The evolution of clay rock/cement interfaces in a cementitious repository for low- and intermediate level radioactive waste |
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| Institution: | 1. RWI, Institute of Geological Sciences, University of Bern, Baltzerstrasse 3, CH-3012 Bern, Switzerland;2. LES, Paul Scherrer Institut, CH-5232 Villigen, Switzerland;3. Nagra, CH-5430 Wettingen, Switzerland;1. Department of Geology and Geochemistry, Faculty of Sciences, Autonomous University of Madrid, Cantoblanco, 28049, Madrid, Spain;2. CIEMAT, Av. Complutense 40, 28040, Madrid, Spain;1. Amphos 21 Consulting S.L., Pg. de Garcia i Fària 49-51, 08019 Barcelona, Spain;2. Andra, 1/7, Rue Jean Monnet, 92298 Châtenay-Malabry Cedex, France;1. Earth Surface Science Institute, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK;2. Research Centre for Radwaste Disposal, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK;3. British Geological Survey, Nicker Hill, Keyworth, Nottingham NG12 5GG, UK;4. Kroto Research Institute, University of Sheffield, Sheffield S10 2TN, UK;5. National Nuclear Laboratory, Birchwood Park, Warrington WA3 6AE, UK |
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| Abstract: | In Switzerland, deep geological storage in clay rich host rocks is the preferred option for low- and intermediate-level radioactive waste. For these waste types cementitious materials are used for tunnel support and backfill, waste containers and waste matrixes. The different geochemical characteristics of clay and cementitious materials may induce mineralogical and pore water changes which might affect the barrier functionality of host rocks and concretes.We present numerical reactive transport calculations that systematically compare the geochemical evolution at cement/clay interfaces for the proposed host rocks in Switzerland for different transport scenarios. We developed a consistent set of thermodynamic data, simultaneously valid for cementitious (concrete) and clay materials. With our setup we successfully reproduced mineralogies, water contents and pore water compositions of the proposed host rocks and of a reference concrete.Our calculations show that the effects of geochemical gradients between concrete and clay materials are very similar for all investigated host rocks. The mineralogical changes at material interfaces are restricted to narrow zones for all host rocks. The extent of strong pH increase in the host rocks is limited, although a slight increase of pH over greater distances seems possible in advective transport scenarios. Our diffusive and partially also the advective calculations show massive porosity changes due to precipitation/dissolution of mineral phases near the interface, in line with many other reported transport calculations on cement/clay interactions. For all investigated transport scenarios the degradation of concrete materials in emplacement caverns due to diffusive and advective transport of clay pore water into the caverns is limited to narrow zones.A specific effort has been made to improve the geochemical setup and the extensive use of solid solution phases demonstrated the successful application of a thermodynamically consistent union of very different materials like hydrated cement and clay phases. A reactive system utilizing a novel solid-solution approach is used, where cation exchange is an intrinsic property of the mineral phase definition. Although such features were not the primary aim of the study, they offer a large potential for studies where ion exchange and changing sorption properties are of interest. |
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