Cs ⁺ /Sr ²⁺ /Na ⁺ ‐Induced Crystallization of Cenosphere's Glass for ¹³⁷ Cs and ⁹⁰ Sr Immobilization in Durable Glass‐Ceramics

ABSTRACT The controlled crystallization of Cs ⁺ /Sr ²⁺ /Na ⁺ ‐admixed aluminosilicate glasses with a mineral‐like composition, based on coal fly ash cenospheres, was studied as a simulation of the immobilization of ¹³⁷ Cs and ⁹⁰ Sr in glass‐crystalline matrices. The incorporation of Cs ⁺ and Sr ²⁺ in mineral‐like phases using cenospheres with SiO ₂ /Al ₂ O ₃ ratios of 1.5–3.4, characteristic of Cs ⁺ ‐ and Sr ^{2 +} ‐bearing minerals, was demonstrated. Cs ⁺ , Sr ²⁺ , and Na ⁺ immobilization was achieved by mixing the cenospheres with predetermined ratios of CsNO ₃ /Sr(NO ₃ ) ₂ /NaNO ₃ solutions, followed by drying and calcination at temperatures between 750°C and 1100°C. Cs ⁺ immobilization via this method resulted in glass‐ceramics incorporating up to 87 wt% of the Cs ⁺ ‐phases pollucite and kalsilite. Processing the Sr(NO ₃ ) ₂ /cenosphere mixtures results in the formation of mineral‐like phases of Sr‐anorthite and Sr‐gehlenite. The Na ⁺ ‐induced transformation of the cenosphere glass was performed to simulate the solidification of ¹³⁷ Cs‐bearing sodium‐enriched radioactive waste, with nepheline acting as the host phase for ¹³⁷ Cs. Depending on the sodium loading and temperature, the NaNO ₃ /cenosphere blends transform into glass‐ceramics with different glass phase/nepheline ratios. The cesium‐normalized leaching rates from the glass‐ceramics were found to be in the range of 10 ⁻⁷ –10 ⁻⁹ g cm ⁻² day ⁻¹ by conducting the chemical durability test. These rates meet the requirements of GOST R 50926–96 and NP‐019‐2000 (Russia). This demonstrates the high durability of cenosphere‐based glass‐ceramics, facilitating their safe geological disposal.