Thermal and Hydrolytic Stability of Silicophosphate Matrices of Langbeinite Structure for Radioactive Waste Immobilization

Crystalline silicophosphates M2Zr2SiP2O12 (M = K, Rb, Cs) of langbeinite structure were synthesized using the sol–gel method. These materials are of interest as candidate matrices for incorporating radioactive elements that are produced during the reprocessing of irradiated nuclear fuel. The composition, morphology, and structure of the samples as a function of the synthesis temperature were studied by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray microanalysis. Single-phase silicophosphates were obtained at 800°C. The sintering of M2Zr2SiP2O12 samples by hot pressing was described in terms of time and temperature. The thermal stability of silicophosphates up to 1200°C was confirmed by thermo-XRD and combined TG–DSC analyses. Silicophosphates are isotropic, moderately expanding materials; the coefficient of linear thermal expansion of Cs2Zr2SiP2O12 is 5.8 × 10–6 °C–1. The 28-day dynamic chemical durability test showed that the leach rate of Cs⁺ ions from the ceramics of langbeinite structure in distilled water at 90°C was approximately ~10–5 g cm–2 day–1.