Abstract
In this study, a promising method of spark plasma sintering (SPS) was investigated for the fabrication of ceramic matrices designed for the reliable immobilization of highly radioactive radionuclides 137Cs and 90Sr. The ceramic matrices were derived from a mixed composition of two aluminosilicate mineral-like phases – pollucite (Cs,Na)AlSi2O6 and gehlenite Sr2Al2SiO7. The originality of the developed approach lies in the utilization of hydrothermal synthesis for obtaining granulated precursor material. Hollow aluminosilicate microspheres (cenospheres) from coal fly ash served as the initial raw material, which were treated with alkaline solutions containing Cs+ and Sr2+ ions as simulants for the corresponding radionuclides. This method facilitated the achievement of high efficiency in cation removal from solutions exceeding 98 %. For a comprehensive examination of the composition and morphology of cenosphere-derived precursor particles, and the influence of sintering temperature on phase and structure formation of ceramic matrices under non-equilibrium conditions of spark plasma heating, various analytical techniques were employed. These included thermogravimetry/differential thermal analysis (TG/DTA), powdered X-ray diffraction analysis (PXRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), as well as the method of low-temperature nitrogen adsorption for determining the specific surface area. The obtained ceramic samples exhibited high values of specific density (2.688–2.915 g/cm³), compressive strength (666–808 MPa), and Vickers microhardness (1.8–2.5 GPa), attesting to their high quality and stability. The results of the hydrolytic stability assessment revealed that the leaching rates of Cs+ and Sr2+ ions from the ceramics are extremely low, within the range of 10−5–10−6 g/cm²·day. These values fully satisfy the requirements of GOST R 50926–96 and the international standard ISO 6961:1982 for solidified forms of high-level waste.
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