This study investigates the long term behavior of glasses used for confinement of nuclear wastes. The results have been obtained from a fruitful collaboration between different CEA laboratories (LLB, LIONS, CEA Marcoule) and the Ecole Polytechnique. The corrosion process of the glasses by water creates at the glass surfaces, an alteration porous layer, hydrated and amorphous, called "gel". This gel, which is the result of the release of soluble elements, of hydrolyses and of silica network recondensation can in specific conditions strongly limit the exchange process between the glass and the solution. Our hypothesis to describe this phenomenon is the closure of the gel porosity. More...