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Diffusion and retention of radionuclides in opalinus clay: results from a long-term migration in-situ test


Congress title :Migration '07
Congress town :Munich
Congress date :27/08/2007

Summary

Deep argillaceous formations are being considered as potential host rocks for radioactive waste disposal because of their favourable hydraulic and retention properties that limit the migration of radionuclides. During the last years, it has been shown by combined laboratory and in-situ migration studies that diffusion is the main transport process in Opalinus Clay [1]. Retention of radionuclides, on the other hand, has been predominantly investigated by batch sorption measurements on disperse material. The use of static batch data for predicting the fate of radionuclides in clay rocks is a priori not straightforward, and migration tests provide a critical verification of the applicability of such sorption data. This contribution reports on the most recent migration test at the Mont Terri Rock Laboratory (Switzerland), the DR experiment (Diffusion and Retention). The objectives are threefold: (i) obtain diffusion and retention data for moderately to strongly sorbing tracers, (ii) improve diffusion data for the rock anisotropy and (iii) quantify effects of the borehole-disturbed zone for non-reactive tracers. The test design is based on previous successful diffusion tests with non-sorbing and sorbing tracers [2, 3]. The injected tracers were tritium, iodide, bromide, sodium-22, strontium-80, caesium (stable), caesium-137, barium-133, cobalt-60 and europium-152. These tracers were injected as a pulse in the borehole and their removal from the circulation fluid - as they diffused into the clay - was continuously monitored by online -detection and regular sampling. An extensive modelling exercise accompanied the DR experiment in which four distinct reactive transport models were benchmarked and applied for design calculations. Modelling of the tracer evolution is in progress and will be accomplished within the next months. [1] E. Tevissen, J.M. Soler, Mont Terri Project. DI Experiment. Synthesis Report. Mont Terri Technical Report TR 2001-05, swisstopo, Wabern, Switzerland, 2003. [2] J.-M. Palut, Ph. Montarnal, A. Gautschi, E. Tevissen, E. Mouche. J. Contaminant Hydrology 61, 203 (2003). [3] L.R. Van Loon, P. Wersin, J.M. Soler, J. Eikenberg, Th. Gimmi, P. Hernán, S. Dewonck, S. Savoye, Radiochimica Acta 92, 757 (2004).
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