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.
Highlights
Bentonites are considered to be the most suitable materials for the multibarrier system of high-level radioactive waste (HLRW) repositories [1,2,3,4]
As indicated by the powder X-ray diffraction (PXRD) study, the heating of the material at 200 ◦ C resulted in the complete dehydration of the interlayer space of the montmorillonite and its structural collapse to the 9.8 Å structure
The results indicate that thermally treated bentonite BCV 2017 fully rehydrates when it is exposed to free water (Figure 7) and its mechanical behaviour is reaching similar values compared to original BCV 2017 bentonite as demonstrated by swelling pressures (Figure 9)
Summary
Bentonites are considered to be the most suitable materials for the multibarrier system of high-level radioactive waste (HLRW) repositories [1,2,3,4]. The favourable behaviour of bentonites is mainly influenced by the presence of smectites having unique physical and physico-chemical properties [3,5]. The stability of smectites is a key factor for all concepts that consider the use of bentonites in the engineered barriers of nuclear waste repositories [3]. The Czech deep geological repository concept is based on the use of bentonites and montmorillonite-rich clays of Czech origin as the buffer and backfill materials. The first bentonite to be tested was extracted from the Rokle deposit, followed by industrially processed B75 bentonite, BAM—a mixture of Rokle, Stránce and Černý vrch bentonites and BCV—bentonite from the Černý vrch deposit
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