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Modeling Eternity in the Rock Laboratory

Friday, March 05, 2021

Cement is one of the key materials for the safe storage of radioactive waste. What is needed is an almost infinite durability of the containers. Empa/CH researchers are therefore analyzing material systems that can handle this task. Barbara Lothenbach from Empa's Concrete & Asphalt Laboratory is investigating cement-based materials, which are suitable for the disposal of radioactive waste.

According to the Nuclear Energy Act, deep geological repositories in Switzerland are to receive low-, medium- and high-level nuclear waste in the future. For this purpose, stable rock layers must be available to enclose the waste containers. Since materials scientists know, however, that no material is unchangeable, a rock formation must be selected that is geologically as stable and dense as possible – over thousands of years. The 180 million-year-old opalinus clay, which extends in Switzerland between Olten and Schaffhausen at a depth of 600 m, for example, has proven to be a suitable host rock. Since it has a low water conductivity, it has excellent insulating properties. But how do the crystalline structures and clay minerals of opalinus clay react with cement-based safety barriers when the gnawing ravages of time lead to changes? The National Cooperative for the Disposal of Radioactive Waste (Nagra) requires data on this issue so that a final repository for nuclear waste can be embedded rock-solid in the earth with regard to environmental protection and safety.

Barbara Lothenbach and her team carry out the required analyses by conducting experiments under realistic conditions at the Mont Terri Rock Laboratory in St. Ursanne, which was constructed in an Opalinus Clay layer. Together with international partners and research groups from Switzerland, such as the University of Bern and the Paul Scherrer Institute (PSI), reactions of cement-based materials and the surrounding Opalinus Clay are simulated. The researchers are investigating and modeling the long-term development of the boundary layers between the very different material systems in experimental approaches lasting several years at different temperatures between 20–70 °C. Of particular importance here is the strongly alkaline pH of cement, which in conventional Portland cement can be as high as pH 13,5 or even higher. To ensure that the alkaline environment does not attack the clay minerals in the surrounding area, a new development, the so-called low-alkali cement, seemed to be a good candidate for durable, cement-based protective barriers. With a pH of 12,2 or lower, it has an alkali concentration that is more than ten times lower. B. Lothenbach and her team therefore compared cement types with different pH values using thermodynamic modeling and X-ray diffraction analysis. This is the first time long-term results are available that allow the cement types and their evolution in the mountain to be characterized. It turned out that low-alkali cement is indeed more gentle on the clay minerals. However, when conventional Portland cement is used, chemical compounds are formed over time that lead to similarly favorable conditions in the safety barrier. As a result, the cheaper and well-established Portland cement has once again become the focus of interest.

Empa researchers have thus investigated radioactive isotopes present in the radioactive waste, such as those of the element selenium, in adsorption studies. The results show that selenium compounds are absorbed by the cement in large quantities. "A protective barrier made of concrete delays the release of radioactivity into the biosphere, since the cement minerals bind the radioactive substances and thus stop their spread.

Eleven nations with various universities and research institutes are involved in the international research program at the Mont Terri Rock Laboratory, including Empa. The underground rock laboratory is located in an opalinus clay layer at a depth of 300 m in Mont Terri near St. Ursanne (JU). The laboratory is operated by the Federal Office of Topography (swisstopo), and the project partners are funding the research programs. Rock formations that could play a role in the storage of radioactive waste have been investigated here since 1996.