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

Abstract

Pellet/powder bentonite mixture is one of the candidate materials for sealing plugs in deep geological high-level radioactive waste disposal. This note presents an investigation on the structure changes of this mixture occurring during the saturation process by means of X-ray computed micro-tomography. The test was performed in an infiltration column (60 mm in inner diameter and 120 mm in height). Water was supplied to the two ends of the column and the changes of the sample morphology were observed during a period of 100 days of hydration. Digital Volume Correlation (DVC) technique was used to determine the vertical displacement field of the bentonite powder. A pressure transducer was used to measure the axial swelling pressure during the hydration. The results show that the initial distribution of powder in the inter-pellet pores was not homogeneous; the powder filled almost completely the pores in the zones close to the two ends while air-filled inter-pellet voids were observed in the middle of the column specimen. When water started to infiltrate inside the specimen from both ends, the pellets and the powder grains started to swell (because of the swelling properties of smectite, the principal mineral of bentonite) and filled the voids. That induced at the same time increase of swelling pressure and downward movement of powder grains. The results allowed a better understanding on the hydro-mechanical couplings, at the pellet scale, in the pellet/powder bentonite mixture upon wetting.

Overview and conceptual constitutive framework for pellet-based buffer materials

“…During hydration, the initially higher density of pellets decreases due to swelling and the initially lower density of powder increases due to soil compression and water content increase. [6] and [8] both noted that a pellet-powder mixture showed a relatively homogeneous structure even prior to full saturation, after several months of hydration. Figure 4 depicts the evolution of the transverse section of the sample during hydration obtained by X-ray computed microtomography.…”

“…This results in a decreased inter-pellet void space and an increased interaggregate void space. While examining a pellet-powder bentonite mixture, [6] showed a downward movement of the powder, which was induced by gravity, towards large air-filled inter-pellet voids. It can be concluded that the larger peak of a PSD measured on a single pellet is influenced by the phenomena of cracking and crushing.…”

“…Pellet samples are heterogenous at the as-compacted state. This heterogeneity is reflected both in the spatial distribution of pellets within the specimen and in each pellet, which contains inclusions, usually made of quartz or pyrite ( [6]), that may be of a comparable size with respect to the dimensions of the pellet itself. Figure 3 shows the appearance of a pellet-powder mixture obtained through X-ray micro-tomography ( [6]).…”

“…This heterogeneity is reflected both in the spatial distribution of pellets within the specimen and in each pellet, which contains inclusions, usually made of quartz or pyrite ( [6]), that may be of a comparable size with respect to the dimensions of the pellet itself. Figure 3 shows the appearance of a pellet-powder mixture obtained through X-ray micro-tomography ( [6]). When employed as backfill material in the boreholes of deep geological repositories for the long-term storage of nuclear waste, pellets allow for a simpler installation, but care must be taken to avoid segregation ( [1]).…”

“…Evolution upon wetting of a pellet-powder bentonite sample (after[6]). [5] investigated the change of the micro-structure of an MX-80 pellet compacted to a dry density of ȡ d = 2.06 Mg/m 3 upon hydration under free swelling conditions.…”