Stable isotopes applied to the study of the concrete/bentonite interaction in the FEBEX in situ test

Abstract

Stable isotope analysis was carried out on a set of samples from the concrete-bentonite interface from the FEBEX real-scale in-situ experiment. The concrete and bentonite that simulated the engineering barriers system of a deep geological repository have interacted for 13 years in saturated conditions up to dismantling. The monitoring of the experiment during the years of operation has made possible to know not only the baseline status but also the evolution of the system and the mass transfer processes. Thus, the FEBEX experiment has been a unique opportunity to study concrete degradation within a broader multi-proxy approach under real-scale repository conditions. Analysis of carbon and oxygen isotopes of carbonates in groundwater, concrete and clay have provided evidence of how dissolution and mass transfer processes occur in the Engineered Barrier System (EBS). Spatial distribution of δ13C values points to the existence of a dominant process of diffusion of carbon species from bentonite towards the concrete/bentonite interface. Variations in δ18O values suggest a non-homogeneous geochemical and hydraulic behavior of the EBS in the vicinity of both, the granite-concrete interface and the concrete-bentonite interface. These differences might be related to different degrees of exposure of each sample to bentonite porewater, groundwater and concrete/bentonite leachates. This seem to be in agreement with the existence of different chemical environments depending on the location of the samples in the experiment.

Keywords: FEBEX in situ; Grimsel underground laboratory; Concrete-bentonite interaction; Carbonation; Stable isotopes; Deep geological repository