No abstract

Bentonites are considered to be the most suitable materials for the multibarrier system of high-level radioactive waste repositories. Since BCV bentonite has been proved to be an ideal representative of Czech Ca-Mg bentonites in this respect, it has been included in the Czech Radioactive Waste Repository Authority (SÚRAO) buffer and backfill R&D programme. Detailed knowledge of processes in the material induced by thermal loading provides invaluable assistance regarding the evolution of the material under repository conditions. Samples of both original BCV 2017 bentonite and the same material thermally treated at 200 °C were characterised by means of chemical analysis, powder X-ray diffraction, infrared spectroscopy, thermal analysis, cation exchange capacity, specific surface area (BET) measurements, the determination of the swell index, the liquid limit, the swelling pressure and water retention curves. The smectite in BCV 2017 bentonite comprises Ca-Mg montmorillonite with a significant degree of Fe3+ substitution in the octahedral sheet. Two main transformation processes were observed following heating at 200 °C over 27 months, the first of which comprised the dehydration of the montmorillonite and the subsequent reduction of the 001 basal distance from 14.5 Å (the original BCV 2017) to 9.8 Å, thus indicating the absence of water molecules in the interlayer space. The second concerned the dehydration and partial dehydroxylation of goethite. With the exception of the dehydration of the interlayer space, the PXRD and FTIR study revealed the crystallochemical stability of the montmorillonite in BCV 2017 bentonite under the selected experimental conditions. The geotechnical tests indicated no major changes in the mechanical properties of the thermally treated BCV 2017 bentonite, as demonstrated by the similar swelling pressure values. However, the variation in the swell index and the gradual increase in the liquid limit with the wetting time indicated a lower hydration rate. The retention curves consistently showed the lower retention capacity of the thermally treated samples, thus indicating the incomplete re-hydration of the thermally treated BCV 2017 exposed to air humidity and the difference in its behaviour compared to the material exposed to liquid water.

It is assumed that clay materials such as bentonites will be used as the buffer and backfill of high-level radioactive waste disposal. Due to the very long timescale, the design can be based only on mathematical models that have been thoroughly validated via the use of high-quality laboratory data. The presentation of an extensive laboratory dataset, accompanied by the validation of a bentonite constitutive model, comprises the main topic of this paper. The objectives of the laboratory research were to determine the basic hydromechanical properties of BCV bentonite and to study the stress path behaviour of this bentonite. The laboratory results revealed that the hydromechanical behaviour is highly dependent on the stress path assumed, as well as the method of water introduction into the sample. The results further indicate that the micro- and macrostructures are constantly evolving, which exerts a significant impact on the determination of the material properties and the development of mathematical models. Moreover, it was demonstrated that swelling pressure tests, although they are typically considered to be element tests (with constant fields of state variables), when adopted by researchers for the calibration of constitutive models, do not in fact constitute element tests. A hypoplastic model of the behaviour of bentonite was implemented and validated using the experimental data obtained. It was demonstrated that the hypoplastic model accurately predicted the main behavioural features of bentonite that are important in terms of the prediction of the behaviour of the buffer in nuclear waste repositories.

The aim of the DOPAS project is to address the design basis, reference designs and strategies for the plugs and seals to be used in geological disposal facilities. The Czech 'Experimental Pressure and Sealing Plug' (EPSP) experiment has the following objectives: to develop, monitor and verify the functionality of an in situ experimental plug, and to determine and describe in detail the materials to be used with respect to experimental plug construction. The functionality of the experimental plug will be verified by means of conducting pressure tests after the construction phase of EPSP. The internal bentonite section of the EPSP will be constructed from bentonite pellets with a dry density of 1850 kg m 23 ; the pellets will disintegrate following saturation and swelling to form a homogeneous material with a dry density of around 1400 kg m 23 . This process leads to the question of the pore space of the bentonite material: that is, whether it will be possible to employ a numerical model for homogeneous compacted bentonite powder porosity instead of that of the double pore/void space of the bentonite pellets used in the experiment. For this reason, a laboratory physical model (PHM -physical hydraulic model) was incorporated into the DOPAS project for the purpose of describing the saturation and disintegration of the bentonite pellets. The aim of this paper is to describe the results of the space distribution of relative humidity in a sample consisting of bentonite pellets with a view to subsequent numerical modelling and to extend the current knowledge concerning the behaviour of the bentonite pellets used in the EPSP experiment. The results show that following water saturation and the swelling of the bentonite the pellets form a homogeneous mass in the same way as does the compacted bentonite powder. After a period of 160 days, the pellets attained 100% relative humidity at a distance of 12.5 cm and swelling pressure recorded at the end of the model was seen to slowly increase over this time period.