Abstract
AbstractConcepts for geological disposal of high-level radioactive waste usually include bentonite buffer materials. Numerous studies have been performed with most usingWyoming bentonite. Gaomiaozi (GMZ) bentonite has been selected as a potential buffer/backfill material for the deep geological repository of high-level radioactive waste in China. In this context, the highly alkaline environment induced by cementitious materials in the repository is likely to alter montmorillonite, the main clay mineral in GMZ bentonite. This alteration may result in deterioration of the physical and/or chemical properties of the buffer material. To acquire quantitative data which would allow us to assess the dissolution of montmorillonite and changes in the diffusivity of hydroxide ions as well as their effects on the swelling pressure and permeability of the compacted GMZ bentonite, an experimental study was conducted under highly alkaline (NaOH solutions with various pH values were used), simulated groundwater conditions. The GMZ bentonite also contains cristobalite which may also have been dissolved. The microstructure of the compacted bentonite samples after the experiments was determined by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDX) was carried out to identify mineralogical changes. At pH >13, the permeability of specimens increased significantly; the swelling potential decreased with increasing pH. Furthermore, the pore volume and pore size of GMZ bentonite changed when exposed to alkaline solution, resulting in an increase in porosity and permeability. The main alteration mechanisms of compacted GMZ bentonite undergoing infiltration by highly alkaline solution are likely to be dissolution and modifications in terms of the microstructure and mineralogy.
Highlights
Numerous experimental and numerical investigations have been conducted to evaluate the behaviour of clay barriers in Deep Geological Repositories (DGR) for the disposal of high-level radioactive waste (HLRW)
Significant alteration of montmorillonite was identified by transmission electron microscopy (TEM)
Several studies reported the dissolution of montmorillonite and an increase in the total porosity in a synthetic alkaline groundwater environment (Lehikoinen et al, 1996; Nakayama et al, 2004; Sánchez et al, 2006; Karnland et al, 2007; Yamaguchi et al, 2007; Cuisinier et al, 2008; Deneele et al, 2010). These laboratory studies did not resolve the question of whether such reactions would occur in a real HLRW storage system affecting the buffering and isolation performance of bentonite
Summary
ABSTR AC T : Concepts for geological disposal of high-level radioactive waste usually include bentonite buffer materials. Gaomiaozi (GMZ) bentonite has been selected as a potential buffer/backfill material for the deep geological repository of high-level radioactive waste in China In this context, the highly alkaline environment induced by cementitious materials in the repository is likely to alter montmorillonite, the main clay mineral in GMZ bentonite. Several studies reported the dissolution of montmorillonite and an increase in the total porosity in a synthetic alkaline groundwater environment (Lehikoinen et al, 1996; Nakayama et al, 2004; Sánchez et al, 2006; Karnland et al, 2007; Yamaguchi et al, 2007; Cuisinier et al, 2008; Deneele et al, 2010) These laboratory studies did not resolve the question of whether such reactions would occur in a real HLRW storage system affecting the buffering and isolation performance of bentonite. GMZ bentonite has a moderately high CEC and is free of soluble Ca minerals such as carbonates or gypsum
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