In-depth characterisation of a mixture composed of powder/pellets MX80 bentonite

Abstract

International audienceMixtures made up of bentonite powder and pellets are a possible candidate for making sealing plugs used in deep radioactive waste disposal due to their low permeability, high swelling capacity, favourable properties with respect to radionuclide retention and operational advantages in terms of placement in situ, which is much easier than that of pre-compacted bricks of bentonite/sand mixture. It is therefore essential to better understand their hydro-mechanical behaviour to optimize the design of the repository. In this context, the French Institute for Radiation Protection and Nuclear Safety (IRSN) has launched the SEALEX project (SEALing performance EXperiments) in which this work has been conducted. Once the initially heterogeneous unsaturated powder/pellet (80/20) MX80 bentonite mixture is put in place, these sealing materials will be subject to coupled hydro-mechanical loadings: hydration due to the infiltration of pore water from the natural barrier and mechanical confinement resulting from the engineered barriers. The present work focuses on the different scales of the material: at the macroscopic scale, it is characterised by a heterogeneous distribution of pellets and powder of bentonite; at the microscopic scale, it is studied by several techniques (MIP, μ-CT observations and SEM). From MIP results, a typical bimodal distribution was found for both pellet and powder. From μ-CT and SEM observations, a heterogeneity was revealed in the internal structure of the pellet: heterogeneous density distribution of the clay minerals and presence of several high density elements

Overview and conceptual constitutive framework for pellet-based buffer materials

“…The smallest pores are deemed intra-aggregate pores, while the larger pores are deemed inter-aggregate-pores. A qualitatively similar result was obtained by [3], who analysed a single pellet (ȡ d = 2.06 Mg/m 3 , suction 135 MPa) of MX-80 bentonite. Interestingly, the inter-aggregate porosity (with an average diameter around 4-5 ȝm) was found to represent barely 7% of the total porosity.…”

“…Pellet samples present a multi-modal porous structure ( [1], [2]), which can be observed using techniques such as Mercury Intrusion Porosimetry (MIP) and X-ray computed microtomography. A single pellet exhibits a double porosity structure ( [1], [2], [3]), which is the result of the compaction on the dry side of optimum moisture content that is employed in the production process. Figure 1(b) from [1] presents the bimodal Pore Size Distribution (PSD) curve of a single, highly compacted pellet (ȡ d = 1.95 Mg/m 3 ) with characteristic pore sizes of 13 nm and 3 ȝm.…”

“…Following compaction, pellets can contain cracks at the scale of the inter-aggregate porosity. These can be shown through X-ray computed microtomography ( [3], [5]), as depicted in Figure 2. Crushed pellets can also be partly reduced to powder ( [1]).…”

Investigation of swelling behaviors of GMZ bentonite pellet mixtures

“…Bentonite pellet mixtures have been proposed as one of the candidate filling materials of tunnels and boreholes in geological disposal of high-level radioactive waste (HLW), not only because of its low permeability, high swelling capacity and high radionuclide adsorption properties, but also its easier backfilling operation, less compaction effort needed and more economic-efficiency expected (Imber and Villar et al, 2006;Alonso et al, 2011;Molinero-Guerra et al, 2016). Once installed in the repository, bentonite pellets will be submitted to coupled hydro-mechanical loadings.…”

Analysis of the structural changes of a pellet/powder bentonite mixture upon wetting by X-ray computed microtomography

“…The swollen mixture will fill different voids and generate swelling pressure, ensuring the sealing of the storage system. Molinero Guerra et al (2017) used X-ray computed tomography (X-ray CT) to investigate the microstructure of pellet/powder bentonite and found that this mixture was characterized by an initial heterogeneous distribution of powder grains inside the inter-pellet pores. This initial state could greatly influence the saturation process and the corresponding structure changes.…”