Deep geological repository: safe underground disposal of radioactive waste
Why deep geological disposal?
Radioactive waste has to be isolated from the human habitat for a very long time period until its radioactivity has decayed to a harmless level. To this end, Switzerland’s Nuclear Energy Act foresees a deep geological repository where the waste is enclosed in a tight and stable rock formation at a depth of several hundred metres. High-level waste, in particular, cannot be kept at the earth’s surface in the long term. The Swiss Confederation has given Nagra the mandate to plan and realise a deep geological repository. We are proud to make a significant contribution to the protection of future generations and the environment.
Protection through multiple barriers
A deep geological repository has to safely enclose the radioactive waste for tens to hundreds of thousands of years. For this reason, it consists of a sophisticated multi-barrier system that ensures the long-term safety of a repository. Geology, in the form of clay-rich rock formations, provides the most important barrier in the Swiss disposal concept.
The Opalinus Clay rock will host the disposal areas for the waste and is therefore also called the host rock. It has a very low hydraulic permeability, can keep water away from the waste and retain radioactive substances. Additional clay-rich, low-permeability rock formations, the so-called confining geological units, are located above and below the Opalinus Clay. Two additional engineered barriers, the thick-walled disposal canister and the backfill that seals any remaining voids, complement the natural geological barrier.
What does a deep geological repository consist of?
A deep geological repository for radioactive waste includes not only underground facilities but also buildings at the surface. This so-called surface infrastructure is where, for example, the transports with the radioactive waste arrive. Access structures connect the surface with the underground areas.
Surface infrastructure: gateway for the radioactive waste
The surface infrastructure consists of a surface facility, auxiliary access facilities, site development infrastructure and landfills for the materials arising from the excavation process. These will be needed for the construction, operation and closure of the repository. The waste is delivered to the surface facility where it is prepared for emplacement in the deep geological repository. High-level waste is transferred from transport and storage casks to disposal canisters. These will then be transported underground via the main access. The disposal containers for low- and intermediate-level waste are provided with an additional layer of packaging before emplacement.
How do you get below ground?
The underground facilities of a deep geological repository are accessed using shafts or ramps. The waste can be transported underground using a lift, or by road vehicle or cog railway. The Swiss Federal Nuclear Safety Inspectorate and Nagra consider both a shaft and a ramp suitable as access methods. These variants offer flexibility for the layout of the surface facility but they also have structural and operational advantages and disadvantages. Both ensure the required nuclear operational safety as well as long-term safety.
Shafts and ramps are commonly used in the mining industry. Not only Nagra, but also other waste management organisations such as in France, Finland and Sweden foresee using them for their disposal projects. The final decision regarding the type of access will be made with the nuclear construction licence.
What does the underground area of the repository consist of?
Below ground, a deep geological repository consists of a main repository, a test area and a pilot repository (see “Diagram of a deep geological repository” above). The purpose of the test area is to definitely determine whether the site is suitable for a repository or not. Specialists also test the technology required in case of the need for retrieval. Most of the waste will be emplaced in the main repository. Low- and intermediate-level waste will be disposed of in emplacement caverns – these are large excavated cavities in the rock – and high-level waste in emplacement drifts, which are basically tunnels that have only one access. A small, representative component of the waste will be placed in the so-called pilot repository. Further underground facilities include space for the vehicle fleet or for experiments.
Radioactive waste has to be transported to an interim storage facility and eventually to a deep geological repository.
Two individual repositories or a combined repository?
What the repository will look like in detail also depends on what type of waste it is designed for. Switzerland primarily has high-level waste and low- and intermediate-level waste. These two waste types have different physical properties and have to be disposed of in separate facilities. This can be done in two individual repositories at two different sites or in a so-called combined repository at a joint site but with separate underground disposal areas. Learn more about the advantages of a combined repository.
Long-term safety has to include option of retrievability
A sealed repository provides passive safety. This means that human intervention or maintenance are no longer required. Drifts and caverns that have been completely filled with waste containers are therefore continuously backfilled and sealed. This also applies to all accesses that are no longer required. Once all of the waste has been emplaced, the monitoring phase will begin and extend over several decades. One access will remain open from the surface to monitor the pilot repository. The final closure of a deep geological repository only takes place once its long-term safety has been demonstrated and the permit for closure is granted by the Federal Council.
Nagra is developing a repository project that provides considerable safety reserves. However, the radioactive waste still has to remain retrievable from a deep geological repository, meaning that it would be possible to bring the waste back to the surface. This is in line with the request of society and is anchored in the legislation. Retrievability measures may neither negatively impact the safety barriers nor long-term safety.
Retrieval for safety reasons is not very likely, but it has to remain possible without undue effort up to the final closure of the repository. After closure, retrieval is still possible, but it would require greater effort. As part of the radioactive waste consists of spent fuel assemblies, future generations might want to retrieve them for use as a resource for energy production. Retrieval of the waste could also come into consideration if an alternative disposal option should become available in the future.