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

A series of Ce-doping nanograin Gd 2 Ce x Hf 2- x O 7 (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 Gd 2 Hf 2 O 7 sample comprised a single pyrochlore structure after sintering at 1673 K, but all the Gd 2 Ce x Hf 2- x O 7 (0.5 ≤  x  ≤ 2.0) samples assumed defective fluorite structures with Ce-doping. As the x Ce 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 Gd 2 Ce 2 O 7 sample after 42 days were slightly lower than those of Hf element in the Gd 2 Hf 2 O 7 sample owing to the lower porosity of the Gd 2 Ce 2 O 7 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.