CFM - A long term repository relevant in-situ experiment on colloid and radionuclide migration, Grimsel test site, phase VI

Full text of publication follows: Earlier in-situ experiments in the Grimsel hard rock laboratory have shown that, under the given conditions, colloids derived from bentonite backfill material can act as radionuclide carriers [1, 2]. Based on the experience gained from this experiment (CRR; Colloid and Radionuclide Retardation Experiment), the Colloid Formation and Migration project (CFM - an international joint project between ANDRA, JNC, FZK and NAGRA) addresses open questions related to colloid processes at the bentonite barrier / groundwater interface. Those are specifically focusing on the behaviour of bentonite colloids and colloid-borne radionuclides under low groundwater flow velocities. Aims of the experiment are, to: - Examine colloid generation rates and mechanisms at the engineered barrier system (EBS) - host rock boundary under in-situ conditions; - Evaluate the long-distance migration behaviour of EBS-derived colloids in a water-conducting feature in a repository-relevant flow system (i.e. with a very low flow rate/water flux); - Study the long-term geochemical behaviour (mobility, mineralisation, colloid formation, etc) of radionuclides at the EBS-host rock boundary; - Examine the influence of radionuclide-colloid interaction mechanisms on colloid-mediated radionuclide migration. An evaluation of various colloid migration experiments clearly demonstrate that colloidal species are retained on rock surfaces at extended residence times in a fractured rock. This observation is made despite the geochemical conditions of the Grimsel groundwater (pH∼9.6, I∼10-3 mol/L) favouring colloid stability and mobility. The paper will present an overview of the results of the new CFM project and PA relevant results from the colloid investigations will be highlighted and the implications of the projects discussed. [1] A. Moeri, W.R. Alexander, H. Geckeis, W. Hauser, Th. Schaefer, J. Eikenberg, Th. Fierz, C. Degueldre, T. Missana, Coll.Surf. 217 (2003), 33-47 [2] Geckeis, H., Schaefer, T., Hauser, W., Rabung, T., Missana, T., Degueldre, C., Moeri, A., Eikenberg, J., Fierz, T., and Alexander, W.R., Radiochimica Acta, 92 (2004), 1-10. (authors)