Microstructural characteristics and aqueous durability of Nd-doped Gd2Hf2O7 nanocrystalline ceramics as nuclear waste forms

A series of Nd-doped nanocrystalline Gd 2- x Nd x Hf 2 O 7 (GNHO, 0.0 ≤ x ≤ 2.0) ceramic samples, serving as nuclear waste forms, were synthesized via the sol-gel method combined with cold pressing and sintering. The microstructure, phase evolution, Raman spectral characteristics, and aqueous durability of these GNHO samples were systematically investigated. Results revealed that all GNHO samples formed a single pyrochlore structure sintering at 1673 K for 20 h. With the increase of x Nd from 0 to 2.0, the average grain size of the samples gradually increased from 162 nm to 294 nm, while their actual density gradually decreased from 8.21 to 7.83 g·cm -3 . After 42 days of testing, the normalized release rates of Nd and Hf elements in the Nd 2 Hf 2 O 7 sample were slightly lower than those of Gd and Hf elements in the Gd 2 Hf 2 O 7 sample. This observation can be attributed to the lower porosity of the Nd 2 Hf 2 O 7 specimen and the higher bond energy of Nd–O bond (∼753 kJ/mol) compared to that of Gd–O bond (∼716 kJ/mol). Furthermore, the normalized release rates of all investigated elements were below 10 −5 g·m -2 ·d -1 , exhibiting excellent aqueous durability. First-principles calculations further confirmed that Nd 2 Hf 2 O 7 ceramic exhibits higher structural stability than Gd 2 Hf 2 O 7 .