Why Switzerland chose Opalinus Clay

When it comes to disposing of its nuclear waste, Switzerland takes a different approach than Scandinavian countries. Instead of granite, she uses opalinus clay. This reduces the technical requirements.

The nuclear power plants in Switzerland produce around 70 tons of highly radioactive waste every year. With a targeted service life of 60 years, that adds up. When the last nuclear power plant is shut down, Switzerland will be sitting on a mountain of 1,400 cubic meters of spent fuel elements. In addition, there is a disproportionately larger amount of low-level and intermediate-level waste. They occur in industry, in hospitals and when nuclear power plants are dismantled.

Until now, this waste has been stored in interim storage facilities. But this is not a permanent solution. When the heat has sufficiently decayed, the radioactive waste must be packaged and stored safely for hundreds of thousands of years. That’s how long it takes for radioactive radiation to drop to a natural level.

Since 1972, the National Cooperative for the Disposal of Radioactive Waste (Nagra) has been trying to find the best solution for the disposal of radioactive waste in Switzerland. Among many options – disposal in marine sediments, in deep boreholes or at the surface – a geological repository in deep rock strata was chosen. This concept promised permanent passive safety that would make human intervention unnecessary in the distant future.

Security through multiple barriers

Safety in a deep geological repository relies on several barriers designed to prevent radionuclides from reaching the earth’s surface. The first barriers are of a technical nature. When the fuel assemblies have cooled sufficiently, the fuel rods are removed from them and packaged in a waste matrix which is very slow to dissolve in contact with water. These bundles are placed in special steel containers, which are then stored in tunnels hundreds of meters below the surface of the earth.

The steel containers are designed in such a way that no radionuclides should escape for the first thousand years. At some point, however, they begin to corrode. Then the second security barrier comes into play. The tunnel in which the steel tanks are stored is filled with bentonite. This is a mixture of different clay minerals that swells when it comes into contact with water and closes the cavities.

The bentonite fulfills a double function. On the one hand, it should bind the radionuclides that escape from the steel containers. On the other hand, it should keep water out and prevent the steel containers from corroding prematurely.

However, no technical barrier is able to hold back the radionuclides for hundreds of thousands of years. It is on these timescales that the geological barrier comes into play. The safety of a repository stands and falls with the fact that the rock layers above the deep repository slow down the diffusion of the radionuclides so much that most of them have decayed before they reach the earth’s surface. To do this, the rock strata must be dense, impermeable to water, homogeneous and stable for the next million years.

After many years of research, Nagra is convinced that the Opalinus Clay best meets these requirements. At the Nördlich Lägern site, this layer of sediment is about 100 meters thick and at a depth of 800 meters. It was formed 175 million years ago when northern Switzerland was still covered by a flat sea. Fine clay mud was deposited on the seabed, which was then overlaid by other layers of rock over time.

Opalinus Clay was not always the first choice for a deep geological repository. First of all, Nagra relied on the bedrock consisting of gneisses, slate and granite, says ETH professor emeritus Simon Löw, who heads the expert commission for deep geological disposal. This rock, which comes to the surface at the Gotthard Pass and in the Reuss area, is several hundred million years old and has already survived several mountain-building cycles.

However, drilling in the 1990s had shown that the bedrock in Switzerland was too heterogeneous. There are local cracks in the rock through which water can penetrate. The Opalinus tone is much more homogeneous, says Löw. That is why Nagra opted for this option.

Finland relies on granite for its repository

Countries like Finland or Sweden have made a different choice. For example, Finland is building its repository for high-level radioactive waste in granite rock strata. Crystalline rock such as granite is less suitable as a geological barrier than Opalinus Clay, says Löw. But in Scandinavian countries there are fewer alternatives than in Switzerland. Finland is trying to make up for these disadvantages with technical measures. The steel containers would be encased in two inches of copper to prevent corrosion for 100,000 years. In Switzerland, the requirements for the containers are less stringent.

Nuclear waste retrieval remains an option

Research into deep geological disposal is to be continued in the future. In addition to the repository for high-level radioactive waste and the repository for low-level to intermediate-level radioactive waste, there will be a pilot repository in the repository in which a small, representative portion of the nuclear waste will be stored and monitored for decades. Only then is the camp completely sealed and left to its own devices.

If it turns out during the monitoring phase that long-term safety is not guaranteed – for example because the steel containers corrode faster than expected – the nuclear waste will be retrieved. The Swiss Nuclear Energy Act stipulates that this must be possible without great effort. Theoretically, the nuclear waste could even be retrieved after the storage facility has been sealed, says Loew.

This bothers the geologist Marcos Buser, who used to belong to the expert group on radioactive waste. Buser fears that the nuclear waste at a depth of a few hundred meters is not adequately protected against access by hostile states. He favors a well-secured underground interim storage facility in which the highly radioactive waste is stored until better final storage concepts are available.

Löw, on the other hand, is convinced that Switzerland is on the right track. There are always new findings that have an impact on deep geological storage. However, as long as these findings are dealt with openly, security can be continuously improved in an iterative process.

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