Nagra has been researching the safe disposal of radioactive waste in deep geological repositories for decades. At the Grimsel Test Site, a new long-term, large-scale experiment “HotBENT” (High Temperature Effects on Bentonite Buffers) has been initiated to investigate effects of high temperatures on bentonite-based barriers. The resulting knowledge will help to optimise deep geological repositories.

The focus of the HotBENT large-scale experiment is on bentonite, which is one of several safety barriers of a deep geological repository. Safety barriers enclose the waste in a repository, thus isolating it from the human habitat. Bentonite consists of clay minerals and is used to fill the voids between the disposal canister and the tunnel wall. Many countries will use bentonite in their repositories, which is why it has been well researched. Bentonite has a very low hydraulic permeability and swells upon contact with moisture. This makes it suitable for sealing repository tunnels because it keeps flowing water away from the radioactive waste and helps to retain it in the repository. The name of the experiment already implies that bentonite will be heated. High-level waste generates heat that is released into the emplacement drifts where the waste is emplaced. In the experiment, the heat is simulated with the aid of heaters.

Schematic representation of the test tunnel with its four heaters. Bentonite is used to fill the voids between the disposal canister (or in this case, the heater) and the tunnel wall.

Excessive heat does not agree with bentonite

In earlier experiments, the bentonite was heated to 80 to 90 degrees Celsius. In HotBENT, the temperatures range from 175 to 200 degrees Celsius, which is considerably higher. When bentonite becomes too warm, there is a risk that it can no longer provide optimum retention of components of the radioactive waste. Nagra aims to use HotBENT to determine what temperatures bentonite can safely withstand and what happens when those temperatures are exceeded. If bentonite can withstand higher temperatures than previously assumed, it will be possible to optimise the repository layout without compromising its safety. The disposal canisters, for example, could be moved closer together, which would reduce the repository’s spatial requirements and costs. Aside from Nagra, eight other international partner organisations are participating in this experiment.

The far end of the test tunnel has been equipped with sensors and is ready.

Initial data are available

The experiment is being conducted in an existing tunnel of the Grimsel Test Site. Since spring 2020, the access to this tunnel has been extended and numerous preparations have been undertaken. In October, the first heater was emplaced in the tunnel and the monitoring equipment installed. In November, the auger machine was used to backfill and seal the tunnel section surrounding the first heater with a granular bentonite material. In December, the second heater was also emplaced and backfilled with bentonite. In addition, construction of the intermediate plug began with which the tunnel will be additionally sealed and closed. This part of the experiment will run for twenty years; the measurement sensors in this section of the tunnel are already gathering data.

By early summer 2021, two additional heaters will be installed, and the tunnel will then be permanently backfilled and sealed. From then on, all the sensors installed at different locations in the tunnel will continuously deliver data. Measurements include temperature, pressure, stress, displacement, moisture saturation and gas composition.

HotBENT will complement and expand existing knowledge on bentonite and repository layout. Underground rock laboratory experiments such as this one contribute to the safety of a future deep geological repository for radioactive waste. Radioactive waste disposal is a project of the century for Switzerland.

The first heater is moved into the test tunnel…

…where it is placed on compacted bentonite blocks at the far end.

This auger machine is used to backfill the tunnel with a granular bentonite material. It enters the tunnel on rails.

The end of the test tunnel has been backfilled with a granular bentonite material. Once all the sensors had been installed and inspected, the first heater could also be covered with the bentonite material. *

Photos: © Comet Photoshopping, Dieter Enz, and (*) Florian Kober, Nagra, published with the kind permission of Flotron AG that inspected the sensors shown here.