Structural investigation of alkali aluminoborosilicate glass containing MoO3 for vitrification of nuclear waste

Abstract

Molybdenum (Mo), one of the fission products abundant in high-level radioactive waste, has low solubility in alumino-borosilicate glasses. When the concentration of MoO3 increases in glass, crystalline phases such as alkali and alkaline-earth molybdates (Na2MoO4, CaMoO4) are generated in the glass during the cooling process. The control of crystallization is important for producing durable nuclear waste glass. In this study, the incorporation of various alkali cations (Li, Na, K, Cs) into alumino-borosilicate glasses was closely investigated for their effect on the crystallization of molybdenum. Several analyses, including XRD, SEM, XAS, XPS, Raman spectroscopy, and solid-state MAS NMR spectroscopy, were used to evaluate the local environment of Mo and the glass structure. The results indicated that the addition of larger alkali cations reduced phase separation and hindered the formation of molybdenum crystalline phases. As the size of the added alkali cations increases, phase separation between the network-forming elements in the glass is suppressed, resulting in a more polymerized and homogeneous glass structure. These structural modifications can inhibit the formation of MoO42- units, causing chemical distortion to the MoO66form, or generate Mo-reducing species. Structural changes in the Mo unit can form direct bonds to the silicate network based on the bond valence model (Mo-O-Si). This study suggests the possibility of structural modification of glasses containing MoO3 to improve the solubility.