Mont Terri rock laboratory, 20 years of research: introduction, site characteristics and overview of experiments

Abstract

Geologic repositories for radioactive waste are designed as multi-barrier disposal systems that perform a number of functions including the long-term isolation and containment of waste from the human environment, and the attenuation of radionuclides released to the subsurface. The rock laboratory at Mont Terri (canton Jura, Switzerland) in the Opalinus Clay plays an important role in the development of such repositories. The experimental results gained in the last 20 years are used to study the possible evolution of a repository and investigate processes closely related to the safety functions of a repository hosted in a clay rock. At the same time, these experiments have increased our general knowledge of the complex behaviour of argillaceous formations in response to coupled hydrological, mechanical, thermal, chemical, and biological processes. After presenting the geological setting in and around the Mont Terri rock laboratory and an overview of the mineralogy and key properties of the Opalinus Clay, we give a brief overview of the key experiments that are described in more detail in the following research papers to this Special Issue of the Swiss Journal of Geosciences. These experiments aim to characterise the Opalinus Clay and estimate safetyrelevant parameters, test procedures, and technologies for repository construction and waste emplacement. Other aspects covered are: bentonite buffer emplacement, highpH concrete-clay interaction experiments, anaerobic steel corrosion with hydrogen formation, depletion of hydrogen by microbial activity, and finally, release of radionuclides Geosci (2017) 110:3-22 DOI 10.1007 into the bentonite buffer and the Opalinus Clay barrier. In the case of a spent fuel/high-level waste repository, the time considered in performance assessment for repository evolution is generally 1 million years, starting with a transient phase over the first 10,000 years and followed by an equilibrium phase. Experiments dealing with initial conditions, construction, and waste emplacement do not require the extrapolation of their results over such long timescales. However, experiments like radionuclide transport in the clay barrier have to rely on understanding longterm mechanistic processes together with estimating safety-relevant parameters. The research at Mont Terri carried out in the last 20 years provides valuable information on repository evolution and strong arguments for a sound safety case for a repository in argillaceous formations.

High-resolution mini-seismic methods applied in the Mont Terri rock laboratory (Switzerland)

“…1 The Mont Terri rock laboratory, located in the Canton of Jura, northwestern Switzerland, lies in the Opalinus Clay. The geological setting of the Opalinus Clay and the mini-seismic experiment sites in the rock laboratory are shown in Bossart et al (2017a) in situ rock-mechanical results, results from laboratory testing, and results stemming from thermo-and hydro-mechanical modelling. Results from MSM closely sample the actual local in situ conditions.…”

“…d Seismic COF section for one receiver distance (R2, 20 cm) with marked P-and Sv-onsets ownwards; c carbonate-rich sandy facies from MI niche, borehole BDM-B2 at a depth of 9.25 m, oriented normal to the bedding, with an azimuth of 150°, dip ?44°upwards; d sandy facies from the TT niche, borehole BLT-A8 at a depth of 5.85 m, oriented parallel to the bedding, azimuth 240°, dip 0°). Location of the niches are shown in Figure 6 of Bossart et al (2017a) The high potential of IVM for resolving substantial spatial variations is demonstrated in three examples (see Figs. 8,9,10,11).…”

“…Figure 1 shows the location of the laboratory within the Opalinus Clay Formation and its complex structural context. The lithological and structural overview of the Opalinus Clay at Mont Terri, its mineralogy, and key parameters is given in Bossart et al (2017a).…”

“…In addition, combining results from MSM with those derived under laboratory conditions (mostly static tests) on cores can improve our understanding and interpretation of rock-mechanical modelling. Bossart et al (2017a), this issue) present an overview of the experiments and sites where the MSM have been applied.…”

5-year chemico-physical evolution of concrete–claystone interfaces, Mont Terri rock laboratory (Switzerland)

“…The experiment is located in the HE-D niche adjacent to Gallery-98 that was excavated in 1998 (see overview map in Bossart et al 2017). This niche is located in the shaly facies of Opalinus Clay (174 Ma marine shale), with a clay content of *60 wt%.…”