Abstract
This laboratory experimental program investigated the impact of variations in the expansive mineral content, the grain size distribution of employed bentonite, the initial dry density and the selected hydration path on the water retention characteristics and swelling properties of processed Callovo-Oxfordian claystone and its mixtures with MX80 bentonite. The French reference concept for the disposal of nuclear waste in deep sedimentary rock formations envisages the reemployment of excavated material as backfill material, which is installed in situ by means of conventional compaction techniques. The investigations were of special interest as the major issues involving in situ compacted backfill materials were portrayed. Experiments showed that the impact of variations in the dry density on the water retention characteristics of all materials vanished as suctions exceeded 100 MPa. The negligible impact of variations in the initial dry density on the collapse behavior of claystone/ bentonite mixtures remained questionable.
Highlights
The French reference concept considers the disposal of intermediate- and high-level nuclear waste in the deep sedimentary Callovo-Oxfordian rock formation, referred to as Callovo-Oxfordian claystone (COXclaystone)
The reduction of the swelling pressure ratios remained essentially comparable, regardless of the initial dry density of the samples. This laboratory experimental program investigated the impact of variations in the expansive mineral content, the grain size distribution, the initial dry density and the suction on the water retention characteristics and the evolving swelling pressures of two different claystone/ bentonite mixtures
Those mixtures are planned to serve as backfill materials in nuclear waste repositories upon installing them directly inside the drifts
Summary
The French reference concept considers the disposal of intermediate- and high-level nuclear waste in the deep sedimentary Callovo-Oxfordian rock formation, referred to as Callovo-Oxfordian claystone (COXclaystone). The nuclear waste is encapsulated in canisters, which are emplaced in drifts being excavated in the host rock formation [1]. Once the operational phase of the repository is terminated, the drifts and shafts are sealed and backfilled. It is planned to process COXclaystone, which is obtained during repository construction, and to reemploy it as backfill material. Crushed and sieved COX-claystone is referred to as COXc. crushed and sieved COX-claystone is referred to as COXc It is installed in unsaturated conditions and swells upon the wetting front originating in the rock formation intrudes into it. The propagation of the excavation damaged/ disturbed zone (EDZ) is limited and hydraulic conductive voids are closed by material swelling under the constant volume conditions. Its manipulation can result in the conformance to defined safety requirements in terms of swelling pressure and hydraulic conductivity
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