Effect of Na2O on aqueous dissolution of nuclear waste glasses

Abstract

Sodium oxide is present in the majority of commercial and waste glasses as a viscosity-reducing component. In some nuclear waste glasses, its source is the waste itself. As such, it can limit the waste loading because of its deleterious effect on the resistance of the glass to attack by aqueous media. The maximum tolerable content of Na2O in glass depends on the presence and concentration of components that interact with it. To assess the acceptability limits of Na2O in the composition region of nuclear waste glasses, we formulated 11 baseline compositions by varying the content of oxides of Si, B, Al, Ca, Zr, and Li. In each of these compositions, we varied the Na2O fraction from 8–16 mass% to 23–30 mass%. To each of 146 glasses thus formulated, we applied the seven-day Product Consistency Test (PCT) to determine normalized B and Na releases (ri, where i ≡ B or Na). Fitting approximation functions ln(ri/gm−2) = Σbijgj to ri data (gj is the j-th component mass fraction and bij the corresponding component coefficient), we showed that the rB (and, consequently, the initial glass alteration rate) was proportional to the glass component mass fractions in the order Al2O3<CaO<SiO2<ZrO2<B2O3<Na2O<Li2O. No threshold was detected at which glass structure would fall apart or beyond which a continuous nondurable phase would be separated. Specific examples are given to demonstrate restrictions imposed on the boundary of the composition region of acceptable glasses by the maximum allowable rB and by the melt viscosity required for glass melter operation. Finally, the role that PCT data may play in understanding the evolution of the glass alteration process is discussed.