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

Abstract

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 (Nd2O3 as a surrogate) variation on feed-to-glass conversion were investigated by means of differential scanning calorimetry, (in situ) X-ray diffraction, 29Si 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 Nd2O3 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 Nd2O3 facilitates the feed-to-glass conversion as all crystalline phases turn to glass in advance with additional Nd2O3. Besides, Si−O units tend to depolymerize with increasing Nd2O3 addition as verified by both 29Si nuclear magnetic resonance spectroscopy and Raman spectroscopy, which indicates that Nd2O3 depolymerizes glass networks and hence promotes the feed reactions.