SON 68 nuclear glass alteration kinetics between pH 7 and pH 11.5

The effect of the pH was investigated on the mechanisms of SON 68 (R7T7-type) glass alteration under `saturationʹ conditions in order to delimit the pH range within which a stable, protective gel is formed. Static experiments were conducted at 90°C with an S/V ratio of 50 cm−1 at various imposed pH values: 7, 8, 9.5, 10, 10.5, 11 and 11.5. An additional experiment was conducted with no restriction on the pH. The kinetic study showed that a protective gel formed in all the test media, although its stability was pH-dependent: at pH 11 or higher, the precipitation of a potassium and sodium aluminosilicate led to the degradation of the gel and the lose of its protective properties. This phenomenon resulted in renewed glass alteration, leading to complete and rapid degradation of the glass into alteration products. The aluminosilicate precipitation was found to be limited by the solubility of aluminum. Below pH 10, this type of secondary phase is unlikely to precipitate and the gel should remain stable. This study, based on thermodynamic and kinetic considerations, suggests that aqueous alteration of the French SON 68 nuclear glass results in the formation of a protective gel with long-term stability between pH 7 and pH 10.