Influence of MnO and ZnO on molybdate crystallization in borosilicate glass

Abstract

Molybdenum (Mo) is an abundant fission product that is present in high-level radioactive waste. The incorporation of Mo into borosilicate glasses developed for the immobilization of radioactive waste causes alkali and alkaline earth molybdates to crystallize because of their low solubility. This study investigated the effects of adding MnO and ZnO to a fabricated simplified nuclear waste glass on the crystallization of molybdates and structure of the glass. The natural cooling of nuclear waste glasses in canisters was simulated by cooling the molten glass slowly at a rate of 1 °C/min. X-ray diffraction and scanning electron microscopy were used to analyze the crystallization behavior, and X-ray absorption spectroscopy was used to identify the local structures of Mn, Zn, and Mo within the glass. The results confirmed that the addition of MnO and ZnO both increased the solubility of Mo in the nuclear waste glass and preferentially inhibited the formation of alkali molybdate crystals, which deteriorate the chemical durability of the glass. Moreover, the solubility of Mo was further improved by the addition of MnO into Nd2O3-bearing borosilicate glasses. Unlike Nd, the Mo-rich phase barely included Mn and Zn. Both Mn and Zn appeared in a tetrahedral structure that comprised a silicate network. On the basis of the bond valence model, it is suggested that MoO42− units can be directly connected to the Mn and Zn.