Clay engineered barrier systems

The proposed Swiss repository for SF, HLW, and ILW is situated in the Opalinus Clay of the Zurcher Weinland in northern Switzerland, where an exploratory borehole was drilled near the village of Benken (Nagra 2002a). The Opalinus Clay formation consists of a well-consolidated clay shale, which is suitable for the construction of small, unlined tunnels and larger, lined tunnels at depths of several hundred metres. The engineered barrier system includes the waste containers and the backfill of construction, operation, and emplacement... 

Figure 13.33 Schematic diagram of the engineered barrier system (BBS) showing the high-level nuclear waste in its metal container, surrounded by a buffer or backfill (usually of compacted bentonite clay), in contact with the host rock. The BBS and rock affected thermally by the waste are sometimes termed the near field, with more distant surrounding rock termed the far field. After The status of near field modeling. Proc. Technical Workshop, copyright 1995 by OECD. Used by permission.

Many national programs plan to surround containers of their nuclear waste in a geologic repository, with a backfill of compacted bentonite clay (Fig. 13.33). A chief function of the clay backfill is to adsorb radionuclides and so retard their release from the engineered barrier system. Conca (1992) measured the apparent diffusion coefficient (D ) and apparent distribution coefficient (K [ml/g]) of some radionuclides in bentonite clay as a function of clay moisture content and compaction density. Measurements were made for clay densities from 0.2 to 2.0 g/cm, which correspond to porosities of 93 to 25%, respectively. With decreasing porosity, values declined by roughly 10 to 10 -fold. However, for the same porosity reduction, values were usually lowered by 10-fold and more, indicating less adsorption with compaction (Fig. 13.38). 

IMPACT OF IN-SITU PARAMETERS AND BOUNDARY CONDITIONS ON THE THERMAL-HYDRO-MECHANICAL BEHAVIOUR OF A CLAY ENGINEERED BARRIER SYSTEM. 

Bamel, N., Lassabat re, T., Le Potier, C., Maugis, P. Mouche, E., 2002 . Impact of a thermal radioactive waste on the thermal-hydraulic behaviour of a clay engineered barrier system. In Auriault, J.-L., et al. (ed.), Poromechanics II, Balkema. 

In Sweden, a repository design of KBS-3 system has been develop (SKB, 1999). The KBS-3 is a multibarrier system to isolate the spent nuclear fuel. The spent nuclear fuel is placed in corrosion-resistant 5-m long copper canisters. Each of the canisters is surrounded by an engineered barrier system (EBS) of bentonite clay in separate deposition holes excavated along tunnels in... 

However, until relatively recently, most workers concerned with waste management have tended to consider chemical processes primarily because they may affect the physical containment properties of engineered barrier systems. Several texts have examined these physical aspects of containment in considerable detail (e.g. Bentley 1996). Implicitly, there has been a tendency to view chemical containment as an aspect of physical containment. For example, any collapse of expandable clay minerals, such as may be caused by interactions involving polar organic molecules, will affect the physical integrity of clay barriers (e.g. Bowders Daniele 1987 Hettirachi et al. 1988). However, this view of containment is simplistic. In reality, chemical and physical processes must be considered holistically. For example, where clay is used to confine a waste, it should be considered as a physico-chemical barrier to contaminant migration (Horseman et al. 1996). 

The canister, the engineered barrier and the host rock are modelled with a ID-axisymetric geometry. The 0.24 m thick canister is not modelled. Next to it stands the 0.8 m thick engineered barrier (EB). The canister and the EB are placed in the host rock. The extension of the whole system is 50 m. The initial conditions are 72.5 MPa of suction in the EB, whereas the host clay is initially saturated at 5 MPa of water pressure upon the hydrostatic level. Numerically, 1% of residual gas content is initially considered in... 

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