1. R Conclusions According to reactive transport calculations carried out in this study,  At an ambient temperature of 25 °C, there will be no obvious mineralogical modifications in the mineralogical composition of the bentonite. This is true at least for the first 10,000 years, which can be understood as the minimum lifetime of the HLW canister,  At 90 °C, The Na-montmorillonite amount increases over 6% of its original volume fraction in two different cases with different initial Na-concentrations. The dissolved minerals include calcite, kaolinite, microcline and quartz. These minerals can provide the necessary Al, Si, and Ca for the new-formed minerals and for Na-montmorillonite precipitation. Illite, saponite-Mg, saponite-Ca and Ca-montmorillonite are other new-formed minerals but with a small total amount, i.e. their contribution is less than 0.1% of total minerals. Geochemical modelling of bentonite-groundwater interactions at 25 °C and 90 o C Zhou Jia 1,2, Bildstein Olivier 2, Tiffreau Christophe 2, Wang Ju 1 1 Beijing Research Institute of Uranium Geology, Chinese National Nuclear Corp., Beijing, 100029, China 2 CEA, DEN, DTN/SMTM/LMTE, Saint Paul lez Durance, F13108, France Objectives: i)develop numerical simulations that help understand the interactions between the Beishan granitic groundwater and GaoMiaoZi bentonite at 25°C and 90°C, ii)study the main features that influence the hydrothermal- geochemical evolution of the near-field of a HLW repository, iii)carry out experimental tests under room temperature to validate the numerical calculations. iv)perform a priori calculations for a mock-up experiment to predict geochemical evolution. Motivations The engineered buffering system of a geological repository is one of the studied subjects as required in the Chinese Guideline for the HLW disposal. GMZ bentonite has been considered as the most promising EB material. BRIUG had spent over 20 years to study GMZ bentonite. A Mock-up test for the thermo-hydro-mechanical-chemical (THMC) study of GMZ bentonite has been installed in BRIUG. Scope and Approach Calculation of the initial porewater composition Redistribution of calcium, carbonate, Na- montmorillonite and new-formed minerals within the bentonite. Reactive transport calculations addressing the geochemical alterations in bentonite when in contact with granitic groundwater have been performed using the code Crunchflow. Introduction Beishan Groundwater chemistry(mmol/L) * Calculated values, unit: bar pHNaKMgCaHCO3-pCO2*SO4--ClAlSiO2(aq) Beishan granitic groundwater Simulated case 1 7.58461.5E-031.72.82.53.0E-059.8347.9E-072.2 Simulated case 2 8.08454.8E-031.72.86.41E-013.0E-049.8347.5E-067.0E-01 Simulated case 3 8.58441.5E-021.72.82.5E-013.0E-039.8347.4E-052.4E-01 MontmorilloniteQuartzCristobaliteK-feldsparKaolinite 75.411.77.34.30.8 GMZ bentonite The Mineral Composition of Bentonite GMZ01 (weight percentage) (source: Wen 2007) The chemical composition (weight percentage) of GMZ bentonite (Wen, 2007) SiO 2 Al 2 O 3 CaONa 2 OK2OK2OMgOFe 2 O 3 FeOMnO 2 TiO 2 P2O5P2O5 69.7914.721.602.121.981.911.280.260.100.070.08 Schematic of the mock-up used in the the 1-D simulation (left). The simulation domain for studying the interactions between bentonite and groundwater in the mock-up was represented by a 1D numerical model (right). The 15-cm long dark zone represents the canister cell; this is an inactive zone in the model. The remaining 35 cm represents the bentonite from the mock-up. Groundwater will be intruding the bentonite blocks from the right side. Schematic view of the Chinese concept for HLW disposal (Wang, 2008) GMZ bentonite porewater at 25 °C and 90 °C (mmol/L) pHNaKCaMgAlFe(tot)SO4--ClHCO3SiO2(aq) 25 °C 8.00442.8E-024.5E-011.1E-042.0E-052.4E-0410E-01Charge3.0E-043.5 90 °C 7..332.8E-031.5E-012.03.6E-031.3E-034.8E-051.0ChargepCO2=0.003 bar 8.6E-01 Primary mineralsSecondary minerals Calcite Kaolinite_B Microcline Montmorillonite-Na Pyrite Quartz Amorphous_silica Anhydrite Chalcedony Clinochlore_14A Cronstedtite_7A,al Daphnite_14A Dolomite Ettringite Ettringite_Fe Greenalite Goethite Gypsum Illite-Mg Magnetite,beta Montmorillonite-K Montmorillonite-Mg Montmorillonite-Ca Nontronite-Ca Nontronite-K Nontronite-Mg Nontronite-Na Pyrrhotite,alpha Saponite-Mg Saponite-Na Saponite-K Saponite-Ca Siderite Sepiolite Minnesotaite Brucite Maghemite_disorder Gyrolite 25 °C 90 °C Initial volume percentage of main minerals. (a) Volume percentage in linear scale. Minerals not visible on the graph are (volume percentage) Illite (6E-7), Ca-Montmorillonite (6E-6), pyrite (1.3E-2), calcite (3.0E-1), and kaolinite (5.1E-1). (b) Volume percentage in logarithmic scale. Volume percentage of main minerals after 10,000 years. (a) Volume percentage in linear scale. Minerals not visible on the graph are Illite, Ca-Montmorillonite, pyrite, calcite, and kaolinite. (b) Volume percentage in logarithmic scale. Bentonite porewater primary minerals neo-formed minerals Primary and secondary minerals used in bentonite-groundwater interaction modelling System description and modelling