ABSTRACT

This paper present the characterization of the mass transfer process in Bure argillite, in terms of moisture diffusivity as a function of relative humidity. A thin plate and hollow cylinder samples are set in a tight box in which relative humidity is imposed. The samples mass continuously recorded. A diffusivity identification method is proposed for case of axial and radial diffusion. This method is based on the measured mass evolution, when the diffusivity is assumed to be constant during each desorption and adsorption step. A significant non-linearity and a high hysteresis are observed during the desorption-adsorption process.

INTRODUCTION

The French Radioactive Waste Management Agency (ANDRA) has selected an argillaccous site in the East of France (Bure, Haute-Marne) as a potential underground nuclear waste storage host medium. An underground research laboratory at a 500 m depth is currently under construction. The excavation of the galleries is expected to create a damaged zone(EDZ) around the galleries, increasing rock permeability by several orders of magnitude. Additional damage could be induced by the desaturation-resaturation process due to the ventilation of the galleries. Such desaturation induced cracks have already been observed in the argillaccous Tournemic tunnel (France) (Ramambasoa et al.2000); and swelling of Opalinus clay around borehole collars causes breakup of shotcrete in the Mont-Terri experimental gallery (Swizerland)(Derek Martin et al. 2001). Both may influence the safety of a disposal. Experimental desorption-adsorption studies were carried out, using saline solutions (Baroghel-Bouny 1994 & Gasc-Barbier 2002) or dryair (Collard 1989, Pel et al. 1996) in the case of desorption process, and saline solutions or direct cantact with water (Draian 1986 & Rollard 2002) in the case of adsorption process. When a complete investigation is considered hydromechanical behavior of the rock, and on the other hand, the mass transfer process during desorption-adsorption steps are needed. In order to have access to the mass transfer kinetics process and the spatial extension of the unsaturated zone, the first steps in understanding such as process consists in studying the moisture transfer in argillite.

We present in this paper the result of desorption-adsorption experiments performed on Bure argillite leading to the characterization of the mass transfer process in terms of moisture diffusion coefficient as a function of air relative humidity.

BURE ARGILLITE

The tested sample were bored from cores sampled at a depth of about 500 m in the Callovo-Oxfordian layer, The mineral content is: 40% of clay materials, 25 to 30% of quartz and 20 to 30% of carbonates (Gase Barbier at al. 1999). Microscopic observations showed that the argillaccous phase is continuous whereas the quartz and carbonates phase are discontinuous and the grains are about 200 μm in size (Bauer-Plaindoux et al. 1998 & Gase-Barbier et al. 1999). The voids of about 1 μm in size and traces of cracks around the grains have been observed(Gase-Barbieretal, 1999). The initial porosity of this argillite is between 10%-14% and their pores radius range from 10 nm to 2000 nm (Gase-Barbieretal, 2002).

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