The Nagra/PNC Grimsel test site Radionuclide Migration Experiment: Rigorous Field Testing of Transport Models
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THE NAGRA/PNC GRIMSEL TEST SITE RADIONUCLIDE MIGRATION EXPERIMENT:RIGOROUS FIELD TESTING OF TRANSPORT MODELS H.UMEKIt, K.HATANAKA1,W.R.ALEXANDER2, I.G.McKINLEY3, U.FRICK3 1.PNC (Power Reactor and Nuclear Fuel Development Corporation), Tokyo, Japan. 2. RWIG (Rock-Water Interaction Group), University of Berne, Berne, Switzerland. 3.Nagra (National Co-operative for the Disposal of Radioactive Waste), Wettingen, Switzerland. ABSTRACT The long-term programme of in-situ radionuclide migration experiments in the underground test site at Grimsel (GTS) involves the development and testing of radionuclide transport models with their associated databases. The field experiments are carried out in a water-bearing shear zone in crystalline rock utilising a suite of tracers of differing geochemical behaviour. A rigorous model testing procedure has been developed for the GTS radionuclide migration experiment. This paper describes application of this testing procedure to a solute transport code developed by PNC. INTRODUCTION Assessing the long-term safety of repositories for radioactive waste utilises a suite of models to examine the consequences of processes which may cause eventual release of radionuclides and their transport to the accessible environment. An important task in nuclear waste management is the testing (verification and validation) of such models and their associated databases. A particular challenge is the testing of models of radionuclide transport through the geosphere, due to its inherent heterogeneity and the practical problems of access to this medium. To this end Nagra, in 1985, initiated in-situ radionuclide migration experiments in its underground test site at Grimsel (GTS), in the Swiss Alps. The major aims of this long-term programme remain the development and testing of radionuclide transport models (in addition to testing the extrapolation of laboratory derived data to field conditions; see [I], for details). The field experiment, now jointly funded by Nagra and PNC, in collaboration with PSI (Paul Scherrer Institute), is carried out in a water-bearing shear zone in crystalline rock (granodiorite) and, to date, has utilised a suite of tracers of differing geochemical behaviour, including uranine, 3/4 He, 82Br, 1231, 22/24 Na, 86Rb, 85Sr, 137Cs and 99mTc. In all cases, the tracers are injected into dipole flow fields where the flow path length, dipole width or shape, and groundwater flow velocity can all be varied. Each tracer is utilised only following extensive laboratory-based experiments (including rock-water exchange, batch sorption experiments, detailed mineralogical and structural analysis of shear zone material, etc.) to determine the behaviour of the tracers under simulated "in-situ" conditions. The GTS migration experiment is well designed so that the rigorous model testing can be carried out. Here, the methodology used is described and the results for a newly developed transport code are presented for the first time and compared with field results. Current limitations to the testing procedure
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