Coupled chemistry-transport modeling of glass alteration using archaeological fractured samples

Complete text of publication follows: To improve the confidence in glass alteration models (in the nuclear and in the natural fields), their long-term predictive capacity has to be validated. For this purpose, we studied archaeological glass blocks weathered for 1800 years in seawater, a known and stable environment. They also presented a good morphological analogy (fracturing) with vitrified nuclear waste. We developed a specific geochemical model of alteration that we coupled with a transport model. The long-term predictive capacity was tested by comparing extended simulation results with the alteration products and kinetics. Three steps describing the glass alteration process were implemented in the chemical model: (1) the formation of a hydrated glass by interdiffusion, whose kinetics are controlled by a pH- and temperature-dependent diffusion coefficient; (2) the dissolution of the hydrated glass, whose kinetics is based on an affinity law; (3) the precipitation of secondary phases when thermodynamic equilibrium is reached. The parameters of the model were measured by performing various laboratory experiments in different conditions (solution composition, pH, temperature, S/V ratio). Then the model was coupled with diffusive transport in solution in order to simulate the alteration in cracks. Simulation results over 1800 years are consistent with archaeological glass block observations concerning (1) the nature of alteration products (hydrated glass, smectites, and carbonates) and (2) crack thicknesses (apparent alteration rates). These results not only improve the understanding of chemistry-transport coupling in fractures but also validate the confidence in glass alteration predictive models