Rhenium volatilization in SiO2-KNO3-KReO4 mixtures at elevated temperatures

Due to its long half-life, high fission yield, and high mobility in the groundwater, Technetium-99 (Tc-99) is a troublesome nuclide in nuclear waste. Moreover, the primary concern is the high volatility of Tc during nuclear waste vitrification. Previous studies demonstrated that nitrate significantly affected the Tc volatilization during the vitrification of Tc-containing waste. However, identifying how nitrate affects Tc volatilization is challenging due to the complexity of the composition of nuclear waste. This study used a simplified SiO₂-KNO₃-KReO₄ feed with rhenium (Re) as a Tc non-radioactive surrogate to study the nitrate effect on the Re volatilization at elevated temperatures. The results showed that KNO₃ could facilitate Re volatilization at temperatures below 800 °C when the molar percentage of KNO₃ in the KNO₃-KReO₄ binary system exceeded 60 mol%, which was attributed to the eutectic reaction between KReO₄ and KNO₃ at 310 °C. However, the increased KNO₃ content could inhibit the Re volatilization at temperature above 1200 °C since adding KNO₃ could increase the amorphous phase content, where Re was encapsulated and hardly evaporated.