Phase stability of Ce, La and U oxides in the presence of hydrogen and water: Thermodynamic modeling and experimental insights

• The effect of the presence of water vapor on the stability of the oxide phase was studied for uranium, lanthanum and cerium and represented graphically using appropriate phase diagrams calculated with Calphad. • The calculated phase diagrams reproduce well the wide range of stability of the oxide phases and their tendency to transform into hydroxide or hydrate in the presence of water vapor. • The calculated phase diagrams are also very efficient in interpreting experimental results on the effect of traces of water vapor on the phases obtained following reduction of the oxide by heating it in the presence of hydrogen. This is illustrated on the reduction of CeO 2 as well as of U 3 O 8 . • The proposed approach may be applied to any metal in order to predict the corrosion products in atmospheres containing oxygen or hydrogen with water vapor. Standard metal-oxygen or metal-hydrogen phase diagrams neglect the presence of water vapor. However, water vapor present in the environment often destabilizes the oxide or hydride phase, leading to the stabilization of hydroxide, oxy-hydroxide or hydrate phases. Appropriate phase diagrams are required to properly describe and predict the chemistry of a metal and its oxide or hydride in realistic conditions. In the present work, M-O-H phase stability diagrams were computed for uranium, lanthanum and cerium (U, La and Ce), based on previously compiled as well as newly assessed thermodynamic data, in CALPHAD (CALculations of PHAse Diagrams) formalism. We show how the ternary phase diagrams relate to diverse experimental observations on the behavior of these elements in environments containing hydrogen or water vapor. The phase diagrams allow the identification of common trends as well as differences between the systems studied. For example, all the elements form hydride phases but minor concentrations of oxygen or water are sufficient to destabilize the hydride. The oxide phases have a wide range of stability, but the oxide may react in the presence of water vapor to form a hydroxide (in the case of La) or hydrates (in the case of U). The range of stability of the hydroxide and hydrate phases increases as the temperature is lowered. M-O-H phase diagrams for predicting and representing the stability of different phases in mixed environment, containing O 2 or H 2 with water vapor at different concentrations, computed for Lanthanum, Cerium and Uranium.