Assessing the Long-Term Behaviour of the Industrial Bentonites Employed in a Repository for Radioactive Wastes by Studying Natural Bentonites in the Field

Abstract

Bentonite makes an important contribution to the performance of the engineered barriers in most radioactive waste repository designs. The choice of bentonite results from its favourable properties for waste isolation and its stability in relevant geological environments. However, the longevity of bentonite (especially the resistance to waste container sinking) has been little studied. Modelling results suggest significant bentonite deformation and associated canister sinking is unlikely and, here, long-term natural system data are used as a reality check on model predictions. Results indicate that bentonite from the investigated site shows no significant deviation in bulk physical parameters from repository bentonite. However, micro-scale shear planes can be seen throughout the sampled cores. The presence of multi-directional S- and C-type shears suggests they originate from loading from the overlying limestone, not gravitational tectonics. The plastic limits and angles of shearing resistance for natural and repository bentonites suggest both are susceptible to shearing. The impact of bentonite shear under load could be minimised by appropriate design, but existing lower activity waste container designs do not consider the potentially high external stresses from the bentonite backfill and this should be addressed in future.

Benefits of applying X-ray computed tomography in bentonite based material research focussed on geological disposal of radioactive waste

“…Sampling of bentonite can cause deformation during extraction from a deposit through heave when confinement is lost or by deformation during drilling due to mechanical strain by the drill bit, even if drilled without water. Drilling itself also deforms soft material especially at the edges of the samples, as observed by Alexander et al (2017). Further deformation may be caused if samples are transported or stored unprotected against humidity changes or by impact damage (e.g.…”

“…from dropping). Even further deformation can be caused by the impregnation methods used in thin section manufacturing (all observed by Alexander et al 2017). These artefacts can often be easily observed in the samples and the damage can be taken into account in the interpretation, but less disturbed samples would be preferred (see discussion in Ewy 2015).…”

“…These artefacts can often be easily observed in the samples and the damage can be taken into account in the interpretation, but less disturbed samples would be preferred (see discussion in Ewy 2015). In this study, the emphasis is set on identifying these artefacts via XCT and assessing their significance to the interpretation of the results (those presented in Alexander et al 2017).…”

“…The Kato Moni bentonite is low grade (smectite content 20.6 to 36.2%) bentonite with calcium as the main exchangeable cation (Alexander et al 2017). It is part of the youngest sequence of the Troodos ophiolite, deposited on top of the pillow lavas (for a description of the full geological setting, see Alexander et al 2017).…”

“…Here, the X-ray computed tomography (XCT) method is applied to bentonites, providing a new, structurally more profound way to interpret the results of physical and chemical analyses of samples. The study is a compilation of three phases: 1) initial assessment of the method supported by experimental data (Alexander et al 2017), 2) applying the method in the initial phase for new NA investigations and 3) applying the method on engineered samples to enable comparison.…”