Objective
The objective is the development of mathematical models for radionuclide migration from underground repositories for radioactive waste to the accessible environment by colloids in groundwater. The model development is to be supported by migration experiments in laboratory, its validity will be evaluated against field data.
The model must be able to interprete laboratory and field experiments, and also to be included in geosphere transport code for safety assessment. For this reason a series of codes, from detailed to simplified, must be developed and validated successively, at different scales.
A simple one dimensional model has been developed, describing the transport of a radionuclide through a saturated porous medium - groundwater - colloid system with stationary groundwater flow. The colloid concentration can be varied in time and space and also the mass per colloid can be specified. Both nonlinear and linear sorption of radionuclides can be included. The following model assumptions are made: one type of colloid/one type of radionuclide; no speciation of the contaminant (sorption excluded); dispersion/diffusion coefficients are different for colloids and radionuclides; radionuclides do not distinguish between mobile and immobile colloids so their sorption is described by the same sorption isotherm; attachment and detachment of colloids to/from the solid phase can be described by a first order reversible equation.
An experimental study has been performed using a chromatographic technique, and concerned the distribution coefficients measurement. Impulse experiments were made to determine the number percentage of particles passing through the bed for the 3 different latex samples at 4 different values of ionic strengths, 2 different values of pH, and for a range of flow rates. Due to surface reaction with the glass beads the pH of the outlet solutions was always higher than the inlet pH. The higher the ionic strength and the lower the pH the greater the possibility for a given size of latex to be captured. In addition, extensive static batch experiments failed to show any detectable adsorption of latexes on glass beads. Another available result is the percentage of the particles passing through the bed as a function of ionic strength and flow rate: it confirms that the capture of colloids is increased by increasing the ionic strength. Another result is that, in a given column, an increase in the flow rate reduces the capture of the colloids. The higher the flow velocity the higher the Sherwood number: the lower the ionic strength and the higher the pH the lower is the Sherwood number with respect to that which would be expected.
With this aim a simple transport model has been formulated to evaluate the migration of pollutant and colloids in a fracture. It takes into account 3 phases: a solute phase containing radionuclides, a colloidal phase, and a porous medium as solid phase. The kinetics are not included in this model. The model is based on 2 types of equations: the equilibrium relationships, linking concentration of radionuclides, radionuclides adsorbed on colloids and radionuclides adsorbed in the porous matrix, and the mass balance equations. This model leads to a nonlinear system. To get a linear model and to determine the retention time of radionuclides, the colloid concentration is assumed to be constant. This assumption allows one to formulate a transfer function and to express the concentration of dissolved radionuclides. The first experimental work focused on the measure of the distribution coefficients between colloidal, solute and solid phases. The parameters which effectively control the transport were found to be the colloid concentration in mobile phase and the values of the linear coefficients.
Work programme:
1. Literature survey.
2. Formulation of a first conceptual model. Screening of phenomena to be included in the model by performing simple calculations of test cases.
3. Planning of laboratory migration experiments with a simplified fixed solid phase. Research of optimal experimental conditions with the help of task 2.
4. Laboratory migration experiments focusing on the study of mechanisms for advection-dispersion of particles, on the interaction between particles and fixed solid phase, and agglomeration and sedimentation of particles.
5. Formulation of a second conceptual model, computer programming, numerical verification of the computer code and tests against the laboratory experiments (task 4 and 6).
6. Planning and performing laboratory experiments using field material as fixed solid phase. Verification of the relevance of the second conceptual model. Compilation on a field data base for model verification.
7. Development of a model for simulating field experiments. Application of this computer code to field experiments and to the compiled data base.
8. Development of a colloid migration model for performance assessment. Application of the code to some relevant performance assessment scenarios.
9. Project management.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
- engineering and technology other engineering and technologies nuclear engineering nuclear waste management
- natural sciences earth and related environmental sciences geology sedimentology
- natural sciences physical sciences condensed matter physics soft matter physics
- natural sciences chemical sciences nuclear chemistry radiation chemistry
- natural sciences mathematics applied mathematics mathematical model
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Coordinator
77305 FONTAINEBLEAU
France
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