Crystalline rock – a host rock with challenges

Crystalline rock must fulfil numerous requirements in order to be suitable as a host rock for the final disposal of high-level radioactive waste. How effectively can we assess this suitability based on the existing data? What special challenges arise for this type of rock in the process of geoscientific and geophysical exploration? And how well can we predict how the rock will change over the next million years? These three central questions were discussed at the Crystalline Workshop in Hanover on 24 and 25 June.

At the invitation of Gesellschaft für Anlagen- und Reaktorsicherheit gGmbH (GRS) and the Bundesgesellschaft für Endlagerung (BGE), around 40 experts on crystalline rock, including several members of the OECD/NEA Crystalline Club (CRC), met to share their knowledge about this type of rock. The workshop was organised in light of the current step in the site selection process for the final repository for high-level radioactive waste. Specifically, the BGE is currently assessing the suitability of sub-areas for final disposal. Crystalline rock differs from the other two host rocks – claystone and rock salt – in key respects.

Fissures – a rock-specific challenge

The participants agreed on the fundamental points. Nobody questioned the fact that it is, in principle, possible to construct a safe repository in crystalline rock, as Finland, Sweden and the Czech Republic are already doing or plan to do in the future. There was also agreement that crystalline rock poses a particular challenge when it comes to the safety analyses.

There were some lively discussions when it came to more-specific questions, such as whether there can actually be areas in crystalline rock where the rock constitutes the essential barrier for radioactive waste – known as containment-providing rock zones (CPRZs). The alternative to the CPRZ – the repository system type II, in which repository containers and geotechnical barriers assume the decisive safety functions – places lower demands on the rock. Whether these demands are significantly or insignificantly lower was another point of discussion. In particular, the rock’s permeability to liquids and gases is a decisive factor in its suitability, and this characteristic is largely related to the fissuring of the rock. However, fissures – which are sometimes microscopically small cracks in the rock – are difficult to predict. Good location-specific data is therefore essential when it comes to evaluating suitability. 
Despite these rock-specific challenges, the participants agreed that crystalline rock should continue through the current Phase I of the repository search as planned. The panel did not support a blanket exclusion of the rock type.

Expertise from the Czech Republic and Switzerland

The German crystalline-rock experts received support from two neighbouring countries on the first day of the workshop. Lukáš Vondrovic, director of SÚRAO, presented the Czech exploration concept, which is intended to determine the most suitable site from four candidates in the crystalline rock. Piet Zuidema, a long-standing member of Nagra’s Executive Board, reported on how much Switzerland was able to benefit from the research and exploration work taking place in Sweden at the same time. He also strongly recommended that a sufficient number of boreholes be drilled in Phase II of the repository search to allow reliable assessment of the local rock. In Switzerland, he said, there were areas where rock had been assumed to be crystalline and it was only thanks to a borehole that it was actually revealed to be sedimentary.

Judith Flügge, head of the Final Storage division at GRS and head of the OECD/NEA Crystalline Club (CRC), says: “We’re delighted that we were able to organise the Crystalline Workshop. Over the two days, we worked together to identify key points regarding the evaluation of crystalline rock as a host rock. However, it also became clear that the evaluation of specific siting regions or sites will largely depend on site-specific data and the underlying safety concept.”
Wolfram Rühaak, head of the Safety Analyses department within the Site Selection division of the BGE, sums up the workshop as follows: “We succeeded in bringing together experts with relevant technical know-how in order to assess the state of knowledge regarding crystalline rock in Germany. I see this as a confirmation that we’re able to make targeted evaluations, even if there are still a lot of unanswered questions.”

About the BGE

The BGE is responsible for proposing the site for a repository for high-level radioactive waste that offers the best possible safety for one million years. It is also constructing the Konrad repository for low- and intermediate-level radioactive waste in Salzgitter. The BGE is decommissioning the Morsleben repository, planning the decommissioning of the Asse II mine following retrieval of the low- and intermediate-level radioactive waste, and closing the Gorleben mine.