Deep geological repository: safe underground disposal of radioactive waste
To protect humans and the environment, the waste will be safely enclosed several hundred metres below the earth’s surface. The safety of the repository will be ensured by a barrier system. A repository consists of buildings at the surface, accesses leading below ground and underground facilities.
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. In Switzerland, the Opalinus Clay has been identified as the most suitable rock.
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 must safely contain the radioactive waste for hundreds of thousands of years. For this reason, it consists of a sophisticated multi-barrier system that ensures long-term, post-closure safety. The most important barrier in the Swiss disposal concept is provided by the geological conditions in the form of clay-rich rock formations.
The Opalinus Clay 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. In addition to this geological barrier, two engineered barriers, the thick-walled disposal canister and the backfill that seals any remaining voids, complement the multi-barrier system.
How the deep geological repository works
What structures does the repository consist of?
A deep geological repository for radioactive waste includes not only underground facilities but also buildings at the surface. This is the so-called surface facility 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 required 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.
Nagra proposes transporting the waste from Würenlingen to the repository. The interim storage facility is already located in Würenlingen, which is why it would make sense to construct the encapsulation plant for spent fuel assemblies at the same site. In this plant, the waste will be repackaged into disposal canisters and prepared for transport. Disposal containers are not only envisaged for high-level waste: the drums containing low- and intermediate-level waste will be provided with an additional layer of packaging before emplacement. After transport to the repository, the waste will be taken underground via the main access.
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 and each has its own structural and operational advantages and disadvantages. Both ensure the required nuclear operational safety as well as post-closure safety.
Shafts and ramps are commonly used in the mining industry. Nagra and 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. The purpose of the test area is to conclusively confirm that the site is suitable for a repository. 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 dead-end tunnels. A small, representative component of the waste will be emplaced in the so-called pilot repository. Further underground facilities include space for the vehicle fleet or for experiments.
Waste must remain retrievable after emplacement
A closed and sealed repository provides passive safety, which means it no longer requires human intervention or maintenance. Drifts and caverns that have been completely filled with waste containers are therefore continuously backfilled and sealed. This also applies to any accesses that are no longer required.
Once all of the waste has been emplaced, the monitoring phase will begin, which can last for several decades. The final closure of a deep geological repository will only take place when its post-closure safety has been demonstrated. Future generations will have to decide when and whether the deep geological repository can be permanently closed. The Federal Council will then issue the closure order.
Nagra is developing a repository project that provides considerable safety reserves. Nevertheless, the radioactive waste still has to remain retrievable from a deep geological repository, meaning that the waste can be brought up to the surface again if necessary. This is in line with the request of society and is legally stipulated. Measures to ensure retrievability may not impair either the safety barriers or post-closure safety.
“We do not expect the need to retrieve the radioactive waste – but we could retrieve it if necessary”
In this interview, Nagra mining engineer Thorsten Steils explains how radioactive waste can be retrieved from a deep geological repository.
