Composition‐Dependent Crystallization Behavior of Molybdate and Oxyapatite in High‐Level Waste Glass

Borosilicate glass, widely used as a host matrix for high-level radioactive waste (HLW) vitrification, encounters substantial difficulties when incorporating waste streams containing elevated levels of molybdenum and rare-earth elements. This challenge arises because molybdenum and rare-earth species, frequently present in HLW compositions, possess a pronounced tendency to initiate crystallization processes. Such crystallization not only causes deviations in the final glass properties from the desired design specifications but also threatens the operational reliability of the glass-melting system. Although the crystallization behavior of glass during vitrification is known to be critically dependent on its composition, a systematic investigation into this relationship remains notably absent. Herein, the studies that address this dependence through compositional design and characterization are still quite limited. In the present work, simulated waste glasses with controlled variations in alkaline earth oxide, B2O3, MoO3, and rare-earth oxide concentrations were prepared and analyzed to examine the role of composition in governing crystallization behavior. Our results reveal the competitive crystallization of two primary phases within the simulated glass systems, namely calcium molybdate (CaMoO4) and oxyapatite (Ca2Ln8(SiO4)6O2, where Ln denotes lanthanide elements). This competition is composition-dependent; in particular, higher levels of B2O3 and MoO3 markedly enhance the precipitation of CaMoO4 while inhibiting oxyapatite. Conversely, enriching the glass with alkaline earth and rare-earth oxides promotes the development of the oxyapatite phase. This observed competitive crystallization mechanism establishes a theoretical framework for tailoring glass crystallization behavior through compositional design of the base glass.