Abstract

The contribution concerns modelling of hydro-mechanical processes in bentonite based sealing plugs installed in a rock environment. The motivation for this investigation is the understanding of the behaviour of bentonite-based barriers for a deep geological repository for high-level radioactive waste. We will define a generic model problem of a bentonite based plug in a vertical borehole that may be considered in different host rock environments and used for investigation of special features connected with the modelling of swelling induced expansion of the plug, which initially does not occupy the whole borehole diameter. This plug expansion influences the hydraulic conductivity as well as the water retention relations. It also influences the mechanical behaviour of the bentonite which in our case is modelled by (nonlinear) elasticity.

1 Introduction

The geological disposal of the spent nuclear fuel assumes construction of geological repositories with a multi-barrier system to isolate high-level radioactive wastes from the biosphere. The multi-barrier system consists typically of a natural barrier - the repository host rock and the engineered barrier system (EBS) - waste canisters, buffer materials, backfill, and seals. Both the natural and engineered barriers are important for the long-term isolation of radionuclides from the biosphere.

In this paper, we consider bentonite-based sealing and particularly behaviour of plugs for sealing tunnels and boreholes. In this respect, experiments in Underground Research Laboratories (URL) as TSX at the AECL, Canada; FEBEX at Grimsel, Switzerland; EB at the Mont Terri, Switzerland and SEALEX at Tournemire URL, France were performed and analysed. The authors are involved in Task 1 of the international Decovalex 2015 project, in which they analyse the behaviour of horizontally deposited cylindrical bentonite based plugs within the SEALEX experiment, see (Barnichon & Deleruyelle 2009) and (Decovalex web site).

In this paper, we restrict the study to a generic (hypothetical) model of a cylindrical plug installed in vertical circular tunnel or borehole with an initial gap between the plug side and the tunnel/borehole surface. This installation enables us to use an axisymmetric model (on the opposite to e.g. the SEALEX). The vertical installation is also used in some experiments, e.g. (Štáska 2014). The plug is assumed to be made from a bentonite/sand mixture (BSM) which is hypothetically close to MX80/quartz sand mixture 30/70. In the model experiment, the plug is saturated by water flooding from the initial technological gap of the width 0.02 m. The saturation results in swelling of the BSM plug and filling the gap. It requires hydro-mechanical modelling of processes of water saturation and swelling and mechanical response, which mutually influence each other. Using this generic model, we mainly demonstrate the use of water retention curves, which vary with the dry density of the bentonite/sand mixture. Such variations, providing a water retention surface instead of a single curve, were also studied in a different content e.g. in (Song & Borja 2014) and the references therein. We also show a specific approach for modelling the closing of the technological gap. Our simulations were realized using the COMSOL software, (COMSOL 2013a, b, c).

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