Generation of Bentonite Colloids Under Repository-relevant Conditions in the CFM Long-term In situ Test, Grimsel Test Site, Switzerland

Abstract:

Colloid-associated radionuclide transport from the Engineered Barrier System may influence the performance of geological repositories for radioactive waste in fractured host rocks. The Colloid Formation and Migration (CFM) experiment at the Grimsel Test Site (GTS) in Switzerland is studying colloid generation from a solid bentonite source in a shear zone under repository-relevant conditions in the CFM Long-term In situ Test (LIT).

Hydraulic gradients in the shear zone are controlled by a tunnel-packer system maintaining stable repository relevant flow-velocities. The tunnel-packer system controls inflow from the MI shear zone to an IAEA radioprotection controlled zone in the AU tunnel at GTS [1]. After detailed characterisation of the shear zone including colloid/radionuclide migration tests [2] over scales from 0.1-6m, a solid bentonite source was installed in a borehole intersecting the shear zone in May 2014.

The source was installed on a purpose-built packer system that can be overcored at the end of the experiment. The bentonite source includes labelled synthetic clays, radionuclide and dye tracers to better characterise colloid generation and near-field solute/colloid-associated radionuclide transport. Three near-field boreholes intersect the shear zone within 10cm of the bentonite source to allow sampling of the groundwater around the bentonite. The experimental setup and first results from monitoring of the LIT are presented. [1] I. Blechschmidt, A. Martin. (2012) In-situ tracer tests and models developed to understand flow paths in a shear zone at the Grimsel Test Site, Switzerland, Abstract H33J-1472 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec.

[2] T. Schäfer, et al. (2013) The latest results on colloid associated radionuclide migration from the CFM Project, Grimsel Test Site (GTS, Switzerland). Abstract H31G-1277 presented at 2013 Fall Meeting, AGU, San Francisco, Calif., 9-13 Dec.