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Nagra, CH


Obayashi, JP

HotBENT Aims & Objectives

HotBENT will provide information and data for repository optimisation with respect to design, space, emplacement strategies and costs. The experiment aims to evaluate current accepted safety functions by investigating the effects of high temperatures on bentonite-based barriers and their safety functions.
The performance of bentonite barriers in the < 100°C temperature range is underpinned by a broad knowledge base built on laboratory and large-scale in-situ experiments. Bentonite parameter characterization above 100°C is sparser (especially for pelletized materials), although up to about 150°C no significant changes in safety-relevant properties are indicated. Information on higher temperatures on buffers is desirable for repository optimization with respect to design, space and costs (e.g., footprint, layout) and to enable more options with respect to the required interim storage time periods. HotBENT corresponds to the initial highest thermal-output period of geological disposal, the data can be used to evaluate strategies for the performance confirmation period. 
The results from HotBENT are not associated with a particular disposal concept.
Specific aims:
  • Increase data base on buffer/host-rock performance under higher T-conditions (optional up to 200°C at the heater surface), at realistic scales and in-situ conditions
  • Investigate the chemical behaviour that can be described with the models and data from existing, albeit limited and contested, laboratory scale investigations at higher temperatures
  • Scrutinize current conceptual and numerical models whether they have sufficient predictive power (i.e., including uncertainty range) to understand the experiment and describing the prevailing processes
  • Upscaling/changing of boundary conditions of bentonite buffers of laboratory and modelling knowledge to large, 1:1 scale with its inherited and commonly observed gradients
Secondary objectives:
Compare different materials, concepts/designs, boundary conditions (e.g., different types of bentonite and canister materials, water chemistries, temperatures, …)
  • Evaluation of microbial activities / corrosion / gases (e.g. buffer/canister)
  • Consider the interaction of bentonite buffer with cement-based liners and/or plugs
  • Integrating of modelling (e.g. THMC) and lab activities (e.g. mock-up experiments), also via the EU funded EJP HITEC program

HotBENT Experiment - location

HotBENT has been emplaced in the well characterized former FEBEX gallery comprising a granitic hostrock.
HotBENT Location 

The HotBENT Experiment set-up

The HotBENT Experiment consists of 4 heaters in two sectors with an intermediate and a final sandwich plug as is shown below. Sector 1 consists of two heaters set either at 200 or 175 °C, while in Sector 2 both heaters are set to 175 °C. The heaters in Sector 1 are fully encased in Wyoming-type bentonite (National Standard), with the heaters resting on a compacted bentonite block pedestal surrounded by granular bentonite material (GBM). In Sector 2, heater 3 is also encased in Wyoming-type bentonite, while heater 4 is encased in Czech bentonite of type BCV (Bentonite Černý Vrch). The construction of the two sectors will allow a partial dismantling of Sector 2 (planned after 5 years of heating, ~2026) without perturbation of Sector 1. Sector 2 is due for dismantling after a heating and hydration phase after about 20 years (~2041), as modelling indicated that these timescales are needed full saturated conditions under the ambient conditions at GTS.
The current HotBENT set-up (in discussion – May 2019)
HotBENT Bentonite variants
The evolution of the buffer (heating and hydration) will be monitored with ~1500 sensors (temperature, relative humidity, displacement, etc) arranged in 36 instrumented cross-sections along the experiment (red lines/numbers below). Additionally, longitudinal line measurements (fibre optics, geoelectric) were installed.
HotBENT Tunnel cross section sectors
The emplacement of the GBM was performed by an auger-machine achieving densities of around 1.40 – 1.42g/cm3 dry density.
 instrumentation 004
HotBENT Experiment -  Timeplan
The HotBENT experiment was constructed between September 2019 and August 2020. The evolution of the buffer of the HotBENT Experiment is planned to be investigated by sampling and analysis at a partially dismantled after 5 years (Sector 2) and a final dismantling after 20 years (Sector 2).

HotBENT timeplan

For more information about this project, or to get in touch, please use our contact page.