In the frame of minor actinide transmutation, americium can be diluted in UO and (U, Pu)O fuels burned in fast neutron reactors. The first mandatory step to foresee the influence of Am on the in-reactor behavior of transmutation targets or fuel is to have fundamental knowledge of the Am-O binary system and, in particular, of the AmO phase. In this study, we coupled HT-XRD (high-temperature X-ray diffraction) experiments with CALPHAD thermodynamic modeling to provide new insights into the structural properties and phase equilibria in the AmO-AmO-AmO domain. Because of this approach, we were able for the first time to assess the relationships between temperature, lattice parameter, and hypostoichiometry for fcc AmO. We showed the presence of a hyperstoichiometric existence domain for the bcc AmO phase and the absence of a miscibility gap in the fcc AmO phase, contrary to previous representations of the phase diagram. Finally, with the new experimental data, a new CALPHAD thermodynamic model of the Am-O system was developed, and an improved version of the phase diagram is presented.

Plutonium–americium mixed oxides, Pu1−yAmyO2−x, with various Am contents (y = 0.018, 0.077, 0.21, 0.49, 0.80 and 1.00) were studied in situ by high‐temperature X‐ray diffraction. In this study, the lattice thermal expansion of the six compounds subjected to heat treatments up to 1773 K under reconstituted air (N2 + 21% O2 + ∼5 vpm H2O) was investigated. The materials remained monophasic throughout the experiments and, depending upon the americium content, the lattice parameter of the face‐centred cubic phase deviated from linear lattice expansion at elevated temperatures as a result of the progressive reduction of Am4+ to Am3+.

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