Characterisation of Radionuclide Migration and Plant Uptake for Performance Assessment
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Characterisation of Radionuclide Migration and Plant Uptake for Performance Assessment Simon A. Mathias, Andrew M. Ireson, Adrian P. Butler, Bethanna M. Jackson, Howard S. Wheater Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, U.K. ABSTRACT Unsaturated vegetated soils are an important component in performance assessment models used to quantify risks associated with deep engineered repositories for underground radioactive waste disposal. Therefore, experimental studies, funded by Nirex over nearly 20 years, have been undertaken at Imperial College to study the transfer of radionuclides (Cl-36, I-129, Tc-99) from contaminated groundwater into crops. In parallel to this has been a modelling programme to aid interpretation of the experimental data, obtain parameter values characterising transport in soil and plant uptake and provide new representations of near-surface processes for performance assessment. A particular challenge in achieving these objectives is that the scale of the experimental work (typically ≤ 1m) is much smaller than that required in performance assessment. In this paper, a new methodology is developed for upscaling model results obtained at the experimental scale for use in catchment scale models. The method is based on characterising soil heterogeneity using soil texture. This has the advantage of allowing hydrological and radionuclide transport parameters to be correlated in a consistent manner. An initial investigation into the calculation of effective (i.e. upscaled) hydrological and transport parameters has been undertaken and shows the results to be potentially highly (and non-linearly) sensitive to soil properties. Consequently, they have important implications for future site characterisation programmes supporting a proposed underground waste repository. INTRODUCTION To date, experimental studies at Imperial College have focussed on understanding the processes that control upward migration and uptake of radionuclides in vegetated soils. Associated with this work has been a modelling programme concerned with analysis of the experimental data and the representation and parameterisation of those processes in physicallybased models [2]. This work has now reached an important juncture, as several of the key elements of interest in radioactive waste disposal (i.e. chlorine, iodine, technetium, and selenium) have been studied using laboratory soil columns and (in the case of chlorine) field lysimeters. However, in order for this work to feed into and inform a future safety assessment, a clear, consistent and defensible methodology is needed for producing model-grid scale parameters from laboratory and lysimeter scale data. There is a vast array of different upscaling techniques in the literature reflected by the correspondingly large number of review papers on the subject (see [7] and references therein). In this paper, we adopt the stream-tube approach to explore the upscaled response of a horizontally heterogeneous field of lysimeter experiments simila
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