Conductivity bentonite barriers

The three-layered clay mineral montmorillonite (bentonite) is characterised by a low-hydraulic conductivity and a capacity to bind water molecules and positively charged ions (cations). As such, water-saturated compacted bentonite powder is used as a hydrological barrier in areas such as waste disposal, for example around land-fill sites where the desire is to prevent leakage of contaminants from the land-... 

The measured power decreases steadily with tintie for the first 400 days after the beginning of the automatic operation. This is consistent with the progressive drying of the inner annulus of the barrier and the associated decrease of thermal conductivity of the bentonite. The slight increase in power in the second part of the period represented is attributed to the progressive hydration of the barrier due to the incoming water. Some predictions reproduce accurately the observed behavior. 

The FEBEX in situ test has been conducted as a multi-national project coordinated by the Spanish organization ENRESA. In essence, the experiment involves the installation of two cylindrical heaters (4.54 m long and 0.97 m in diameter) centered in a tunnel (diameter 2.27 m) and surrounded by a barrier (thickness 0.64 m) made of highly compacted unsaturated blocks of bentonite (Figure 1). The tunnel is located in a mountainous area of massive saturated granite about 420 m below ground surface. 

Geological disposal of high-level radioactive waste (HLW) in Japan is based on a multibarrier system composed of engineered and natural barriers. The engineered barriers are composed of vitrified waste encapsulated in a canister, metal overpack and buffer material. Highly compacted bentonite is considered as the candidate buffer material, mainly because of its low hydraulic conductivity and high sorption capacity of radionuclides. 

The primary engineered barrier is the spent fuel itself. It is a sparingly soluble oxide (U02(s)) which dissolves extremely slowly under anoxic conditions, but can corrode to more soluble forms in the presence of dissolved oxygen (O2) (summarized in Miller et al. 1994, Section 4.2). This is also the case for the copper canister, and the steel lining of the canister, should it be breached. The copper canister can also undergo sulphidic corrosion (described below) under reducing conditions. The buffer material is intended to be compacted bentonite clay, with a mixture of bentonite and sand to be used as fill material for the tunnel and shafts. The compacted bentonite buffer is designed to maintain low hydraulic conductivity in order... 

Geotextile—bentonite composites are well known as GCLs. They are excellent water-barrier materials. However, a disadvantage of GCLs is that it has an increase in hydraulic conductivity in sea water owing to the swelling mechanism of bentonite. This section introduces how to improve the swelling behavior by modification (Table 18.1). 

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