Chemical Interactions in the Near-Field of a Repository for Spent Nuclear Fuel - A Modeling Study
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INTRODUCTION It has been planned to dispose of spent nuclear fuel in Finland in a repository at a depth of about 500 m in crystalline bedrock. The near-field barriers in the excavated space comprise a compacted bentonite buffer surrounding a copper-lined iron canister, the canister itself and the spent fuel matrix (U0 2). These barriers interact via groundwater and evolve towards thermodynamic equilibrium with each other and with the geochemical environment. The canister is predicted to remain intact for a very long period of time in the base case of the safety assessment. In one conceivable scenario, in which a defect in a canister has remained undetected, the groundwater could come into contact with the spent nuclear fuel. The fuel matrix (U0 2 ) and the iron inside the copper canister, may act as redox buffers and help to maintain some of the radionuclides in their low-solubility forms, thus contributing to lower radionuclide releases. The evolution of the near-field chemistry towards thermodynamic equilibrium depends on both the chemical reactions and mass transport phenomena. The transport of solutes in the very slowly moving groundwater is, in practice, the only way by which the radionuclides of the spent fuel matrix can be released into the far-field. When assessing the safety of the repository, knowledge of the rates at which radionuclides will be released into the groundwater is needed. In safety assessment, the radionuclide solubilities are important parameters, which depend on the chemical conditions that develop in the near-field. The objective of this work is to model the near-field chemistry arising from interactions between the groundwater, bentonite, canister and the spent fuel using the groundwater and bentonite considered in the Finnish nuclear waste disposal concept. The outcome of this study is the chemistry in the solution contacting the spent fuel, the actinide solubilities and the effect of the type of synthetic groundwater on them. MODELLING AND RESULTS The near-field chemical interactions have usually been modelled by taking into account coexisting transport phenomena and chemistry [1]. These coupled models often calculate the transport of solutes in porous media relatively accurately, but frequently oversimplify the 123 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
chemical interactions of the aqueous species and their heterogeneous reactions with the solid phases. In this study, the equilibrium modelling of chemical interactions in the near-field has been realized by means of a sequential-closed-systems approach [2], which ignores solute transport but provides a detailed description of the equilibrium chemistry. The groundwater and the three barriers of the near-field were accounted for by this approach with particular emphasis given to the pH, Eh and actinide solubilities in the water in contact with spent fuel. The effects of water chemistry inside the canister as well as water radiolysis on fuel dissolution were included in the model. The groundwater (either f
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