Immobilization of simulated An4+ in radioactive sludge via microwave sintering: Mechanistic and performance

In this study, we investigate radioactive sludge containing An ⁴⁺ ions via simulation. Ce ⁴⁺ is employed as a surrogate isotope for An ⁴⁺ ions, and silicate glass particles are incorporated as stabilizing agents. Continuous microwave sintering technology is adopted to sinter the simulated radioactive sludge with varying CeO ₂ contents co-doped with 60 wt.% glass particles at temperatures of 1100 °C, 1200 °C, and 1300 °C to achieve sample vitrification. The phase evolution, microstructure, morphology, chemical stability, sample density, and porosity of the sintered specimens are analyzed comprehensively. The results indicate distinct maximum solubility limits at different sintering temperatures: 5 wt.%, 6 wt.%, and 13 wt.% for 1100 °C, 1200 °C, and 1300 °C respectively. Under these conditions, Ce is immobilized within the glass network structure. Notably, the Ce ³⁺/Ce ⁴⁺ ratio in the 1300 °C-Ce10 specimen reaches 2.04, and the combined percentage of Q ³ and Q ⁴ glass structural units exceeds 50 %. Beyond the optimal solubility limit, tetragonal CeO ₂ crystals are formed on the glass sample surface. The normalized leaching rate of Ce from the samples remains at 5.0 × 10 ⁻⁶ g m ⁻² d ⁻¹. Additionally, the sintered samples indicate densities ranging from 2.65 to 3.01 g/cm ³ and porosities from 0.18 % to 1.8 %. This study introduces a novel perspective for addressing radioactive sludge treatment.