A calculation code was developed by the Commissariat à l'Energie Atomique to estimate the influence of the major parameters of a geologic repository site on the alteration of a high-level vitrified waste package. The model is based on a first-order kinetic law and on a deviation concerning saturation with respect to H4SiO4. Glass alteration is governed by the coefficient of silicon diffusion in the interstitial water of the gel layer and by the leachate renewal rate (i.e., the flow rate in the repository) if it is of very low magnitude. The effects of the other parameters are much less significant. When applied to the alteration of natural basalts, the code seems to indicate that the gel conserves its diffusion barrier properties for a long time. Finally, the validity of the underlying hypotheses of the code is discussed.