Engineered barrier system: EBS

The near field of the repository includes the engineered barrier system (EBS, i.e., canister and buffer) and the waste form. Also included in the near field is the interface between the buffer and the host rock, denoted as excavation disturbed zone (EDZ). In terms of waste/water interactions, the geochemical evolution of the near field is essential as it controls the composition of the fluids that will eventually contact the waste. 

The FEBEX is the full-scale in-situ Engineered Barrier System (EBS) Experiment performed in Grimsel Test Site (GTS) in Switzerland (enresa (2000)). The simulation of the coupled thermal, hydraulic and mechanical (THM) behaviour of FEBEX is the task of the DECOVALEX (DEvelopment of COupled models and their VALidation against Experiments) III. 

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... 

AR226 Engineered barrier systems (EBS) in the context of the entire safety case. Radioactive Waste Management, Workshop proceedings, Oxford, 25-7 September 2002. 

The engineered barrier system (EBS) for the Swiss concept is characterized by the use of large quantities of rather simple, well-understood materials. Design optimization is certainly possible (e.g. McKinley Toyota 1998) but has not yet been extensively studied, although the consequences of reducing the diameter of the emplacement tunnel has been assessed for the crystalline host rock option. Examples of the inventories of materials for vitrified HLW (for two different tunnel diameters) and spent fuel are presented in Table 2. The inventories for the sedimentary host rock option are similar to those for the 2.4 m diameter tunnel crystalline case with the addition of a tunnel liner. 

Fig. 1. Potential colloid generation processes in a nuclear waste repository near field (for explanation, see text) (HLW, high-level waste EBS, engineered barrier system).

The experimental setup consists of a concrete container with gas vent surrounded by an engineered barrier system made of a 20/80 % bentonite/sand mixture, placed in layers, a granular backfill of the upper cavern and a concrete plug. In total, 12 different materials are considered in the numerical model (cf. Fig. 1). As the surrounding rock matrix is very impervious, only the relatively high-permeable shear zone is considered for flow outside of the EBS. Most important material parameters are given below. These parameters are obtained from independent laboratory test for the sand/bentonite (materials 6 and 8), from the literature for the other materials there is no back-estimation from the calculation results or calibration involved. 

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... 

An important role of the EBS is the control of chemical conditions to enhance barrier preservation and radionuclide immobilization, and maximize its capacity to withstand external perturbations to these conditions. In the geosphere, chemical characteristics cannot be engineered and must be assessed via site characterization efforts. In this regard, it is important to distinguish between simply characterizing ambient chemical conditions in a particular water-rock system and defining the control or buffer of... 

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