Effects of rare earth variation on feed-to-glass conversion during nuclear waste vitrification

Vitrification has been considered the primary option for immobilization of the high-level radioactive liquid waste (HLLW); however, the compositional variation of HLLW makes the reactions between wastes and glass frits rather complex. Rare earth elements are one of the most enriched elements in HLLW; thus, effects of rare earth (Nd ₂O ₃ as a surrogate) variation on feed-to-glass conversion were investigated by means of differential scanning calorimetry, ( in situ) X-ray diffraction, ²⁹ Si nuclear magnetic resonance spectroscopy, and Raman spectroscopy in this work. Differential scanning calorimetry analysis indicates that phase evolution occurs between ∼600 °C and ∼1000 °C and can hardly be affected by Nd ₂O ₃ addition. ( In situ ) X-ray diffraction demonstrates that the HLLW calcines and the glass frits start reacting above 600 °C and intermediate phases appear between 700 °C and 900 °C. The addition of Nd ₂O ₃ facilitates the feed-to-glass conversion as all crystalline phases turn to glass in advance with additional Nd ₂O ₃. Besides, Si−O units tend to depolymerize with increasing Nd ₂O ₃ addition as verified by both ²⁹ Si nuclear magnetic resonance spectroscopy and Raman spectroscopy, which indicates that Nd ₂O ₃ depolymerizes glass networks and hence promotes the feed reactions.