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

Abstract

The hydration behavior of gadolinium oxide (Gd2O3) 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 (CeO2) additions to improve the hydration resistance of Gd2O3. Pellets containing 0-25 at.% Ce were exposed to pressurized water at 200 degrees C, 1.5 MPa, for 48 h. Gd2O3 pellets are originally monoclinic under 1600 degrees C sintering temperature, whereas CeO2 additions progressively suppress the phase transformation; at 15 at.% or higher Ce doping, a single cubic phase forms. XRD showed that undoped Gd2O3 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 Gd2O3, by vanishing Gd-O-H bands with the Ce addition. XPS indicated that Ce ions are predominantly Ce4+ in 15 at.% or higher Ce-doped Gd2O3, suggesting that Ce4+ substitution on Gd3+ sites prevents oxygen-vacancy-assisted hydroxylation. Conversely, a lower Ce4+ 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 Gd2O3 and completely suppresses hydration under the tested conditions, as a result of a reduced oxygen vacancy concentration.