In-situ X-ray diffraction study of phase transformations in the Am–O system

In the frame of minor actinides recycling, americium can be transmuted by adding it in UO2 or (U, Pu)O2 fuels. Americium oxides exhibiting a higher oxygen potential than U or Pu oxides, its addition alters the fuel properties. To comprehend its influence, a thorough knowledge of the Am–O phase equilibria diagram and of thermal expansion behavior is of main interest. Due to americium scarcity and high radiotoxicity, few experimental reports on this topic are available. Here we present in-situ high-temperature XRD results on the reduction from AmO2 to Am2O3. We show that fluorite (Fm-3m) AmO2 is reduced to cubic (Ia-3) C′-type Am2O3+δ, and then into hexagonal (P63/mmc) A-type Am2O3, which remains stable up to 1840 K. We also demonstrate the transitional existence of the monoclinic (C2/m) B-type Am2O3. At last, we describe, for the first time, the thermal expansion behavior of the hexagonal Am2O3 between room temperature and 1840 K.