Microstructural features of compacted MX-80 bentonite after the long-time experiment

Bentonite clay is proposed to be a buffer material in the KBS-3 method of deep geological disposal of spent nuclear fuel in Sweden and Finland. It is essential to know the long term behaviour of the material when evaluating the safety performance of the barrier system. Therefore, various experiments and modelling have been carried out with bentonite in different conditions during the last decades. However, most of the laboratory experiments in controlled conditions last less than few years only.

In this study 15 years old compacted MX-80 bentonite samples were studied. They stayed closed in a copper canister in contact with low salinity groundwater both in aerobic and anaerobic conditions. After dismantling the experiment a thorough characterization was carried out, including analysis of chemical composition of bentonite and external solution, clay microstructure, mineralogy and microbial activity.

In this presentation the focus is on the microstructural features of the samples. Small-angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR) and anion exclusion measurement were used to investigate the nano and micro porosity features of the material. The results are compared with similar analysis of MX-80 samples prepared in the laboratory and with short time equilibration with external solution. Calculations based on SAXS data give similar results but there are differences for NMR based results. This could be caused by dissimilarities of between the samples at the mesoscale level. No differences were observed between the oxic and anoxic conditions suggesting no influence of redox conditions on the bentonite microstructure.