Abstract
The Callovo-Oxfordian claystone is a material with notoriously complex hydro-mechanical behaviour. Combined neutron and x-ray tomography modalities are used for the first time to characterise the dynamics of water absorption in this material by comparing material deformation as well as water arrival. Exploiting recent work on multimodal registration, neutron and x-ray datasets are registered pairwise into a common coordinate system, meaning that a vector-valued field (\textit{i.e.} neutron and x-ray reconstructed values) is available for each timestep, essentially making this a 5D dataset. The ability to cross-plot each field into a \textit{joint histogram} (an inherent input into the registration) allows an improved identification of mineral phases in this complex material. Material deformation obtained from the application of Digital Volume Correlation on the x-ray timeseries data is \textit{locally} compared to changes in water content available from the neutrons, opening the way towards a quantitative description of the hydro-mechanics of this process.
Highlights
The beginning of the study of clay-rich formations as a potential geological barrier for the storage of nuclear waste dates back to a couple of decades (Bonin, 1998; Andra, 2005; Delay et al, 2010)
The mechanical response of the COx claystone is strongly related to its water content (Armand et al, 2017), the variation of which has been related to the modification of its microstructure (Montes et al, 2004; Robinet et al, 2012)
The swelling properties of the Callovo Oxfordian are linked to the presence of smectite in the clay matrix, which is very sensitive to water content changes and triggers swelling at high degrees of relative humidity (Homand et al, 2006)
Summary
The beginning of the study of clay-rich formations as a potential geological barrier for the storage of nuclear waste dates back to a couple of decades (Bonin, 1998; Andra, 2005; Delay et al, 2010). In France, the Callovo Oxfordian claystone (COx) is being studied by Andra (The French radioactive Waste Management Agency) through an impressive volume of in-field and lab-scale work. This claystone’s properties (low permeability, self-sealing properties) make it a favourable host rock, the complex couplings that govern its hydromechanical behaviour are still under investigation. Stavropoulou et al (2019) combined x-ray tomography and neutron radiography to study wate imbibition in COx (albeit on twin samples) linking the cracking and propagation to the water penetration. This paper presents a significant technical evolution of Stavropoulou et al (2019) with the analysis of a wetting test on Callovo Oxfordian claystone using a simultaneous x-ray and neutron tomography on the same sample
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