Precipitation and growth behavior of RuO2 crystals in simplified nuclear waste glass

Ruthenium (Ru), an important fission product in spent nuclear fuel, has low solubility in borosilicate glass and tends to precipitate during the vitrification process, which may adversely affect the stability and performance of the final waste form. In this work, the precipitation behavior of RuO<inf>2</inf> in a simplified borosilicate glass system was systematically studied. The results showed that at 1200 °C, the solubility of Ru was <200 ppm, and nanocrystalline RuO<inf>2</inf> began to form when the concentration exceeded this limit. About 80 % of the excess Ru was retained in the glass in the form of crystals, whereas about 20 % volatilized. RuO<inf>2</inf> initially precipitated as flower-like clusters and then transformed into granular particles through a diffusion-controlled growth process. When the melting time exceeded 50 min, the crystal growth was significantly enhanced because of surface energy-driven particle aggregation. This study offers valuable insights into the precipitation and growth mechanism of RuO<inf>2</inf> in borosilicate glass melts, which is crucial for optimizing the vitrification process and controlling noble metal behavior in nuclear waste immobilization.