State of the art on thermo-hydro-mechanical behaviour of clay buffers at high temperature

María Victoria Villar^1*Katerina Cernochova²Jaime Cuevas³Antonio Gens⁴Natalia Gimeno¹Caroline Graham⁵Jon Harrington⁵Vlastislav Kaspar⁶Stephan Kaufhold⁷Olivier Leupin⁸David Mašín⁹Jan Najser⁹Markus Olin¹⁰Heini Reijonen¹¹Šárka Šachlová⁶Sergey Sayenko¹²Daniel Svensson¹³Jiri Svoboda²Gianni Vettese¹⁴Janne Yliharju¹⁵Borys Zlobenko¹⁶

• ¹Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Madrid, Spain
• ²Czech Technical University in Prague, Prague 6, Prague, Czechia
• ³Universidad Autónoma de Madrid, Madrid, Spain
• ⁴Universitat Politecnica de Catalunya, Barcelona, Catalonia, Spain
• ⁵British Geological Survey, Nottingham, United Kingdom
• ⁶ÚJV Rež, a. s., Husinec, Czechia
• ⁷Federal Institute For Geosciences and Natural Resources, Hanover, Lower Saxony, Germany
• ⁸National Cooperative for the Disposal of Radioactive Waste (NAGRA), Wettingen, Aargau, Switzerland
• ⁹Charles University, Prague, Prague, Czechia
• ¹⁰VTT Technical Research Centre of Finland Ltd, Espoo, Uusimaa, Finland
• ¹¹Geological Survey of Finland, Espoo, Uusimaa, Finland
• ¹²Kharkov Institute of Physics and Technology, Kharkiv, Kharkiv Oblast, Ukraine
• ¹³Swedish Nuclear Fuel and Waste Management, Stockholm, Stockholm, Sweden
• ¹⁴University of Helsinki, Helsinki, Uusimaa, Finland
• ¹⁵University of Jyväskylä, Jyväskylä, Central Finland, Finland
• ¹⁶Institute of Environmental Geochemistry of National Academy of Sciences of Ukraine, Kyiv, Ukraine

Most safety cases for radioactive waste disposal concepts consider a temperature limit of 100°C for the clay buffer. Given that being able to tolerate higher temperature would have significant advantages, the work package HITEC of the EURAD project aimed at determining the influence of temperature above 100°C on buffer properties, trying to establish if the safety functions are unacceptably impaired. A synthesis of the state of knowledge on the thermo-hydro-mechanical and chemical behaviour of different buffer materials at different temperatures is presented, along with the progress made in this area during HITEC. The change of properties of preheated material and the hydro-mechanical properties of the bentonite at high temperatures were assessed.To cover the first instance, bentonite was heated at 150°C in dry and wet conditions for different periods of time up to 2 years. Clay mineralogy was remarkably preserved. The slight changes observed in other properties were opposite depending on the heating conditions: if evaporation was allowed, decreases in cation exchange capacity, specific surface area, sorption coefficients and sometimes swelling pressure were observed. The changes likely resulted from the strong drying induced by the elevated temperature. Bentonite was also subjected to hydration under thermal gradient in field and laboratory tests. No postmortem structural modifications of the smectite were observed, but dissolution and precipitation of species occurred, conditioned by the kind of bentonite and hydration water. These processes were accompanied by modification of the exchangeable cation complex.The determination of hydro-mechanical properties of expansive clay at elevated temperatures is challenging, with experimental and interpretation issues. In most cases a reduced swelling pressure was obtained when temperature increased, particularly for the higher dry densities. The results may be affected by the experimental protocols, the use of bentonite or purified smectite, and the exchangeable cations. Even at the highest temperatures the bentonite had the ability to fill voids and was able to develop large swelling pressures at high densities.Thermo-hydro-mechanical models were developed or upgraded during the project to include thermal phenomena and dependencies and were applied to the simulation of new laboratory thermo-hydraulic tests in cells.