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Abstract: The solution selected by some countries to isolate radioactive wastes from the biosphere for up to one million years in deep geological repositories includes a multi-barrier disposal design, with steel canister, bentonite and cement materials. The geochemical contrast between such materials and the host rock formation creates perturbations potentially altering the confinement properties of the formation. In this context, the French Institute for the Radiological protection and Nuclear Safety (IRSN) have developed an in situ experimental programme based on the study of cement/argillaceous formation interfaces in their Underground Research Laboratory at Tournemire (Aveyron, France). An in situ engineered analogue of a cement/clay-rock interface which has undergone 15years of interaction has been characterised. Such important interaction time for an in situ engineered analogue provides a bridge between laboratory-derived data and the long time scale of safety assessment modelling. As the mineralogical and petrological investigations have already been published, this work presents for the first time a quantitative characterisation of the spatial distribution of the porosity in the cement and the clay-rock in terms of time scale and design. Interfaces have been characterised using an autoradiography technique in addition to petrophysical measurements. This technique enables visualisation and quantification of the spatial distribution of the porosity using 2D mapping of decimetric-scale specimens. Thus autoradiographs allow highlighting the relationship between the field heterogeneities and the pore space evolution in each material in contact. Moreover, the porosity measurements show a clogging of the porosity in the clay-rock while the porosity increases in the cement. The extension of the porosity evolution extends to a centimetre on both sides of the interface but is heterogeneously distributed in space as a function of the fissure network and interface geometries. The connected fissure network visualised using autoradiography in the clogged area could permit solute (e.g. radionuclide) transport and may also be interpreted as an evolution of the mechanical properties of the clay-rock formation upon alkaline perturbation. This set of data, with the spatial quantification of the porosity in both cement and clay materials will be useful to constrain reactive transport modelling and thus to predict long term evolution of an engineered barrier. [Copyright &y& Elsevier]

The Callovo-Oxfordian mudstone (Meuse/Haute-Marne, France) is currently considered as the host rock barrier for a deep geological repository. The intimate relationships between the porosity and mineralogy of this host rock were investigated at the small scale (μm–mm) and large scale (m–hm). At the small scale, we have adapted the 3 H-PMMA autoradiographic method to map the porosity of the Callovo-Oxfordian mudstone. The 3 H-PMMA autoradiographic method was improved in terms of its spatial resolution. 3 H-PMMA porosity maps were then compared to homologous mineral maps (clay minerals, carbonates and tectosilicates) built from scanning electron microscopy images (using back-scattered electron imaging). Based on an inversion procedure, the specific porosity of each mineral group was estimated from the mineral and porosity maps. We found that the spatial distribution of porosity at the small scale is mainly controlled by the spatial distribution of the clay matrix (the average porosity of the clay matrix is 40–45%), whereas quartz and carbonate mineral grains have low porosities (0–4%). At the geological formation scale, the porosity and mineralogy distributions were determined by logging tool techniques (nuclear magnetic resonance and spectral gamma-ray). The coupled evolution of clay content and porosity with depth was analyzed according to the porosity/mineralogy relationship defined at the small scale. Finally, we modeled the evolution of the porosity of the Callovo-Oxfordian mudstone with depth by considering the clay content and the effect of physical compaction during burial. [ABSTRACT FROM AUTHOR]