To understand the role of water diffusion on the long-term nuclear waste glass alteration, dynamic experiments were conducted with the borosilicate SON 68 glass in synthetic solutions enriched in Si, Na and B at 50 and 90 °C. The water entering the glass exists to 80% in the form of molecular H2O and to 20% in the form of SiOH. The ratio of H/Na was 2.6 ± 0.3, indicating a complex mechanism including water diffusion and ionic-exchange. It was in agreement with model calculations based on glass structural units such as reedmergnerite and B2O3. Water diffusion coefficients in the glass, determined by modeling of the experimental data, were between 2 × 10−21 and 6 × 10−23 m2 s−1. Finally, under HLW disposal conditions, where interaction of nuclear glass with groundwater is expected to maintain saturation conditions, it is likely that water diffusion will contribute to the control of the glass alteration and the release of radionuclides.