Technical Report NTB 15-02

The Full-scale Emplacement (FE) Experiment at the Mont Terri underground research laboratory (URL) simulates aspects of the construction, waste emplacement, backfilling and early-stage thermo-hydro-mechanical (THM) evolution of a spent fuel / high-level waste (SF / HLW) emplacement tunnel in a clay-rich formation (Opalinus Clay), using heaters instead of SF / HLW disposal canisters. The entire experiment implementation and the post-emplacement THM evolution are monitored using several hundred sensors. These are distributed in the host rock, on the tunnel lining, in the buffer and tunnel plug, and on the surface of the heaters.

The objectives of the FE Experiment are:

• To investigate SF / HLW repository-induced THM coupled effects on the host rock at full scale and to validate existing coupled THM models.
• To verify the technical feasibility of constructing an emplacement tunnel using standard industrial equipment.
• To optimise the bentonite buffer material design and production, in particular to produce bentonite blocks that are capable of resisting the ambient conditions during the emplacement and operation phases.
• To investigate (horizontal) canister and buffer emplacement procedures for underground conditions.

The FE Experiment was designed to replicate the emplacement tunnel of the reference repository design at 1:1 scale.  The FE tunnel has an inside diameter of approximately 3 m and a length of 50 m, and is divided into four zones. In the main Test Section of the experiment, three heaters with dimensions similar to those of the SF / HLW disposal canisters were emplaced on top of bentonite block pedestals. The remaining space was backfilled with highly-compacted granulated bentonite material (GBM). In this zone, the rock is supported by mesh-reinforced low-pH shotcrete. At the far end of FE tunnel is the Interjacent Sealing Section (ISS), which comprises a concrete wall, a two-metre-long bentonite block wall consisting of manually installed bentonite blocks and a section filled with GBM. A concrete plug section and an access section comprise the two sections closest to the FE cavern, which provides the entrance to the FE tunnel and hosts the monitoring data acquisition systems during experiment operation.

During the production of the GBM, the material treatment, pelletisation, grinding and mixing were studied in a systematic way and optimised in order to obtain properties that fulfil all requirements. The resistance of the bentonite blocks to varying tunnel climate conditions was improved by optimisation of the production parameters.

Experience from previous experiments led to the prototype backfilling machine being designed with five screw conveyors, allowing the horizontal backfilling of disposal tunnels with GBM as densely and homogeneously as possible. After construction, this machine underwent intensive testing. The minimum bulk dry density of 1.45 g/cm3, as targeted for the bentonite backfill according to the Swiss repository concept, was exceeded without any break-downs or accidents. The optimisation, industrialisation and automation of these processes can be completed in the decades remaining until the start of repository operation.

The heaters were turned on between December 2014 and February 2015 and are planned to constantly emit 1,350 W each for the first few years of heating. Afterwards, it is planned to decrease the power according to a decay function typical for SF.

For monitoring the effects of this full-scale heating on the backfill and the host rock, sensors were installed in and around the FE tunnel. These sensors measure various parameters, including temperature, pressure, deformation, humidity / water content and gas composition. After 18 months of heater operation, temperatures on the surface of the heaters are approximately 117 - 132 °C, and approximately 45 – 70 °C at the rock surface. At the end of the continuous heating period, temperatures of approximately 130 – 150 °C at the surface of the middle heater and approximately 60 – 80 °C at the rock interface are expected.

In general, the THM response observed over the first 18 months of heating is in line with expectations based on the experience from previous heating experiments and the scoping and predictive calculations made prior to the FE Experiment.

The FE Experiment is an important step in the design of the Swiss SF / HLW repository. For example, the outcomes may impact the design of disposal tunnels and the loading of the disposal canisters.

This report covers the design, construction and first 18 months of heater operation for the FE Experiment. During this period, significant progress has been made with respect to the overall Experiment objectives:

• With respect to the objective “to investigate SF / HLW repository-induced THM coupled effects on the host rock at full scale and to validate existing coupled THM models”, the first 18 months of heater operation have provided information for improving the understanding of early post-emplacement performance of the EBS and near-field rock prior to saturation. The monitoring instrumentation is operating successfully and is providing detailed information on the THM response of the system to heating. 3D models have been developed of the evolution of the host rock and the engineered barriers. The modelling has included calibration and validation using measured data, which is ongoing.
• With respect to the objective “to verify the technical feasibility of constructing an emplace­ment tunnel using standard industrial equipment”, the tunnel has been constructed and tunnel support has been installed consistent with requirements on construction related to the long-term performance of the disposal system. These include requirements on the tunnel dimen­sions, the rate of support installation and the composition of materials (e.g. shotcrete), and a requirement that no water shall be used during the excavation process. Therefore, the technical feasibility of constructing an emplacement tunnel in an over-consolidated claystone using standard industrial equipment has been successfully verified at the Mont Terri URL.
• With respect to the objective “to optimise the bentonite buffer material design and production, in particular to produce bentonite blocks that are capable of resisting the ambient conditions during the storage and operation phases”, detailed investigations have been undertaken of bentonite block and bentonite pellet production and production parameters determined. The impact of bentonite water content and tunnel relative humidity has been identified. The associated parameter ranges have been determined to ensure that bentonite block pedestals perform to the standards required during the emplacement and backfilling of disposal tunnels. Production parameters for GBM have been determined, including water content, compaction process and grain size distribution.
• With respect to the objective "to investigate (horizontal) disposal canister and buffer emplacement procedures for underground conditions", work has focused on the horizontal emplacement of the GBM. A prototype backfilling machine has been designed, tested and utilised to install GBM at the required density. Continued monitoring of the THM performance of the FE Experiment will provide further insights into the influence of the heterogeneity of the buffer on its performance.