Abstract
Artificial and singular geochemical environments are created around the engineered barrier systems (EBS) designed to isolate high level nuclear wastes in deep geological repositories. A concrete-bentonite interface takes place within the EBS and it builds a significant chemical gradient (pH), approximately from pH 8 (bentonite) to pH 12 (low alkali concrete), in a few millimetre thickness. This disequilibrium triggers dissolution and precipitation reactions and form a thin altered region. In this area, poorly ordered authigenic clay minerals, mainly hydrated magnesium silicates, are formed adjacent to hydrated calcium silicates and calcite precipitates adhered to the interface with concrete. This paper presents the development of this authigenic mineral layer comparing 6–18 months to 13 years interfaces. Scanning Electron Microscopy with Energy Dispersive X-ray spectroscopy (SEM-EDX) morphological and chemical characterization with the aid of ternary plots, X-ray diffraction (XRD) and infrared (IR) data show the young to old interface evolution from single brucite layers to stevensite-saponite silicates composition. Geochemical calculations indicate that this layer acts as a pH~11 buffer useful to minimize bentonite alteration and to favour the retention of amphoteric metal ions.
Highlights
Deep geological repositories (DGR) are the available solution for the long-term storage of high-level radioactive waste (HLRW) [1]
Mg rich smectites have been proposed elsewhere as authigenic mineral indicators for alkaline lacustrine environments or basic rocks alteration and diagenesis. It has been studied the consequences of a rapid alkaline alteration induced by the contact of concrete and bentonite
An initial disequilibrium condition drives to the consumption of CO2 with the formation of calcite at the concrete interface and the precipitation of Mg and Mg-Al hydroxides intimately mixed with montmorillonite
Summary
Deep geological repositories (DGR) are the available solution for the long-term storage of high-level radioactive waste (HLRW) [1]. The waste will be isolated from the biosphere by a system of engineered and natural barriers. The engineered barrier system (EBS) usually consist of a cylindrical metal canister containing the waste, surrounded by a compacted bentonite clay barrier. The host rock, where access galleries are excavated, will need concrete for mechanical support of the walls in clay rock formations or concrete plugs in crystalline rock formations. Concrete is necessary to seal the excavation accesses and to maintain the swelling pressure of the hydrated bentonite backfill inside the galleries. DGR would gain confidence according to complementary multi-scale analysis and characterization carried out after the dismantling of short-term laboratory tests (months-years) and long-term (10–30 years) in-situ simulated experiments
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