Abstract
The study investigated the basic geomechanical and mineralogical evolution of the bentonite barrier under various experimental boundary conditions which replicated the near-field Thermo-Hydro-Chemico (THC) conditions in a repository. The relationships between the physicochemical alterations and changes in the geotechnical properties have seldom been studied, especially on a consistent dataset. This paper attempts to link the physicochemical properties of Na-bentonite (MX-80) to the macro-scale engineering functionality of the bentonite post THC exposure. Experiments investigated the impact of THC variables on the engineering and physicochemical functionality of the bentonite with respect to its application within a High-Level Waste (HLW) engineered barrier system. Intrinsic alterations to the MX-80 bentonite under relatively short-term exposure to hydrothermal and chemical conditions were measured. Additionally, two long-term tests were conducted under ambient conditions to consider the impact of exposure duration. The intrinsic measurements were then related to the overall performance of the bentonite as a candidate barrier material for application in a UK geological disposal facility. Findings indicate that exposure to thermo-saline-corrosion conditions (i.e., corrosion products derived from structural grade 275 carbon steel) inhibits the free swell capacity and plasticity of the bentonite. However, the measured values remained above the design limits set out for the Swedish multi-barrier concept, from which the UK concept may take a lead. Corrosion alone does not appear to significantly affect the geotechnical measurements compared with the influence of thermal loading and high saline pore water after relatively short-term exposure. Thermal and corrosion exposure displayed no impact on the intrinsic swelling of the smectite component, indicating that no significant structural alteration had occurred. However, when exploring more complex saline solutions i.e., mixed Na, K and Ca, rather than the reference NaCl, divalent cation replacement was observed within the interlayer exchange site. This was accelerated in higher thermal loading conditions.
Highlights
The aim of the work presented here was to provide insight into geomechanical and corresponding mineralogical alterations to the MX-80 bentonite under variable THC exposure scenarios that are relevant to the canister/barrier interface within a deep geological disposal facility for nuclear waste.Geosciences 2017, 7, 69; doi:10.3390/geosciences7030069 www.mdpi.com/journal/geosciencesCorrelation of the mineralogical alterations with changes in the measured engineering parameters has seldom been conducted with previous research focusing on one of these aspects; more usually the mineralogical alterations
The MX-80 was subjected to saline solutions for approximately one month under ambient
This work examined the influence of salinity, temperature loading and duration considered for use within the engineered barrier system (EBS)
Summary
The aim of the work presented here was to provide insight into geomechanical and corresponding mineralogical alterations to the MX-80 bentonite under variable THC exposure scenarios that are relevant to the canister/barrier interface within a deep geological disposal facility for nuclear waste.Geosciences 2017, 7, 69; doi:10.3390/geosciences7030069 www.mdpi.com/journal/geosciencesCorrelation of the mineralogical alterations with changes in the measured engineering parameters has seldom been conducted with previous research focusing on one of these aspects; more usually the mineralogical alterations. The EBS for the UK’s current concept for disposal of intermediate (ILW) and high level wastes (HLW) is comprised of the waste matrix (vitrified, processed nuclear waste), the waste container (presently considered to be S275 carbon steel), the intermediate compact clay barrier (MX-80, Na-bentonite) and the outer host rock. The purpose of the engineered barrier is to isolate the radioactive waste, prevent canister displacement or breach due to externally imposed stresses and retard long-life radionuclide transport in the long-term. These functions are achieved by ensuring a low permeability, ductile and self-healing environment, provided by the high swelling and unique physicochemical nature of the MX-80
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