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In many countries, compacted bentonite is a candidate engineering barrier material for safe disposal of high-level radioactive waste. The Swedish Nuclear Fuel and Waste Management Company (SKB) set up an in situ experiment (the ABM project) to compare the stability of different bentonites under the conditions of exposure to an iron source and to elevated temperature (up to 130°C) at the Äspö Hard Rock Laboratory, Sweden. Results for the Japanese bentonite (Kunigel V1) are summarized in the present study.Mineralogical investigation using X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) suggested no transformation of smectite or neoformed clay phases. However, a distinct change in exchangeable cations of smectite was indicated (i.e. from Na type to Fe type and/or Ca type) in the bentonite in the vicinity of the steel heater.Measurements of hydraulic conductivity and swelling properties suggest that no significant changes occurred in the bentonite even in the vicinity of the steel heater. This is attributed to the limited portion of the bentonite affected by the iron–bentonite interactions and the incomplete ion-exchange reactions. The methylene blue cation exchange capacity and the determination of the exchangeable cations showed that the lateral distribution for these parameters was constant. However, the total exchangeable cation population has changed significantly compared to the initial sample.

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Swelling pressure tests have been conducted to understand the swelling properties of bentonite which is planned to be used as a buffer material in repositories for the geological disposal of radioactive waste. It has been reported that the swelling pressure obtained by swelling pressure test increases monotonically or temporarily decreases after increasing with the passage of the time depending on the initial water content. In this study, swelling pressure tests were conducted under several different initial water content conditions, and the change in wet density distribution inside the specimen during the swelling pressure tests was observed by X-ray CT measurement. It was confirmed that the presence or absence of collapse behavior and its magnitude affect the shape of the change in swelling pressure. When the collapse deformation was large, the amount of temporary decrease in swelling pressure was large. On the other hand, swelling pressure increased monotonically when there was no deformation due to collapse.