Estimated longevity of performance of Portland cement grout seal materials

The sealing of boreholes, fractures and underground workings of repositories is a common concern for all programs investigating the deep burial of nuclear waste as a disposal mechanism. Two grouting materials, bentonite and portland cement, have been identified by many programs as likely candidate seal materials. The longevity of performance of both of these materials is currently being investigated under the auspices of the Stripa Project. These investigations comprise coordinated laboratory, field and modeling studies to produce fundamental data, practical experience and estimates of long-range performance, respectively. Long-term performance is an especially sensitive issue for cement because the phases that comprise cement are metastable. Accordingly, it may be assumed that cement grout performance will degrade with time. For a simplified cement system, two mechanisms for chemical degradation have been considered: phase change and dissolution. When considering dissolution, both equilibrium (slow flow) and open (fast flow) systems have been analyzed to establish bounds. Granitic terrain groundwaters ranging from fresh to saline have been taken as solvents. To assess the consequences in terms of flow, an empirical relation between cement permeability and porosity has been developed. Predictions of performance changes with time have been produced by making conservative estimates of local hydraulic head conditions for various periods of repository history. For the crystalline rock environments considered, preliminary results indicate that cement grout performance may be acceptable for tens of thousands to millions of years providing its initial hydraulic conductivity is on the order of 10-12 m/s