discussed that presence of grain boundaries could enhance the self‐healing process due to the annihilation of defects by interstitial‐vacancy recombination near grain boundaries.…”