The DR-A in-situ diffusion experiment at Mont Terri: Effects of changing salinity on diffusion and retention properties.
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The DR-A in-situ diffusion experiment at Mont Terri: Effects of changing salinity on diffusion and retention properties. Josep M. Soler1, Olivier X. Leupin2, Thomas Gimmi3,4 and Luc R. Van Loon3 1
IDAEA-CSIC, Barcelona, Catalonia, Spain. NAGRA, Wettingen, Switzerland. Paul Scherrer Institut, Villigen, Switzerland. 4 University of Bern, Switzerland. 2 3
ABSTRACT In the new DR-A in-situ diffusion experiment at Mont Terri, a perturbation (replacement of the initial synthetic porewater in the borehole with a high-salinity solution) has been induced to study the effects on solute transport and retention, and more importantly, to test the predictive capability of reactive transport codes. Reactive transport modeling is being performed by different teams (IDAEA-CSIC, PSI, Univ. Bern, Univ. British Columbia, Lawrence Berkeley Natl. Lab.). Initial modeling results using the CrunchFlow code and focusing on Cs+ behavior are reported here. INTRODUCTION Clay formations are being considered internationally as potential host rocks for radioactive waste disposal due to their low permeability, diffusion-controlled solute transport and favorable retention properties. After a series of successful in-situ experiments in the Opalinus Clay (OPA) at the Mont Terri Underground Rock Laboratory, the new DR-A test, funded by Nagra (Switzerland), NWMO (Canada) and DOE (USA), has been designed to provide further insight into solute transport and retention, and more importantly, to test the predictive capability of reactive transport codes. In previous experiments (DI, DI-A1, DI-A2, DR; [1-6]) the behavior of different tracers (water, cations and anions, conservative and sorbing) was studied using solutions at equilibrium with the host rock. Those results showed the value of the experimental setup (synthetic porewater in equilibrium with the rock; circulation of the synthetic porewater containing the tracers in a single borehole; closed circulation loop) and the importance of processes such as anion exclusion and sorption of cations. The first stage of the DR-A test consisted of a conventional diffusion experiment using a synthetic version of the OPA porewater, including several tracers (HTO, I-, Br-, Cs+, 85Sr2+, 60 Co2+, Eu3+). Reactive transport modeling is being performed by several teams (IDAEA-CSIC, PSI, Univ. Bern, Univ. British Columbia, Lawrence Berkeley Natl. Lab.). Predictive modeling is first performed based on existing knowledge (diffusion coefficients, accessible porosities, sorption parameters). Model calibration is performed later using the measured experimental data. After 189 days, the solution in the borehole was replaced with a higher-salinity solution (0.50 M NaCl + 0.56 M KCl, approx.), with clear effects such as the back-diffusion of Cs+ from the rock to the borehole due to desorption from the cation exchange complex in the rock. These results are now being modeled by the different teams. Initial modeling results using the CrunchFlow code and focusing on Cs+ behavior are reported here.
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SETUP AND DIMENSIONS Figure 1
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