Coupled thermo-hydro-chemical models of compacted bentonite after FEBEX in situ test

FEBEX (Full-scale Engineered Barrier EXperiment) is a demonstration and research project for the engineered barrier of a high-level radioactive waste (HLW) repository. FEBEX is based on the Spanish reference concept for radioactive waste disposal in crystalline rock according to which canisters are emplaced in horizontal drifts and surrounded by a compacted bentonite clay barrier. The project includes two main large-scale tests which started in February 1997: an in situ full-scale test performed at Grimsel, Switzerland, and a mock-up test operating at CIEMAT facilities in Madrid, Spain. Coupled thermal, hydrodynamic and chemical (THC) models for the engineered barrier have been developed from laboratory experiments and the mock-up test. Dismantling of section 1 of in situ test, which took place in the summer of 2002, provides a unique opportunity to test THC model predictions. Here a numerical THC model of FEBEX in situ   test is presented. Predictions performed with three conceptual geochemical models are compared to measured temperature, water content and geochemical data obtained from aqueous extract tests of samples collected after dismantling of heater 1. These data require the use of inverse hydrogeochemical models for their interpretation in order to account for the geochemical processes suffered by bentonite samples during aqueous extraction. Model predictions reproduce temperatures and capture the trends of water content and most chemical species. However, there are some discrepancies near the heater which are especially significant for SO42- and HCO3-. The THC model could be improved by taking into account different types of water in bentonite, bentonite swelling, protonation/deprotonation by surface complexation, and CO2 degassing and dissolution processes.