Phase evolution and aqueous durability of nanograin Gd2Hf2O7 ceramic as a potential nuclear waste immobilization matrix

A nanograin (Gd2Hf2O7, GHO) ceramic is prepared using a sol-gel and cold-pressing sintering method as a potential curing matrix material for the high-level radioactive waste (HLW). The microstructure, phase evolution, Raman spectra and aqueous durability of the nanograin GHO samples are comprehensively studied. Results show that the sintered powder of the GHO ceramic undergoes a transition from defective fluorite to a pyrochlore structure in the process of increasing the sintering temperature from 1273 to 1673 K. As the pressing pressure increases from 5 to 30 MPa, the density of the GHO sample increased from 7.25 to 8.25 g cm-3, and the corresponding open porosity decreases from 7.61 to 3.95%. XRD, Raman, and microstructure analysis confirmed that the GHO sample pressed at 30 MPa and sintered after 1673 K exhibits a typical nanograin pyrochlore structure with an average grain size of 167 nm. The normalized release rates of Gd and Hf elements in the optimized GHO sample after 42 days are 6.7 x 10-6 g m- 2 d-1 and 7.9 x 10-7 g m- 2 d-1, respectively, which exhibited good aqueous durability. The results can provide an alternative candidate for the development of HLW curing matrix material with enhanced environmental stability.