Over the last few decades, understanding the kinetics of nuclear waste glass dissolution/alteration in aqueous media has advanced through a large number of the experimental and atomistic simulation studies performed by a large number of researchers in the world, which has contributed successfully to an advance of modeling the glass dissolution/alteration for the geological disposal. For assessing the long-term glass performance in geological disposal with reliability, however, we need a greater understanding of the glass dissolution/alteration kinetics under the actual repository conditions in consideration of the potential long-term evolution of the repository conditions. The greater understanding of the dissolution/alteration kinetics is also required for parameterization of a mechanistic model used to determine the glass dissolution/alteration rate with the radionuclide release rate as a function of key environmental variables such as the solution composition, pH, Eh, temperature, and time.

For the understanding of glass dissolution/alteration kinetics, we need experimental data on the glass dissolution rate measured precisely, consistently and systematically under well-constrained test conditions. Along with the data on the dissolution rate, which is usually determined from solution analysis, we also need the data on the altered glass surface properties, such as morphology of alteration layers, chemical composition, elemental depth profile, crystallinity, stability, density, porosity, permeability of water and glass constituent elements, etc., analyzed for the glass altered under well-constrained test conditions. For the last few decades, a large number of dissolution tests have been performed for the waste glasses, however, there have existed only a few data available for evaluation of the dissolution kinetics, because of difficulties in precise and systematic measurement. With respect to pH dependence of the glass dissolution rate, for example, we have only a few data available for the systematic evaluation, because the test conditions such as the solution composition, pH, and the glass surface area can change easily during the test period against the expectations as a consequence of the nature of the current standard test methods such as MCC-1 and PCT tests. Therefore, we have developed a new test method, “Micro-channel flow-through (MCFT) method”, to measure the glass dissolution rate precisely and systematically under various well-constrained test conditions, and applied it to measurement of the glass dissolution rate as a function of environmental parameters such as the solution composition, pH, temperature.

The MCFT method has been applied first to measurement of the initial dissolution rate, r0, as a function of pH and temperature for an international reference glass ISG and a Japanese reference glass P0798. The r0 has been measured successfully to provide the systematic data of r0 as a function of pH, where ISG showed a “V-shaped” pH dependence with a minimum at pH4, while P0798 glass shows a “U-shaped” pH dependence with a minimum at pH6. While the systematic data of r0 as a function of temperature at each constant pH has provided the apparent activation energy, Eact, for the initial dissolution. The Eact was evaluated to be 60-70 [kJ/mol] at acidic to weakly basic pH for ISG, which suggests a surface-reaction controlled-dissolution mechanism. For P0798, the Eact was evaluated to be 60-70 [kJ/mol] at neutral to weakly basic pH as well as ISG, however, the Eact at acidic pH was evaluated to be 50 [kJ/mol], which suggests a diffusion -controlled dissolution mechanism.

Currently, we have been trying to measure the glass dissolution rate as a function of solution concentration of Si by using the MCFT method and Si-isotopes. The concentration of Si dissolved in solution has been considered to be one of fundamental factors dominating the glass dissolution kinetics, and the rate equation based on the first-order dissolution rate law of pure amorphous silica has been proposed to be applied to modelling the glass dissolution/alteration as follows,
r = r0 (1-[H4SiO4]/K) (eq.1)
where r is the glass dissolution rate, r0 is the initial dissolution rate, [H4SiO4] is the activity of orthosilicic acid at the interface between glass surface and solution, and K is the equilibrium constant (which equals the
activity of orthosilicic acid at saturation).

We have been accumulating the data on the glass dissolution rate as a function of solution concentration of Si for ISG and P0798, and some of the interesting results will be introduced in the lecture along with discussions on the glass dissolution/alteration kinetics.