Harmonisation of Site Characterisation and Performance Assessment Modelling - The Relative Importance of Surface Sorptio
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James Crawford, Luis Moreno and Ivars Neretnieks Department of Chemical Engineering and Technology Royal Institute of Technology, S-10044 Stockholm, Sweden $%675$&7 In the context of geological repositories for nuclear waste disposal, the goal of site characterisation (SC) is to obtain input data that can later be used as a basis for performance assessment calculations (PA). Performance assessment is required to give an indication as to whether the repository will behave as intended over the geological timescales relevant for risk analysis. Processes that may be important in a PA setting for constraining radionuclide transport may not necessarily be dominating, or indeed may not even be observable during SC investigations. In this paper it is shown that the migration of sorbing tracers is governed largely by surface sorption phenomena in typical SC-type field experiments whereas in a PA scenario, matrix interaction instead can be expected to play an overwhelmingly important role. This study uses data and settings that are representative for fractured rocks in Sweden, but the method used and the conclusions may have more general applicability. ,1752'8&7,21 The goal of site characterisation (SC) is to obtain input data that can later be used in performance assessment (PA) calculations. This input data generally takes the form of parameter values describing the physical and geochemical properties of the studied rock volume. Performance assessment calculations are frequently based upon the premise that there may be one, or more, initially defective containment structures that can leak radionuclides to the surrounding rock. If this scenario occurs, then the properties of the surrounding rock volume that comprises the natural barrier for radionuclide migration is of overwhelming importance for the operational safety of a high level waste (HLW) repository after closure. In the present context we will focus upon an HLW repository of the type that is being planned for construction in Sweden. The Swedish concept for the disposal of high-level nuclear waste (the KBS - 3 system) involves encapsulation of spent fuel rods in corrosion-resistant ironcopper canisters surrounded by a bentonite clay buffer at a depth of approximately 500 metres in crystalline bedrock. The canisters are to be located in deposition holes that will be bored in the floors of a system of tunnels comprising the repository. Frequently, the parameter values necessary for PA are obtained from interpretation of field tests and experiments using theoretical models that attempt to capture the essential physics of the flow and transport system. Solute transport retardation processes that may be important in a PA setting may not necessarily be dominating processes or may not even be observable in the SC phase. There is therefore a continuing need to bridge the conceptual gaps between the models used to inte
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