Multiple barriers provide safety

Switzerland will dispose of its radioactive waste in a deep geological repository. For the protection of humans and the environment, this type of repository will have multiple safety barriers that contribute to its long-term safety.

In the future, Switzerland will safely dispose of its radioactive waste in a deep geological repository over a very long time period. A system consisting of several engineered and natural safety barriers will reliably enclose the waste. Far removed from the human habitat, the radioactive substances contained in the waste can decay until they no longer pose a hazard. The containment period required for high-level waste is around 200 000 years and around 30 000 years for low- and intermediate-level waste.

We plan to construct the repository in a manner that ensures the safety barriers offer the highest possible safety during the entire containment period. To provide protection from glaciers and erosion, the repository is located at a depth of several hundred metres. We avoid active zones with deformed rock layers, so-called fault zones. We also adapt the repository to the geological conditions at the site and continuously optimise the overall process. Due to its robust safety barriers, a deep geological repository can be left unsupervised, i.e. without the need for human intervention, once it has been permanently closed.

Clay-rich rock formations as natural barriers

The safety barriers mutually complement each other. They protect against negative impacts such as flowing water from the waste and by retaining radioactive substances in the repository. This prevents unacceptable amounts of radioactive substances from being dissolved in water and then migrating through the surrounding rock to the earth‘s surface. The barriers also effectively shield against direct radiation: at a distance of just one to two metres from the tunnel wall of the repository, the radiation level is already below that of the rock’s natural radiation.

Host rocks being considered in European waste management programmes include crystalline, salt and clay. Switzerland’s most important barriers consist of clay-rich rock formations, which will eventually host the repository. These include the Opalinus Clay (host rock), which has a very low permeability, and the confining geological units located above and below it which have an equally low permeability.

Nagra geologist with Opalinus Clay from a drill core obtained from the Stadel borehole. Photo: Boris Baldinger

What is Opalinus Clay?

Opalinus Clay originated in the Jurassic Period some 173 million years ago. At that time, Northern Switzerland was covered by a shallow sea. Islands washed sediments into the sea. In the area presently covering Strassburg–Stuttgart–Zürich–Bern, fine clay muds were deposited on the seabed where they solidified to form the Opalinus Clay. Its name is derived from the ammonite “Leioceras opalinum” found in it.

Where in Switzerland can the Opalinus Clay be found?

The requirements relating to thickness and depth of the layer of Opalinus Clay are best met in a section extending from Olten to Schaffhausen. This is where the potential siting regions for a repository are located: Jura Ost, Nördlich Lägern and Zürich Nordost. In these regions, the Opalinus Clay layer is around 110 metres thick and lies at a depth ranging from 400 to 900 metres. Its properties remain similar over large areas. All three siting regions have sufficient space for a deep geological repository and are characterised by a stable geological situation.

The Opalinus Clay is being thoroughly researched as part of Nagra’s deep borehole investigations and in swisstopo

Properties of the Opalinus Clay

The Opalinus Clay consists mainly of microscopically small, plate-like clay minerals, which make it very tight. This means that, on the one hand, hardly any water flows through the intact rock, and when the rock does come into contact with water, many of the clay minerals swell. As a result, any fissures that form, for example as a result of tunnel excavation deep underground, are sealed again. Consequently, it is almost impossible for harmful substances to be washed out.

On the other hand, negatively charged clay platelets in the Opalinus Clay give it the capacity to bind positively charged substances. Most radionuclides in the waste are positively charged and attach themselves to the clay minerals. The scientific term for this process is sorption. Any negatively charged radionuclides migrate very slowly through the rock, which is known as diffusion. The Opalinus Clay therefore has the capacity to confine (radioactive) substances very well and over a very long time period.

Opalinus Clay under the scanning electron microscope: one gram of Opalinus Clay has a surface area of approximately 100 square metres. This is roughly equivalent to the size of a badminton court (photo width around 0.05 mm).

As a comparatively soft rock, the Opalinus Clay poses structural challenges when constructing deep underground. Engineering solutions can be developed but construction will remain complex. In addition, the high-level waste is still hot in the early emplacement period. It is therefore important to ensure that the Opalinus Clay is not exposed to excessive heat as this could compromise its containment properties. For this and other reasons, careful planning and construction of the repository are essential.

In Northern Switzerland, the Opalinus Clay and the rock formations above and below it, the so-called confining geological units, are often located at a depth of several hundred metres. At outcrops such as those found at the Frick clay pit, it is possible to see these natural barriers at the surface.

Rock formations above and below the Opalinus Clay

The clay-rich confining geological units also contribute to significantly delaying the radioactive substances from reaching the earth’s biosphere. This gives most radioactive substances sufficient time to decay to a harmless level.

Examples of confining geological units include the ‘Brauner Dogger’ and Effingen Member. Compared to the Opalinus Clay, these geological units are not evenly composed and have a slightly different composition of rocks and minerals. Both mainly consist of marl, which is a mixture of clay and limestone, but they also contain sand- and limestones or, as in the case of the ‘Brauner Dogger’, iron oolites (iron ore). These layers have a lower clay content, making them slightly less impermeable than the Opalinus Clay.

Engineered and natural safety barriers in a deep geological repository for high-level waste. Source: Nagra

Engineered safety barriers

To ensure the highest possible protection, the engineered safety barriers are adapted to the natural safety barrier.

Engineered barriers in the repository section for high-level waste:

  1. High-level substances from spent fuel assemblies are enclosed in fuel pellets that consist of uranium oxide. High-level waste from reprocessing is embedded in glass. This waste matrix has a very low solubility and forms the first of the engineered barriers. Even when water enters the disposal canisters, the radioactive substances can only be released into the water at a very slow rate.
  2. Spent fuel assemblies are packaged in thick-walled steel disposal canisters, which, as the second barrier, prevent the release of radioactive substances over a period of at least 10 000 years.
  3. The disposal canisters are placed on a bentonite pedestal in the emplacement drift, and the entire drift is backfilled with granular bentonite material. Bentonite mainly consists of clay minerals, has a very low permeability and forms the third barrier. It swells upon contact with moisture, which seals any fissures or fractures. Its clay minerals also bind radioactive substances, thereby retaining them.
Glimpse into a test tunnel at the Mont Terri Rock Laboratory. As will eventually be the case in a deep geological repository, the steel canister is placed on a bentonite pedestal and then completely backfilled with granular bentonite material. Photo: © Comet Photoshopping, Dieter Enz

Engineered barriers in the repository section for low- and intermediate-level waste:

  1. The waste is solidified in a matrix consisting of cement, glass or bitumen and enclosed in drums (first barrier).
  2. The drums are transferred to a concrete disposal container, which is then filled with cement mortar (second barrier).
  3. The concrete containers are then stacked on top of and adjacent to each other in large caverns, and the spaces between the containers are backfilled with a special mortar (third barrier).
The drums are emplaced in the disposal container, which is then filled with cement mortar (cross-section view). Source: Nagra