Long-Term Release Rates of Borosilicate Glass Waste Forms

Evaluating the durability of nuclear waste glass material in terms of leach test results requires that one make reasonable extrapolations from laboratory experiments performed over a few years’ duration to repository behavior over time scales ranging up to tens of thousands of years. These require an understanding of the mechanisms that govern the leaching of glass as well as an accompanying predictive capability. By comparing the measured behavior with the predictions of mechanistic models, it can be concluded that at high flow rates, kinetic factors are predominant, while at low flow rates, saturation of the aqueous medium with respect to major matrix elements, particularly with respect to silica present in the glass and in its alteration products, becomes a controlling factor. A mathematical framework in which this synthesized picture can be expressed is presented. A careful analysis of the situation indicates that under likely repository conditions the fractional annual release rates can be expected to fall below the U.S. Nuclear Regulatory Commission criterion of 10⁻⁵ yr⁻¹.