Direct immobilization of simulated nuclear waste in preformed Gd2Zr2O7 pyrochlore via spark plasma sintering reaction

Abstract

A rapid and efficient method is preferred for immobilizing nuclear wastes. In this study, Ce4+, a simulated tetravalent actinide (An4+) nuclear waste, was directly introduced into the Gd2Zr2O7 matrix (as-prepared) via spark plasma sintering (SPS) at different temperatures (1725, 1750, 1775, and 1800 °C) without a complicated lattice occupancy design. The solubility of CeO2 could be achieved to 18.8 wt%. Most of the sintered specimens exhibited the fluorite phase, but two samples sintered at 1775 °C exhibited the pyrochlore phase due to the Ce4+→Ce3+ transformation. Moreover, the samples sintered at various temperatures exhibited highly densified microstructures, in which Gd, Zr, and Ce were evenly distributed, indicating adequate immobilization results. By increasing the doping content to 60 mol%, the grain boundary of the sample sintered at 1800 °C became almost indistinguishable and exhibited slight vitrification, which was confirmed by transmission electron microscopy results. These results afforded a rapid and effective method for the immobilization of nuclear wastes in the as-prepared matrix without considering the lattice occupation.