Clays engineered barriers

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. 

Adapted from Manassero et al, 1997 Fig. 5. Clay engineered barrier specifications for some countries. 

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). 

Villar, M.V., and Lloret, A., Temperature influence on the hydromechanical behavior of a compacted bentonite. Proc. Int. Meeting on Clays in Natural and Engineered Barriers for Radioactive Waste Confinement. Reims 2002. 

The work presented here is being performed in the framework of FEBEX (Full-scale Engineered Barriers Experiment in Crystalline Host Rock), which is a project for the study of the near field for a HLW repository in crystalline rock according to the Spanish concept the waste canisters are surrounded by a clay barrier constructed from... 

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... 

Compacted swelling clays are often envisaged as the main component of engineered barriers for radioactive waste disposal. These barriers are subjected to thermal loading due to the heat emitted by the waste and to hydration from water coming from the adjacent rock. As a consequence of these thermo-hydraulic phenomena, mechanical and chemical changes arise that, in turn, may affect all other aspects of behaviour. A correct understanding and prediction of these barriers would require, therefore, the performance of fully coupled thermohydro-mechanical and chemical (THMC) numerical analyses. 

AfTEMIN (2001). Engineered Barrier Emplacement. Experiment in Opalinus Clay EB" Experiment. Internal Report. AfTEMIN, Madrid... 

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... 

The initiated radioactive inventory for spent reactor fuel consists of actinides, fission products and activation products. As noted previously, (Gi. 21) the shorter lived fission products, such as Sr and Cs, and transuranic elements, such as Pu, Pu, are the main contributors to the radioactivity. However, performance assessments strongly indicate that the waste form matrix and the near field engineered barriers (e.g. clay backfill, etc.), can successfully retain and prevent any migration to the far field viromnent for one thousand years and probably much longer (> lO years). After the first thousand years the long lived nuclides such as Cs, Sn, Tc and Se among the fission products and the actinides Np, Pu, Pu, and Am become the major concern. 

Engineered barriers generally include a waste form (spent nuclear fuel or glass containing radioactive wastes) encapsulated within a metallic container (e.g., steel and copper), which may be surrounded by a low-permeability, clay-rich buffer and backfiU. Natural barriers include the repository host rock and a volume of rock between the repository and biosphere. The engineered barriers and immediately adjacent host rock are referred to as the near field [1]. 

K. Yokozeki, K. Watanabe, N. Sakata and N. Otsuki, Modeling of leaching from cementitious materials used in underground environment, clay in natural and engineered barriers for radioactive waste confinement, Appl. Clay Sci., 26 (2004) 293. 

The engineered barriers of a repository (waste form, container, clay buffer and backfill material and repository stmcmre) which will degrade with time due to various physico-chemical processes, strongly affect the geochemistry of fluids and rocks of the near field environment (redox properties, sorption properties, solubility limits, equilibrium chemistry, etc.) and determine the speciation and the subsequent migration behaviour of the radionuclides into the far field. Extensive... 

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