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Thermodynamic Sorption Modelling in Support of Radioactive Waste Disposal Safety Cases
NEA Sorption Project Phase III
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- Table of contents
- Overview and purpose
- + 1. Thermodynamic sorption models and radionuclide migration
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2. Theoretical basis of TSMs and options in model development
- 2.1. Conceptual building blocks of TSMs and integration with aqueous chemistry
- 2.2. The TSM representation of sorption and relationship with Kd values
- 2.3. Theoretical basis of TSMs
- 2.4. Example of TSM for uranyl sorption
- 2.5. Options in TSM development
- 2.6. Illustration of TSM development and effects of modelling choices
- 2.7. Summary: TSMs for constraining Kd values – impact of modelling choices
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3. Determination of parameters for TSMs
- 3.1. Overview of experimental determination of TSM parameters
- 3.2. Theoretical estimation methods of selected model parameters
- 3.3. Case study: sorption modelling of trivalent lanthanides/ actinides on illite
- 3.4. Indicative values for certain TSM parameters
- 3.5. Parameter uncertainty
- 3.6. Illustration of parameter sensitivity and uncertainty analysis
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4. Approaches for applying TSMs to intact and complex materials
- 4.1. Introduction
- 4.2. Real substrates (What makes them complex?)
- 4.3. Determination and estimation of TSM parameters in real systems
- 4.4. TSM application for Kd estimation on chemically complex substrates
- 4.5. Sorption modelling in clay rocks and compacted clay systems
- 4.6. TSM applications to crystalline host rocks
- 4.7. Application of generalised TSMs to complex and intact materials:concluding remarks and key messages
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5. General guidelines and recommendations
- 5.1. Framework for TSM applications in the context of radioactive wastedisposal
- 5.2. Purposes and predictive capabilities of TSMs
- 5.3. Strategies for building TSMs
- 5.4. Recommendations
- 5.5. Building a scientifically defensible, calibrated TSM for radioactive wasteapplications
- 5.6. Concluding comments
- 6. References
- + 7. Annexes
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List of figures
- 1.1. Sensitivity of Kd for U(VI) sorption on geochemical conditions
- 1.2. Generic radioactive waste disposal concept
- 2.1. Inner sphere and outer sphere surface complexes on a metal oxidesurface are distinguished by the types of chemical bonding between the aqueousion and the surface site
- 2.2: Illustration of the co-ordination of hydroxyl groups on a particular (110)surface of goethite
- 2.3: Schematic representation of a 2:1 clay mineral platelet with surface sites indicated
- 2.4: Model of U(VI) sorption on ferrihydrite, showing the computation of both(a) percentage sorbed and (b) Kd using a TSM approach
- 2.5: Schematic representation of sources of uncertainty
- 2.6: Generalised decision tree for TSM modelling choices
- 2.7: Results from five different TSM fitting strategies
- 3.1: Sorption edge data for Zn measured on Na-montmorilloniteat 0.1 M NaClO4
- 3.2: LFERs for cation adsorption on clays
- 3.3: Titration data for Na-illite carried out in 0.01 M, 0.1 M and 0.5 M NaClO4
- 3.4: Eu(III)/Am(III) sorption edges on Na-illite measured in 0.1 M NaClO4
- 3.5: Sorption isotherm of Eu(III) (labelled with 241Am) on Na-illitein 0.1 M NaClO4 at pH 6.1
- 3.6: SSA histogram for amorphous mineral coatings (upper), clay minerals(central) and for crystalline oxides (lower)
- 3.7: Normalised ranking of the sensitivity of input parameters as a function of pH, for two different solid-liquid ratios
- 4.1: Illustration of various aspects of complexity using a hypothetical substrate
- 4.2: Normalised ranking of the sensitivity of input parameters for the CA approach test case as a function of pH
- 4.3: Normalised ranking of the sensitivity of input parameters for the GC approach test case as a function of pH
- 4.4: Experimental data (circles) for adsorption of uranium(VI) on a weathered schist as a function of pH
- 4.5: Experimental data (circles and diamonds) for adsorption of uranium(VI) on a weathered schist as a function of pH
- 4.6: Experimental data (filled symbols) for adsorption of uranium(VI) on analluvial aquifer sediment as a function of U(VI) concentration and partial pressure ofCO2 gas equilibrated with synthetic groundwater solutions
- 4.7: Simplified sketch of the Opalinus Clay structure at the micrometre scale
- 4.8: Comparison of Kd (Rd) values of Na and Sr on Opalinus Clay [Mont Terri (MT)and Benken (BE)] obtained in diffusion measurements (in the laboratory and in thefield), batch sorption experiments, and from geochemical calculations
- 4.9: Cs sorption isotherm on Opalinus Clay (OPA)
- 4.10: Cs sorption on Horonobe mudstone; comparison between Kd valuesmeasured by batch sorption and transient diffusion (TD) experiments for GW-A (ionicstrength; 0.024M) and GW-B (0.24M), and modelled by the clay-based CA model
- 4.11: Diffusion-derived Kd values, modelled Kd values, and surface speciationof Np(V) in compacted montmorillonite in different pore water soluti
- 4.12: Schematic representation of RN migration pathways and sorption incrystalline rock
- 5.1: Overview flowchart of TSM modelling framework and resulting approaches
- 5.2: Detailed flowchart to determine the expected range of geochemical conditions for TSM model setup
- 5.3: Detailed flowchart to develop and parameterise a specific TSM
- 5.4: Detailed flowchart to evaluate a specific TSM
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List of tables
- 2.1: Summary of surface reactions of uranium sorption on ferrihydrite
- 2.2: Summary of model choices applied to Np(V) sorption onto hematite
- 3.1: Number of additional EDL parameters required to describe surfaceproperties in contact with a 1:1 electrolyte
- 3.2: A typical range of system properties considered in the numerical optimisation process
- 3.3: Indicative values for some TSM parameters of important minerals ormineral groups (marked by *), uncertainties represent 2σ (95% confidence level)
- 3.4: Illustrative example – parameter set
- 3.5: Kd distributions (logarithmic) of Np (mean value and uncertainties) as a function of pH and solid-liquid ratio (SLR)
- 4.1: Parameter set for test case of the component additivity (CA) approach (upper) and for the test case of the generalised composite (GC) approach (lower)
- 4.2: Kd distributions (logarithmic) of Co (mean value and uncertainties) as a function of pH for both the GC and the CA approach
- 4.3: Overview of some major characteristics of clay rich rock types and bentonite
- 5.1: Generic recommendations
- 5.2: Advice on an acquisition programme for sorption data and TSM parameterisation
- 5.3: Advice on model development and application
A central safety function of radioactive waste disposal repositories is the prevention or sufficient retardation of radionuclide migration to the biosphere. Performance assessment exercises in various countries, and for a range of disposal scenarios, have demonstrated that one of the most important processes providing this safety function is the sorption of radionuclides along potential migration paths beyond the engineered barriers. Thermodynamic sorption models (TSMs) are key for improving confidence in assumptions made about such radionuclide sorption when preparing a repository's safety case. This report presents guidelines for TSM development as well as their application in repository performance assessments. They will be of particular interest to the sorption modelling community and radionuclide migration modellers in developing safety cases for radioactive waste disposal.
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Publication year : 2012
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