The significant distance between the deep geological repository and the earth’s surface provides a high level of safety in itself. Deep below the surface, the Opalinus Clay rock will ensure the long-term safety of humans and the environment. This is possible because the clay has three key properties.
First, the Opalinus Clay is practically water-impermeable. Second, it can self-seal any potential fissures. And third, radioactive particles, so-called radionuclides, effectively stick to it.
All of these factors slow down the migration of radioactive particles to such an extent that only minute, harmless quantities could be released from the deep geological repository into the groundwater or the environment.
While a system of multiple engineered barriers will help to prevent the diffusion of radioactive substances, the Opalinus Clay is by far the most important barrier with regard to the post-closure safety of the deep geological repository. Decades of research and numerous scientific experiments have demonstrated how reliably the clay rock can enclose radioactive waste deep underground.
Tiny particles, great impact
In the Jurassic Period, large parts of Europe were covered by a shallow sea. The average depth was only 20 to 50 metres. Around 175 million years ago, tiny rock particles carried down from rivers were deposited on the seabed. These included many clay minerals from weathered granite. Over the course of several hundred thousand years, a layer of mud formed, which eventually became the 100-metre-thick Opalinus Clay.
The three decisive properties of the clay rock can best be studied at the microscopic scale. The inside of clay minerals is made up of tiny platelets between which water accumulates. Small pores between the clay minerals also contain water. While the Opalinus Clay contains a good ten per cent water, this water hardly moves at all. For this reason, the dense clay rock is practically water-impermeable.
If water were nevertheless to penetrate from the outside, for example through a fissure, this would cause the Opalinus Clay to swell, thereby increasing its volume and effectively self-sealing the leak. This self-sealing capacity has been demonstrated in various scientific tests.
If the tiny clay platelets could be joined together, the surface area would be enormous: one gram of Opalinus Clay would cover around 100 square metres. As these platelets are negatively charged, positively charged particles stick to their large surface. This is the case with radioactive waste, which is mostly made up of metals and as such is positively charged.
Around 200 years after the emplacement of the radioactive waste, the radiation in the disposal canisters will have decreased to only a few per cent of the initial level. The thick-walled steel canisters will completely enclose the high-level waste for at least 1,000 years. When they can no longer do so completely, radionuclides will be able to reach the backfilled emplacement drifts. However, almost 90 per cent of these radioactive particles will be retained by the tunnel backfill. This consists of a clay granulate that closes the void between the canisters and the Opalinus Clay.
There are different types of radionuclides. Most of them do not get very far and already decay in the disposal canister. Some manage to migrate a little further. And only a few succeed in covering an even greater distance until they too eventually “run out of steam”.
Considering the unlikeliest scenarios
To demonstrate the post-closure safety of the deep geological repository, Nagra calculates even highly unlikely scenarios. For example, safety analyses show that the legally stipulated protection of humans and the environment would remain ensured even if the Opalinus Clay were to be only 50 instead of 100 metres thick. Or if an incompletely sealed fissure in the clay rock were to run right through the emplacement drift: even then, the emitted radiation dose would still be well below the maximum permissible value.
These assumptions are referred to as conservative, and they cover a variety of highly unlikely scenarios. It is not possible to demonstrate that they will never occur or to make reliable predictions about them. While including conservative assumptions means that negative consequences for humans and the environment are very likely to be overestimated, they serve to bridge any existing uncertainties.
Three regions in Switzerland are suitable for the construction of the deep geological repository: Jura Ost, Nördlich Lägern and Zürich Nordost. In recent years, Nagra conducted in-depth geological investigations of these siting regions. These investigations confirmed that, even when taking into account conservative assumptions, the maximum permissible radiation dose would not be reached in any of the three regions.
As Nördlich Lägern has the greatest safety reserves, Nagra wants to construct the deep geological repository there. To this end, Nagra submitted the so-called general licence application to the federal government in November 2024.
Where does the name Opalinus Clay come from?
The Opalinus Clay is named after the fossil Leioceras opalinum, a so-called ammonite, which is frequently found in this rock. This squid-like creature lived in a shell shaped like that of a snail. The fossilised shell shimmered like an opal, giving the ammonite its name. Thanks to air chambers in the “snail shell”, the ammonites could move through the water like submarines. When they died, they sank to the bottom of the sea and fossilised.