Observed heterogeneities after hydration of MX-80 bentonite under pellet/powder form

Highlights

• A number of variable size swelling tests on granular MX80 bentonite are presented.

• Stress and dry density heterogeneities of the final state are demonstrated.

• First results of a metric-scale resaturation experiment are presented.

Abstract

Bentonite is considered for engineered barriers in radioactive waste geological repositories due to its low permeability, swelling pressure and radionuclide retention properties. The French concept favors the use of bentonite under granular form - pellets and crushed pellets mix - for an easier building of plugs and seals relatively to bricks. However, this introduces a significant degree of macroscopic initial heterogeneity (at several scales) that is expected to disappear via mass redistribution during resaturation in order to obtain homogeneous hydraulic and mechanical performance. This also may have practical effects on the reliability of results from tests with small volumes of bentonite compatible with the time scales manageable at the laboratory. In this study, isochoric swelling pressure tests at various densities and sample sizes were carried out and evidenced bias mainly due to the size of the cell compared to the size of the pellets, although the average density is equivalent, pointing towards incomplete homogenization. Large-scale swelling pressure tests, however, converged towards values previously measured for more finely grained materials. Local pressure measurements also evidenced some stress heterogeneities that do not seem to disappear at medium time-scales. These stress heterogeneities could be correlated to local density variations of the final state assessed using destructive measurements and X-ray computed tomography.

Introduction

Preventing the release of radionuclides into the biosphere is the central issue of high- and intermediate-level nuclear waste geological repositories. The large swelling capacity, low permeability and radionuclide retention properties of bentonites make them desirable materials for building engineered barriers (plugs and seals) (Delage et al., 2010; Cui, 2017). Several bentonite-based materials (pure bentonite, bentonite-sand, etc.) are thoroughly studied in research programs of a number of countries. In the French CIGEO concept, pure MX-80 bentonite is proposed to build the seals in access galleries and shafts.

In order to facilitate the placement of large volumes of bentonite underground with a target density constraint, it is proposed to use bentonite as a granular form, such as compacted pellets/powder mix, thus avoiding the relatively complex building operations associated for instance with compacted bentonite bricks. The feasibility of implementing such a full-scale seal concept has been recently assessed in the framework of the European DOPAS project with the FSS experiment (Bosgiraud et al., 2015; Conil et al., 2015) (Fig. 1). This large-scale mock-up allowed to investigate technologies required for the construction of a bentonite plug, as well as the density attained and properties of the remaining technological voids. This surface demonstrator made use of a 36 m long concrete structure mimicking an underground gallery of 7.6 m in diameter that was sealed with a bentonite core, between low-pH concrete containment plugs. No resaturation of the bentonite was attempted. The granular material used is composed of approximately 80% of 32 mm diameter pellets and 20% of crushed pellets in dry mass. Although the hydromechanical behavior of analogous granular materials has already been studied (Imbert and Villar, 2006; Hoffmann et al., 2007), the evaluation of the homogeneity of the final saturated state requires additional work. Indeed, the performance of such a structure can be dependent on the homogenization of the bentonite core at resaturation. Lower density regions could lead to higher hydraulic conductivity or reduced hydromechanical properties that should not degrade the expected performance of these structures (Nasir et al., 2017). The resulting heterogeneities in the hydraulic properties of such a structure may be due to (Fig. 1):

• Large gaps and technological voids – decimeter to meter scale – (Mokni et al., 2016) in top areas due to incomplete filling, or large scale density gradients due to possible segregation of pellets for example; issues mitigated by a careful filling process,

• Local density variations – millimeter to centimeter scale – (pellet/powder/macroporosity), that are expected to resorb through resaturation, as evidenced for example through X-ray microtomography (Van Geet et al., 2005),

• Interface effects, which may have different hydraulic properties than the bulk due to thermal or, in the present case, chemical alteration, in particular with concrete structures (Herbert et al., 2004, Herbert et al., 2008; Sanchez et al., 2006; Yamaguchi et al., 2007; Marty et al., 2010; Fernandez et al., 2017), that could be mitigated by the choice (if possible) of compatible materials.

In this paper, laboratory characterizations of the pellet-powder bentonite material designed for the DOPAS-FSS demonstrator are presented. The objective is to investigate in representative conditions (constant volume, controlled water compositions, and targeted density values) the residual small scale heterogeneities (2. in the list above) in a resaturated granular bentonite material. These heterogeneities are investigated through the effect of sample size on the swelling pressure kinetics and reached values, culminating with a currently ongoing metric scale experiment (called “REM”) implemented in the framework of the DOPAS project (Conil et al., 2015). Local total pressures and local density were also evaluated for an intermediate scale test.