Engineered barrier systems

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. 

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 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). 

THM SIMULATION OF THE FULL-SCALE IN-SITU ENGINEERED BARRIER SYSTEM EXPERIMENT IN GRIMSELTEST SITE IN SWITZERLAND... 

Abstract Coupled THM simulation of the FEBEX, which is the full-scale in-situ Engineered Barrier System Experiment performed in Grimsel Test Site in Switzerland, is one Task in the international cooperation project DECOVALEX III. In the Task, the simulation of the thermal, hydraulic and mechanical behaviour in the buffer during heating phase is required, e.g. the evolutions and the distributions of stress, relative humidity and temperature at the specified points in bentonite buffer material. 

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. 

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. 

Figure 1. THM(C) processes and parameters associated with engineered barrier system.

The first part of the GMT experiment after the completion of the experimental set-up was devoted to the natural and artificial resaturation of the engineered barrier system. This part of the experiment has been completed by end of 2002. 

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. 

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... 

P. K. Nair, Manager, Engineered Barrier System, Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166... 

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

Liner system An engineered barrier system that is composed of combination of one or more liners and drainage layers. The drainage layers are placed immediately above the liner(s) to drain the leachate that has percolated through the waste to a collection system for removal and treatment. 

After a pubhc review by an Environmental Assessment Panel, the Panel acknowledged that from a technical perspective, the safety of the Canadian concept for nuclear fuel waste disposal was, on balance, adequately demonstrated. However, the Panel recommended that broad public support be demonstrated before proceeding to repository site selection. Work continues at the URL, and the intent of this work is to increase confidence in proposed methodologies for designing repository excavations and engineered barrier systems. 

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