Building Confidence in Radionuclide Transport Models for Fractured Rock: The Nagra/JNC Radionuclide Retardation Programm
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Building Confidence in Radionuclide Transport Models for Fractured Rock: The Nagra/JNC Radionuclide Retardation Programme K. Ota1, W.R. Alexander2, P.A. Smith3, A. Möri4, B. Frieg2, U. Frick2, H. Umeki5, K. Amano1, M.M. Cowper6 and J.A. Berry6 1 JNC (Japan Nuclear Cycle Development Institute), Tono Geoscience Centre, Toki, Gifu 509-5102, Japan. ([email protected]) 2 Nagra (National Co-operative for the Disposal of Radioactive Waste), 5430 Wettingen, Switzerland. 3 SAM (Safety Assessment Management Ltd), Bingham, Nottinghamshire NG13 8HQ, UK. 4 Geotechnical Institute, 3007 Bern, Switzerland. 5 JNC, Tokyo Office, Tokyo 100-8245, Japan. 6 AEA Technology, Harwell, Oxfordshire OX11 0RA, UK. ABSTRACT The joint Nagra/JNC Radionuclide Retardation Programme has now been ongoing for 15 years with the main aim of direct testing of radionuclide transport models in as realistic a manner as possible. A large programme of field, laboratory and natural analogue studies has been carried out at the Grimsel Test Site in the central Swiss Alps and the Kamaishi In Situ Test Site in north-east Japan. The understanding and modelling of both the processes and the structures influencing radionuclide transport/retardation in fractured host rocks have matured as has the experimental technology, which has contributed to develop confidence in the applicability of the underlying research models in a repository performance assessment. In this paper, the successes and set-backs of this programme are discussed as is the general approach to the thorough testing of the process models and of model assumptions. In addition, a set of key findings is presented, involving discussions on the enhancement of confidence through the program. INTRODUCTION Nagra (Swiss National Co-operative for the Disposal of Radioactive Waste) and JNC (Japan Nuclear Cycle Development Institute) have developed modelling approaches to radionuclide transport in fractured rocks with detailed geological, hydrological and geochemical characterisation of the geosphere. These approaches have been applied, for example, in the performance assessment (PA) of the proposed L/ILW (low- and intermediate-level radioactive waste) repository at Wellenberg, central Switzerland [1] and in the H-12 PA to establish the scientific and technical basis for HLW (high-level radioactive waste) disposal in Japan [2]. Despite the massive computing power now available, the current models of geosphere transport, which have been developed by these approaches, are abstractions of reality. The processes which are incorporated in the model and the geometry (or structure) within which these processes operate must be simplified because of the complexity of natural systems and the impossibility of complete characterisation of the geosphere. Within a repository PA, it is necessary to have confidence either that the simplifications have a negligible impact on the results or, if bounding estimates are acceptable, that the simplifications are conservative, leading to an overestimation of radionuclide releases and co
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