Improving Molybdenum and Sulfur Vitrification in Borosilicate Nuclear Waste Glasses Using Phosphorus: Structural Insights from NMR.

Reclaiming residual fissile materials from spent nuclear fuel rods results in highly radioactive waste that must be immobilized in durable materials to protect the biosphere from harmful environmental exposure. The glasses typically used for this purpose are less effective at incorporating highly charged cations than cations in lower oxidation states. Molybdenum and sulfur solubilities in model aluminoborosilicate nuclear glasses were studied using solid-state nuclear magnetic resonance (NMR) spectroscopy. The extent of molybdenum and sulfur incorporation was significantly improved by doping the glasses with phosphorus, a high-field-strength cation capable of competing with Mo6+ and S6+ for oxygen, resulting in more oxygen sharing and a higher degree of Mo and S integration into the glass network. The addition of 3 mol % each of molybdenum and sulfur to phosphorus-free glasses led to the separation of crystalline sodium molybdates and sodium sulfates, whereas the replacement of 5 mol % SiO2 by P2O5 resulted in complete Mo and S incorporation into the glass, enhancing their respective loading limits by several times. NMR spectroscopy indicates that this improvement originates from the availability of chain phosphates as binding sites. This work demonstrates the value of using phosphate additives to improve Mo and S integration in nuclear glasses, providing insight into its specific structural role and foundational understanding for further development as a material design principle.