Concrete is increasingly being used for encapsulating dangerous wastes although the experience with modern reinforced concrete is not longer than 100 years. In order to predict service lifes of 300–500 years for LLW (low level waste) facilities, some kind of extrapolation is therefore necessary. Due to values of corrosion intensity recorded when steel actively corrodes in concrete, it is not recommended to consider a propagation period in the design of LLW disposals. The design hypothesis should be to keep the steel passive unless unexpected changes in service life, occur. If the steel remains around values of 0.01 μA/cm2 (normal in a passive steel embedded in concrete) the stability of the structure could be provided along the foreseen expected life. A final comment is presented on the erroneous assumption usually made that relates corrosion rates to the oxygen diffusion rate through concrete cover.