Effect of Ce-doping on the phase composition and aqueous stability of Gd2Hf2O7 ceramic as nuclear waste forms

A series of Ce-doping nanograin Gd2CexHf2-xO7 (0.0 ≤ x ≤ 2.0) ceramic samples as nuclear waste forms were prepared, and their microstructure, phase evolution, and aqueous durability were systematically studied. The results revealed that the original Gd2Hf2O7 sample comprised a single pyrochlore structure after sintering at 1673 K, but all the Gd2CexHf2-xO7 (0.5 ≤ x ≤ 2.0) samples assumed defective fluorite structures with Ce-doping. As the xCe value increased from 0.0 to 2.0, the average grain size of the samples gradually increased from 162 nm to 599 nm, whereas the relative density (measured density/theoretical density) of the samples gradually increased from 91.12% to 93.93%. The normalized release rates of Ce element in the Gd2Ce2O7 sample after 42 days were slightly lower than those of Hf element in the Gd2Hf2O7 sample owing to the lower porosity of the Gd2Ce2O7 sample, and the bond dissociation energy of Ce–O (∼795 kJ/mol) was slightly higher than that of Hf–O (∼791 kJ/mol). Moreover, the normalized release rates of all considered elements were lower than 10−5 g m−2 d−1, indicating that the samples had excellent chemical stability.