Thermochemistry of borosilicate glasses. Contribution to the study of alteration of confinement glasses for radioactive wastes

This work contributes to the study of the thermochemical properties of nuclear waste glasses. Results are used to discuss mechanisms and parameters integrated in alteration models of storage matrix. Glass is a disordered material defined thermodynamically in a non-equilibrium. Taking into account one order parameter to characterise its configurational state, we consider a glass in a metastable equilibrium and we can measure its main thermochemical properties. Calorimetric techniques have been used to measure heat capacities and formation enthalpies of borosilicate glasses (from 3 to 8 constitutive oxides). Formation Entropies have been measured too: using the entropy theory of relaxation processes proposed by Adam and Gibbs (1965), the configurational entropy contribution has been determined from viscosity measurements. This set of data allowed to us the calculation of Gibbs free energies of dissolution of investigated glasses in pure water. By comparison with leaching experiments, we showed that the decreasing of the dissolution rate at high reaction progress cannot be associated to the approach of an equilibrium between the safe glass and the solution. We must take into account the composition changes of the reaction area and consider an equilibrium between hydrated de-alkalinized glass and/or gel layer with solution to evaluate the saturation state of the solution with respect to alteration products. (author)