Cement-bentonite-iron interactions on small scale tests for testing performance of bentonites as a barrier in high-level radioactive waste repository concepts

Abstract

Nuclear power plants produce high-level radioactive waste, which has to be stored. Many countries plan to encapsulate their high-level radioactive waste (HLRW) in iron or steel canisters, which will be surrounded by highly compacted bentonite. The bentonite may be in contact with cement. Bentonite, discussed as the geotechnical barrier, is currently tested to find out which type has the best properties for a final repository. Considering the variety of bentonites in their behaviors regarding processes in contact with iron or cement, comparative methods are exceedingly valuable. In addition, when operating a repository a quality management (QM) regarding bentonite qualities will be required.In this study a small scale setup was tested that included the performance of bentonite in contact with the bentonite-iron and bentonite-cement interfaces. The tests proved that the different bentonites reacted differently. The mini-tests could be an appropriate way to test the behavior of bentonite in the final repository taking into account the appropriate proportions of cement, bentonite, and iron considered in the repository. The excess of cement used in this study has shown that the cement reacts very aggressively, which masked the reactions taking place at the bentonite-iron interface. A first overview of quantitative data about bentonite performance can be an example gained from both the mass loss of the metal pellet and the drop of cation exchange capacity (CEC) of the bentonite after the tests. Taking both these values into account does not permit deep understanding of the complex processes but allows for a quantitative comparison of the performance of different bentonites. Amongst the three bentonites tested the Ca-bentonite B36 from Slovakia showed on average less loss of swelling capacity (CEC) and hence is regarded superior to both other materials.