Large-scale HotBENT experiment
High-level waste and spent fuel assemblies disposed of in emplacement drifts radiate heat. With the HotBENT experiment conducted at the Grimsel Test Site, the scientists want to find out how heat can be generated in an emplacement drift without compromising the function of the safety barriers.
In the experiment, heaters are used to simulate this heat. The specific focus of the investigations is on the bentonite, a clay-rich material suitable for sealing the emplacement drifts of a repository. Bentonite has properties similar to those of the Opalinus Clay: it can prevent flowing water from coming into contact with radioactive waste, bind radioactive substances and confine them. It thus constitutes one of the engineered barriers in the deep geological repository.
The name of the experiment already implies that bentonite will be heated. When bentonite becomes too warm, there is a risk that it can no longer optimally enclose the radioactive waste. The aim of the experiment is to ascertain what temperatures bentonite can safely withstand and what happens when those temperatures are exceeded. The outcome of HotBENT will complement existing knowledge on bentonite and repository layout.
TS experiment
“TS” stands for “testing tunnel support systems in the Opalinus Clay”.
A deep geological repository has to have tunnels in the Opalinus Clay. Clay rock is malleable and hence places higher demands on structural engineering during the construction of these tunnels than other more stable rocks such as granite. At the Mont Terri Rock Laboratory in Canton Jura, we are investigating the best approach to tunnel construction, particularly with regard to the associated excavation and support systems.
The TS experiment involved lining three tunnels segments with different support systems such as steel arches (see photos) and shotcrete. The experiment provides information on the behaviour of the Opalinus Clay and the support structures during the excavation of a tunnel. The objective of the researchers is to predict how and to what degree Opalinus Clay reacts to external impacts and whether it will be robust enough to withstand the overburden pressure. They need this information to calculate the dimensions of the tunnel support in a future repository.
GAST experiment
In a repository, the corrosion of metals and the degradation of organic materials produces gases such as hydrogen, carbon dioxide and methane. If the resulting pressure becomes too high, this could impact one of the safety barriers.
Gas can escape through the tunnel walls and infiltrate the surrounding clay rock. Processes also take place in the tunnels that consume gas. In addition, gas can migrate along the backfilled caverns and tunnels at the disposal level, which counteracts the build-up of pressure. Gas-permeable tunnel seals facilitate this gas transport. The production, consumption and transport of gas occur slowly and are being investigated in several experiments.
Since 2012, the GAST experiment at the Grimsel Test Site has been testing a gas-permeable tunnel seal on a large scale. The seal consists of a mixture of sand and clay. The investigation focuses on the saturation behaviour and the controlled gas transport through the backfill material in a realistic environment. In addition, the experiment provides know-how on whether these sealing structures are technically feasible.
FE experiment
The “Full-scale Emplacement” (FE) experiment at the Mont Terri Rock Laboratory allows Nagra to gain practical experience for the later emplacement of high-level waste. The 50-metre-long emplacement tunnel contains three heater canisters simulating the heat production of high-level waste and spent fuel assemblies. The tunnel was sealed and backfilled with bentonite. The dimensions and work processes correspond to those planned for the future deep geological repository. This experiment measures the impact of the heat on the bentonite tunnel backfill and the Opalinus Clay.
Metrologists have equipped the tunnel and surrounding rock with hundreds of measuring instruments. These can detect minimal changes in temperature, humidity and pressure as well as deformation and gas composition in the bentonite and the surrounding rock. The instruments record around one million readings per day. These are compared to existing computer simulations and modelling studies and provide the basis for repository calculations.
With the FE Experiment, researchers can obtain hands-on experience for the emplacement and backfilling processes in the future repository.
Title photo: Maria Schmid