The Department of Advanced Laser and Optoelectronic Functional Materials at the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences, has recently made significant progress in understanding the mechanism behind the suppression of molybdate crystallization in nuclear waste glass. For the first time, the suppression mechanism at the atomic scale has been revealed, identifying the critical role of aluminum-oxygen polyhedra in inhibiting molybdate crystallization. This breakthrough provides a theoretical basis for the compositional design of highly stable nuclear waste glass. The findings were published in The Journal of Physical Chemistry C under the title "Impact of Glass Compositions on Molybdate Crystallization in Borosilicate Glasses."

Borosilicate glass used for immobilizing nuclear waste faces a major challenge: high-level radioactive waste can contain up to 12 wt% molybdenum (Mo), whereas the solubility of Mo in conventional glass matrices is typically less than 2 wt%. Excess Mo tends to precipitate as crystalline molybdates (e.g., Na₂MoO₄, CaMoO₄), which severely undermines the chemical and thermal stability of the glass. Effectively suppressing molybdate crystallization is essential for ensuring the long-term safe storage of nuclear waste. Although previous studies have attempted to enhance Mo solubility by modifying glass compositions, the microscopic mechanism by which specific components influence Mo behavior remained unclear.

In this study, the researchers employed advanced techniques such as solid-state nuclear magnetic resonance (SSNMR) and transmission electron microscopy (TEM) to systematically investigate the relationship between glass composition and molybdate crystallization. The study reveals that Ca²⁺ ions can simultaneously coordinate with tetrahedral aluminum (Al^[4]) and octahedral molybdenum (Mo^[6]) to form Al^[4]–Ca²⁺–Mo^[6] linkages—structures that cannot be formed by Na^⁺ ions. These linkages facilitate the dispersion of Mo⁶⁺ and Ca²⁺ ions, thereby inhibiting CaMoO₄ crystallization.

Moreover, the research proposes a competitive hierarchy among high-valence cations for free oxygen and highlights the distinctive roles of different network modifier cations. These insights offer a theoretical framework for optimizing nuclear waste glass formulations and enhancing their long-term performance.

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences [grant number XDB0650000] and the China National Project titled “Development and Verification of Two-Step Cold Crucible Vitrification Engineering-Scale Prototype” [grant number FKY1683ZHG001SSJS-B01-001].

Left: ²⁷Al MAS NMR spectra of glasses with different CaO contents.

Right: Schematic illustration of the Al^[4]–Ca²⁺–Mo^[6] structure.

Article link: https://pubs.acs.org/doi/10.1021/acs.jpcc.4c08048

Contact: REN Jinjun

Advanced Laser and Optoelectronic Functional Materials Department,

Shanghai Institute of Optics and Fine Mechanics, CAS

Email: jinjunren@siom.ac.cn@siom.ac.cn