Accommodation of multivalent cations in fluorite-type solid solutions: Case of Am-bearing UO2

Melting behaviour of americium-doped uranium dioxide

“…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. On the contrary, no variation of the The melting/freezing temperature of UO 2 was determined in a previous work [21].…”

“…It was shown that the compounds are fluorite solid solutions for all Am contents. Regarding the charge distribution, equimolar proportions of Am +III and U +V have been measured in U 0.85 Am 0.15 O 2±d [17,18,20] meaning that the O/M ratio is close to 2.00. On the contrary, only U(IV) has been reported for the U 0.90 Am 0.10 O 2±d and U 0.80 Am 0.20 O 2±d compounds used in this work, suggesting the hypo-stoichiometry of these materials [5].…”

“…Fits were performed between 1.6 and 6 Å at both the U L II and Am L III edges, with a k-weight value of 3. As often employed for An L III /L II [18,[38][39][40], the amplitude factor (S 0 2 ) was set at 0.90 for U and Am shells. The shift in the threshold energy (DE 0 ) was varied as a global parameter.…”

“…The radiotoxicity and heat load of ultimate nuclear waste would thus be decreased as well as the ecological footprint of deep geological repositories [2,3]. In this context, several studies have been dedicated to U 1Àx Am x O 2±d compounds, not only to investigate synthesis methods [4][5][6][7][8][9][10][11] and assess their behaviour under irradiation in reactors [4,7,12,13], but also to determine their structural and thermophysical properties, the latter remaining scarcely known [14][15][16][17][18][19][20]. Among them, no data regarding the U 1Àx Am x O 2±d melting behaviour has been reported, despite the importance of such information with respect to safety margins during irradiation, notably in accidental conditions.…”

“…The samples were also characterised after melting using powder X-ray Diffraction (XRD) and X-ray Absorption Spectroscopy (XAS). A comparison between pre and postmelting structure (local and long-range) and cationic charge distribution can thus be proposed, based on XRD/XAS characterisation of the same compounds already published in the literature [5,17,18,20].…”