Improving hydration resistance of gadolinium-based burnable absorbers via cerium oxide additions

The hydration behavior of gadolinium oxide (Gd 2 O 3 ) under high-temperature water conditions presents a significant challenge for the development of soluble-boron-free small modular reactor (SMR) fuels. To overcome this issue, we investigated cerium oxide (CeO 2 ) additions to improve the hydration resistance of Gd 2 O 3 . Pellets containing 0–25 at.% Ce were exposed to pressurized water at 200 °C, 1.5 MPa, for 48 h. Gd 2 O 3 pellets are originally monoclinic under 1600 °C sintering temperature, whereas CeO 2 additions progressively suppress the phase transformation; at 15 at.% or higher Ce doping, a single cubic phase forms. XRD showed that undoped Gd 2 O 3 fully hydrates, whereas specimens with 15 at.% or higher Ce doping retain the cubic structure with no detectable hydroxides. FT-IR results also provide evidence of hydration resistance of Ce-doped Gd 2 O 3 , by vanishing Gd-O-H bands with the Ce addition. XPS indicated that Ce ions are predominantly Ce 4+ in 15 at.% or higher Ce-doped Gd 2 O 3 , suggesting that Ce 4+ substitution on Gd 3+ sites prevents oxygen-vacancy-assisted hydroxylation. Conversely, a lower Ce 4+ fraction in pellets sintered in an Ar atmosphere correlated with an increased hydration tendency. These results indicate that 15 at.% or higher Ce doping stabilizes the cubic phase of Gd 2 O 3 and completely suppresses hydration under the tested conditions, as a result of a reduced oxygen vacancy concentration.