Alteration effects and leach rates of basaltic glasses: Implications for the long-term stability of nuclear waste form borosilicate glasses

Abstract

One of the most critical aspects of the evaluation of radioactive waste forms is the extrapolation of short-term laboratory experiments (usually less than one year) over long periods of geologic time (greater than 10 000 y). The extrapolation requires not only accurate experimental measurements, but the selection of the correct model for the mechanism of dissolution. Basaltic glasses (SiO2 = 45 to 55 wt%) and borosilicate nuclear waste form glasses (SiO2 = 35 to 50 wt%) appear to be similar in their corrosion rates and mechanisms, as the SiO2 content of glass has been shown to have an important effect on the kinetics and reaction mechanism. This study presents data on a rhyolitic glass from Glass Mountain, Siskiyou County, California (SiO2 = 76 wt%, less than 500 y old) and two basaltic glasses from Kilauea, Hawaii (SiO2 = 50 wt%, ≈ 24000 y old) for comparison son to data for a simulated borosilicate waste glass, C-31-3EC-SPF-Na (SiO2 = 35 wt%). All samples (1 × 1 × 0.1cm) are leached at 200 °C in synthetic NaCl-solution (saturated at 55 °C) in Teflon-lined autoclaves for 3, 10 and 30 days. The sample surfaces were examined (before and after leaching) by SEM, EPMA and EDS. The results of these experiments were compared to alteration layers (palagonites) observed for naturally altered basaltic glasses of great age.