Dr. Andreas Jenni

Research Interest: Interactions in the near-field of waste repositories

Repository concepts comprise fundamentally different materials, e.g.: host rock, concrete, clay-based backfill, metallic waste containment, and the waste itself. After saturation of the nearfield with surrounding groundwater, the different materials lead to gradients in the porewaters, followed by solute transport, and subsequent phase reactions. Neoformations or dissolution of minerals might compromise the barrier functionality.

Experiments and analytics

Research in this field requires an interdisciplinary approach containing chemical, mineralogical, physical, material science, and engineering aspects. Field experiments in underground rock laboratories involve long-term compatible designs and a profound knowledge of the geological setting. Confining pressures are in the range of the clay swelling pressures (Mega-Pascals range) and have to be taken into account in lab experiments. Standard water and clay-specific geochemical analytics, as well as spatially resolved chemical-mineralogical characterisation methods are needed in the setting-up, running, and post-mortem phase of such experiments. We develop novel analytical approaches to characterise reaction fronts in the µm scale in clay and cement, where solid-solutions, amorphous phases, and cation exchangers are frequent.

Conceptual understanding and numerical modelling

The negatively charged surface of the clay layers fractionates the aqueous species mainly according to their charge. Anions are depleted, cations strongly enriched in the porewater close to the clay sheets. Numerical modelling of solute transport in such media has to account for this effect combined with the porosity distribution. Some of the constraints needed for modelling are not yet known and certain mechanisms still under debate. We are trying to close these gaps with specific experiments. We implement derived mechanisms into reactive transport codes in order to understand and predict experimental data.

Key topics and current projects

1. Bentonite under different geochemical conditions

If bentonite is exposed to host-rock porewater, glacial melt water, cementitious porewater, to heat, or corroding canister surfaces, phase reactions occur, swelling pressure might change, and the porosity distribution is altered. We conduct core infiltration experiments, participate in large-scale rock lab experiments, and implement dual porosity transport or chemo-mechanical coupling into reactive transport codes.

• Analysis of iron-bentonite interfaces (ABM experiments)
• Experimental characterisation and quantification of cement-bentonite interaction using core infiltration techniques coupled with x-ray tomography
• Modelling chemical-mechanical coupling in bentonite
• Benchmarking reactive transport codes: subsurface environmental simulation benchmarking workshop series (SSBench)
• Participation in the international multi-organisation Engineered Barrier Systems Task Force (EBS TF)

2. Opalinus Clay – cement interaction

Deep geological waste repositories require concrete fortifications, and cementitious waste matrices are planned for low/intermediate level waste. Possibly malicious scenarios include clay mineral alterations, pore clogging at the interface, or concrete deterioration during the waste emplacement phase. Experiments and numerical modelling help to understand the complex interaction processes.

• Cement – clay interaction (Mont Terri CI experiment)
• Shotcrete – Opalinus Clay interface (Mont Terri FE experiment)

3. Analytical and code developments

• Scanning electron microscopy combined with energy dispersive spectroscopy (SEM EDX) is pushed to its limits with respect to sensitivity and sample size: novel approaches allow for detection of weak chemical gradients over centimetres. Sample preparation of soft clay or cement materials involves specific drying and polishing methods (including broad ion beam milling).
• Synchrotron micro X-ray diffraction (µXRD) allows for mineralogical characterisation in the µm range. Preparation of micro-thin sections on very thin supports is developed.
• Further spatially resolved methods (µRaman, µFTIR) are applied to the same samples measured by the methods above.
• For specific bulk methods, interaction layers have to be scratched off and the limited amount of sample is measured (Mössbauer, TGA/DTA, aqueous extracts, cation occupancy, etc.).
• Dual porosity transport in clay is implemented into CrunchFlowMC (Carl Steefel, BNL) and FLOTRAN.
• Implementation of chemical-mechanical coupling into reactive transport codes aims for the representation of observed alterations of swelling pressure and porosity caused by the changing porewater composition.

Research group Rock-Water Interaction