Coupled Thermo-Hydro-Geochemical Models of Engineered Barrier Systems: The Febex Project
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ENRESA, with the collaboration of the Swiss’s sister organization, NAGRA, as well as other European organizations [1]. FEBEX includes two main experiments: the in situ and the mock-up heating and hydration tests. The in situ full-scale test is being performed at Grimsel (GTS). The test layout consists of a drift excavated in a granitic massif back-filled with a full scale mock-up of the bentonite EBS. Embedded within the bentonite, two heaters were installed along with 624 different sensors to monitor the thermo-hydro-mechanical evolution of the bentonite and the surrounding rock. The mock-up test has been operating since 1997 at the CIEMAT facilities in Madrid (Spain) and consists of a steel sheath filled with bentonite blocks and two heaters, everything being instrumented with several hundred sensors [2]. One of the objectives of the FEBEX Project is the development and testing of conceptual and numerical models for the thermal, hydrodynamic, and geochemical (THG) processes expected to take place in engineered clay barriers. A large number of lab tests were performed by several R&D institutions such as CIEMAT and CSIC for the identification of relevant THG processes and derivation of parameters. These tests, most of which were interpreted numerically, include: 1) batch tests to derive distribution coefficients, 2) through- and in-diffusion tests to estimate accessible porosity and molecular diffusion coefficients, 3) permeation tests intended to explore double porosity structures and dispersive parameters, 4) infiltration tests devised to estimate relative hydraulic conductivities, 5) exchange tests to better understand exchange phenomena and derive selectivity coefficients, 6) porewater squeezing experiments to derive representative porewater geochemistry, and 7) heating and hydration experiments which were most useful for testing and calibrating THG numerical models. Data from the two large scale tests were also used to derive difficult-to-measure hydrodynamic and thermal parameters such as vapor tortuosity. A significant improvement in coupled THG modeling of the clay barrier has been achieved both in terms of (1) a better understanding of THG processes and (2) more sophisticated THG computer codes. In a first stage, THG models were constructed based on the information provided by lab tests in which hydrogeochemical information was primarily collected at test completion. Here we report the main features of the THG model of the FEBEX clay barrier and some of the most relevant results achieved in small-scale experiments. Later, the thermohydrodynamic parts of the model were improved based on data provided by the large-scale tests. THG MODELS OF COMPACTED BENTONITES The main features of the conceptual model for water flow through the clay barrier are [3]: 1. Darcy’s Law is applicable in terms of pressure heads and intrinsic permeability. 2. The clay experiences an increase in water content, eventually getting fully saturated. Water flow occurs under variably saturated conditions. Once the clay barrier is saturated,
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