Insight into the Am–O Phase Equilibria: A Thermodynamic Study Coupling High-Temperature XRD and CALPHAD Modeling

In the frame of minor actinide transmutation, americium can be diluted in UO2 and (U, Pu)­O2 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 AmO2–x 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 AmO2–x –AmO1.61+x –Am2O3 domain. Because of this approach, we were able for the first time to assess the relationships between temperature, lattice parameter, and hypostoichiometry for fcc AmO2–x . We showed the presence of a hyperstoichiometric existence domain for the bcc AmO1.61+x phase and the absence of a miscibility gap in the fcc AmO2–x 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.