Browsing by Author "Ochs, M"

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    Comparative evaluation of surface complexation models for radionuclide uptake by diverse geologic materials
    (Elsevier, 2006-09) Payne, TE; Davis, JA; Ochs, M; Olin, M; Tweed, CJ; Altmann, S; Askarieh, MM
    This chapter summaries a major international modelling exercise, co-ordinated by the OECD Nuclear Energy Agency, in which independent scientific teams applied thermodynamic sorption models (TSMs) to a number of experimental adsorption data sets. A wide variety of models was employed to simulate and predict the data. In all the test cases, reasonable, broadly similar TSM approaches were adopted, but based on wide diversity of assumptions and methods of parameter estimation. The models were able to realistically, and with some predictive power, simulate the experimental data for a range of substrates, radionuclides and chemical conditions. However, sorption modelling has not reached a stage approaching standardisation. Basic features such as the stoichiometry and structure of surface complexes and mathematical formulations for such model components as the EDL are subject to debate. In addition, key model input parameters such as site populations are not well defined (particularly for natural substrates). As a result, the numerical values of optimised model parameters are highly model- dependent, which means that, in the present study, it has not been meaningful to compare individual model parameters (such as log K values for surface complexes). If a consensus is reached on model components, and uniform modelling approaches are adopted, it will be appropriate to do such a comparison. Until that time, modellers need to recognise that model parameters can typically not be used directly in other models, but that they need to be scaled or re-fitted. Although the modelling strategies differed among the teams, all were guided by a single principle, representation of sorption in terms of mass action and mass balance laws. The generally satisfactory results of this intercomparison suggest that these types of models have inbuilt chemical plausibility and predictive capabililty. © 2006, Elsevier Ltd.
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    Guidelines for thermodynamic sorption modelling in the context of radioactive waste disposal
    (Elsevier, 2013-01-01) Payne, TE; Brendler, V; Ochs, M; Baeyens, B; Brown, PL; Davis, JA; Ekberg, C; Kulik, DA; Lützenkirchen, J; Missana, T; Tachi, Y; Van Loon, LR; Altmann, S
    Thermodynamic sorption models (TSMs) offer the potential to improve the incorporation of sorption in environmental modelling of contaminant migration. One specific application is safety cases for radioactive waste repositories, in which radionuclide sorption on mineral surfaces is usually described using distribution coefficients (K-d values). TSMs can be utilised to provide a scientific basis for the range of K-d values included in the repository safety case, and for assessing the response of K-d to changes in chemical conditions. The development of a TSM involves a series of decisions on model features such as numbers and types of surface sites, sorption reactions and electrostatic correction factors. There has been a lack of consensus on the best ways to develop such models, and on the methods of determination of associated parameter values. The present paper therefore presents recommendations on a number of aspects of model development, which are applicable both to radioactive waste disposal and broader environmental applications. The TSM should be calibrated using a comprehensive sorption data set for the contaminant of interest, showing the impact of major geochemical parameters including pH, ionic strength, contaminant concentration, the effect of ligands, and major competing ions. Complex natural materials should be thoroughly characterised in terms of mineralogy, surface area, cation exchange capacity, and presence of impurities. During the application of numerical optimisation programs to simulate sorption data, it is often preferable that the TSM should be fitted to the experimentally determined K-d parameter, rather than to the frequently used percentage sorbed. Two different modelling approaches, the component additivity and generalised composite, can be used for modelling sorption data for complex materials such as soils. Both approaches may be coupled to the same critically reviewed aqueous thermodynamic data sets, and may incorporate the same, or similar, surface reactions and surface species. The quality of the final sorption model can be assessed against the following characteristics: an appropriate level of complexity, documented and traceable decisions, internal consistency, limitations on the number of adjustable parameter values, an adequate fit to a comprehensive calibration data set, and capability of simulating independent data sets. Key recommendations for the process of TSM development include: definition of modelling objectives, identification of major decision points, a clear decision-making rationale with reference to experimental or theoretical evidence, utilisation of a suitable consultative and iterative model development process, testing to the maximum practicable extent, and thorough documentation of key decisions. These recommendations are consistent with international benchmarks for environmental modelling. Copyright © 2013, Elsevier
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    Use of thermodynamic sorption models to derive radionuclide Κd values for performance assessment: selected results and recommendations of the NEA sorption project
    (Oldenbourg Verlag, 2006-03) Ochs, M; Davis, JA; Olin, M; Payne, TE; Tweed, CJ; Askarieh, MM; Altmann, S
    For the safe final disposal and/or long-term storage of radioactive wastes, deep or near-surface underground repositories are being considered world-wide. A central safety feature is the prevention, or sufficient retardation, of radionuclide (RN) migration to the biosphere. To this end, radionuclide sorption is one of the most important processes. Decreasing the uncertainty in radionuclide sorption may contribute significantly to reducing the overall uncertainty of a performance assessment (PA). For PA, sorption is typically characterised by distribution coefficients (K-d values). The conditional nature of Kd requires different estimates of this parameter for each set of geochemical conditions of potential relevance in a RN's migration pathway. As it is not feasible to measure sorption for every set of conditions, the derivation of K-d for PA must rely on data derived from representative model systems. As a result, uncertainty in Kd is largely caused by the need to derive values for conditions not explicitly addressed in experiments. The recently concluded NEA Sorption Project [1] showed that thermodynamic sorption models (TSMs) are uniquely suited to derive Kd as a function of conditions, because they allow a direct coupling of sorption with variable solution chemistry and mineralogy in a thermodynamic framework. The results of the project enable assessment of the suitability of various TSM approaches for PA-relevant applications as well as of the potential and limitations of TSMs to model RN sorption in complex systems. © 2006, Oldenbourg Verlag