Résumé
We have performed TEM, Raman microspectroscopy, and NanoSIMS characterization of borosilicate glass samples altered for nearly 26 years at 90 degrees C in a confined granitic medium in order to better understand the rate-limiting mechanisms under conditions representative of a deep geological repository for vitrified radioactive waste. For the first time, we show a thick interphase that behaves like a diffusion barrier between the pristine glass and the other alteration products (porous gel and crystalline phases). Our findings indicate that the glass undergoes two distinct irreversible reactions: (i) hydration of the pristine glass controlled by water diffusion with a diffusion coefficient of 2 x 10(-21) m(2)/s and (ii) transformation of the hydrated glass into a macroporous gel with major structural changes. Both materials are nonstoichiometric and metastable. A final reversible reaction leads to the formation of crystalline phases that consume elements forming the gel layer and the hydrated glass. All these reactions must be combined in a model to predict long-term rates of nuclear glass in natural environments.