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Schlieren Zurich, Switzerland

Tachi Y.,Japan Atomic Energy Agency | Ochs M.,BMG Engineering Ltd. | Suyama T.,Japan Atomic Energy Agency
Journal of Nuclear Science and Technology | Year: 2014

To predict the long-term migration of radionuclides (RNs) under variable conditions within the framework of safety analyses for geological disposal, thermodynamic sorption models are very powerful tools. The integrated sorption and diffusion (ISD) model for compacted bentonite was developed to achieve a consistent combination of clay-water interaction, sorption, and diffusion models. The basic premise considered in the ISD model was to consistently use the same simple surface model design and parameters for describing RNs sorption/diffusion as well as clay surface and porewater chemistry. A simple 1-site non-electrostatic surface complexation model in combination with a 1-site ion exchange model was selected to keep sorption model characteristics relatively robust for compacted systems. Fundamental parameters for the proposed model were evaluated from surface titration data for purified montmorillonite. The resulting basic model was then parameterized on the basis of selected published sorption data-sets for Np(V), Am(III), and U(VI) in dispersed systems, which cover a range of key geochemical conditions such as pH, ionic strength, and carbonate concentration. The sorption trends for these RNs can be quantitatively described by the model considering a full suite of surface species including hydrolytic and carbonate species. The application of these models to the description of diffusive-sorptive transport in compacted bentonites is presented in Part 2. © 2014 Atomic Energy Society of Japan. All rights reserved.

Tachi Y.,Japan Atomic Energy Agency | Yotsuji K.,Japan Atomic Energy Agency | Suyama T.,Japan Atomic Energy Agency | Ochs M.,BMG Engineering Ltd.
Journal of Nuclear Science and Technology | Year: 2014

It is important to understand the coupled processes of sorption and diffusion of radionuclides (RNs) in compacted bentonite, and to develop mechanistic models that can aid in the prediction of the long-term performance of geological disposal systems of radioactive waste. The integrated sorption and diffusion (ISD) model was developed based on the consistent combination of clay-water interaction, sorption and diffusion models. The diffusion model based on the electrical double layer theory describing relative ionic concentrations and viscoelectric effects at the negatively charged clay surface was coupled with porewater chemistry and sorption models. This ISD model was successfully tested for various actinides with a complex chemistry (Np(V), Am(III), U(VI) under conditions where variably charged carbonate complexes are formed) considered in Part 1, by using published diffusion and sorption data ((D a, De, Kd) as a function of partial montmorillonite density. Quantitative agreements were observed by considering uncertainty in porewater chemistry and dominant aqueous species. It can therefore be concluded that the ISD model developed here is able to adequately explain the sorption and diffusion behavior of various RNs with a complex chemistry in compacted bentonites. The performed modeling indicates that uncertainties are mainly related to porewater chemistry and RN speciation and that these parameters need to be carefully evaluated. © 2014 Atomic Energy Society of Japan. All rights reserved.

Tachi Y.,Japan Atomic Energy Agency | Nakazawa T.,Mitsubishi Group | Ochs M.,BMG Engineering Ltd. | Yotsuji K.,Japan Atomic Energy Agency | And 4 more authors.
Radiochimica Acta | Year: 2010

Diffusion and sorption of radionuclides in compacted bentonite/ montmorillonite are key processes in the safe geological disposal of radioactive waste. In this study, the effects of carbonate and salinity on neptunium(V) diffusion and sorption in compacted sodium montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients (D e) and distribution coefficients (Kd) of 237Np(V) in sodium montmorillonite compacted to a dry density of 800 kgm-3 were measured under four chemical conditions with different salinities (0.05/0.5M NaCl) and carbonate concentrations (0/0.01M NaHCO 3). De values for carbonate-free conditions were of the order of 10-10-10-11 m2 s-1 and decreased as salinity increased, and those for carbonate conditions were of the order of 10-11-10-12m2s-1 and showed the opposite dependence. Diffusion-derived Kd values for carbonate-free conditions were higher by one order of magnitude than those for carbonate conditions. Diffusion and sorption behaviors were interpreted based on mechanistic models by coupling thermodynamic aqueous speciation, thermodynamic sorption model (TSM) based on ion exchange, and surface complexation reactions, and a diffusion model based on electrical double layer (EDL) theory in homogeneous narrow pores. The model predicted the experimentally observed tendency of De and Kd qualitatively, as a result of the following mechanisms; 1) the dominant aqueous species are NpO+ 2 and NpO2CO- 3 for carbonate-free and carbonate conditions, respectively, 2) the effects of cation excess and anion exclusion result in opposite tendencies of De for salinity, 3) higher carbonate in solution inhibits sorption due to the formation of carbonate complexes. ©by Oldenbourg Wissenschaftsverlag, München.

Payne T.E.,Australian Nuclear Science and Technology Organisation | Brendler V.,Helmholtz Center Dresden | Ochs M.,BMG Engineering Ltd. | Baeyens B.,Paul Scherrer Institute | And 9 more authors.
Environmental Modelling and Software | Year: 2013

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 (Kd values). TSMs can be utilised to provide a scientific basis for the range of Kd values included in the repository safety case, and for assessing the response of Kd 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 Kd 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. © 2013 Elsevier Ltd.

Tachi Y.,Japan Atomic Energy Agency | Ochs M.,BMG Engineering Ltd. | Suyama T.,Japan Atomic Energy Agency | Trudel D.,BMG Engineering Ltd.
Materials Research Society Symposium Proceedings | Year: 2014

The use of generic sorption data in PA requires the transfer of the data to the PA-specific conditions. A site-specific Kd setting approach for PA calculations was tested, comparing two data transfer procedures. First transfer of sorption data can be done through semi-quantitative estimation procedures, by considering differences between experimental and PA geochemical conditions (sorption capacity, radionuclide speciation, competitive reactions, etc.). On the other hand, thermodynamic sorption models allow to estimate Kd variations directly, based on quasi-mechanistic understanding. The present paper focuses on illustrating example calculations regarding the derivation of Kd values, and their uncertainties, of Cs, Ni, Am and Th, for the mineralogical and geochemical conditions of the mudstone system at the Horonobe URL. Clay minerals (illite and smectite) were considered as sorption-relevant minerals in all cases. The Kd setting results were compared with Kd measured for Horonobe mudstone by batch experiments. The results indicate that Kd can be quantitatively evaluated from generic sorption data when adequate data and models are available. The careful evaluation and conjunctive use of calculated and measured Kd values can enhance the reliability of Kd setting and uncertainty assessments. © 2014 Materials Research Society.

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