High Temperature Effects on Bentonite Buffers (HotBENT) - Introduction

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Partners (Detailed Design Phase and partially full project):

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. That information 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 heater surface temperatures) and at realistic scales
  • 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)
  • Integrating of modelling (e.g. THMC) and lab activities (e.g. mock-up experiments), also via the EU funded EJP HITEC program

HotBENT location

HotBENT will be in the well characterized former FEBEX gallery comprising a granitic hostrock.
Current planning (January 2020) builds on a 4 module heater experiment, where heaters will be place on compacted bentonite blocks, surrounded by granular bentonite material (GBM). Two bentonites are under consideration: Wyoming type bentonite and BCV (Czech) bentonite.  Emplacement of the GBM is planned by an auger-machine achieving densities of higher than 1.45 g/cm3 dry density.

The start of construction of the HotBENT experiment has started in spring 2020.

The current HotBENT set-up (March 2020) including the currently forseen instrumentation sections

The current HotBENT set-up (in discussion – May 2019)
Current activities provisional timeplan
A number of different tasks have to be completed before turning on the heaters including excavation of the HotBENT cavern, concreting of the floor, drilling of enhanced geosphere pressurization boreholes, tunnel wall instrumentation, bentonite block production, granular bentonite material production, adjustements on the emplacement machine, development of a Data Acquisition System, emplacement of the bentonite pedestals, the heater, the instrumentations, the corrosion tests, routing of the cables, backfilling of the experiment and finally construction of the plugs. Current onsite efforts are focused on the excavation of the HotBENT cavern, which is hindered by the competent surrounding rock mass.

The GTS underground facilities are also available to interested 3rd parties for underground testing and research. The GTS offers cost-effective access to a fully developed, well characterised underground research facility with round the year logistical support - please contact Dr. Ingo Blechschmidt, Head of the Grimsel Test Site, for further details.