Melting behavior of uranium-americium mixed oxides under different atmospheres
Résumé
In the context of a comprehensive campaign for the characterisation of transmutation fuels for next gen-eration nuclear reactors, the melting behaviour of mixed uranium-americium dioxides has been experi-mentally studied for the first time by laser heating, for Am concentrations up to 70 mol. % under differenttypes of atmospheres. Extensive post-melting material characterisations were then performed by X-rayabsorption spectroscopy and electron microscopy. The melting temperatures observed for the variouscompositions follow a markedly different trend depending on the experimental atmosphere. Uranium-rich samples melt at temperatures significantly lower (around 2700 K) when they are laser-heated in astrongly oxidizing atmosphere compressed air at (0.300 $\pm$ 0.005) MPa, compared to the melting points(beyond 3000 K) registered for the same compositions in an inert environment (pressurised Ar). Thisbehaviour has been interpreted on the basis of the strong oxidation of such samples in air, leading tolower-melting temperatures. Thus, the melting temperature trend observed in air is characterized, inthe purely pseudo-binary dioxide plane, by an apparent maximum melting temperature around 2850 K for 0.3 < x(AmO$_2$) < 0.5. The melting points measured under inert atmosphere uniformly decreasewith increasing americium content, displaying an approximately ideal solution behaviour if a meltingpoint around 2386 K is assumed for pure AmO$_2$. In reality, it will be shown that the (U, Am)-oxide systemcan only be rigorously described in the ternary U-Am-O phase diagram, rather than the UO$_2$-AmO$_2$ pseudo-binary, due to the aforementioned over-oxidation effect in air. Indeed, general departures fromthe oxygen stoichiometry (Oxygen/Metal ratios–2.0) have been highlighted by the X-ray AbsorptionSpectroscopy (XAS). Finally, to help interpret the experimental results, thermodynamic computationsbased on the CALPHAD method will be presented.
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