The paper contains a coupled numerical analysis of the hydration of a potential sealing system for isolating shafts, ramps and connecting galleries in a deep geological repository for high level nuclear waste. The modelling adopted for the different components of the seal are described first together with the basic features of the numerical model. Subsequent the results of the analyses, concerning both mechanical and hydraulic parameters, are presented and discussed. Finally, it is shown that the use of a double structure model for the behaviour of the bentonite affects the evolution of the mechanical response of the seal but it does not modify significantly the basic hydration process.