Geochemical and coupled transport modelling

In the research field of geochemical and coupled transport modelling numerical computer codes are used to study the behaviour of contaminants in and surrounding a disposal or storage system. These codes use basic principles of thermodynamics, kinetics and solute transport to simulate processes such as:

• Water-rock interactions
• Evolution of engineered barriers (waste form, container, backfill – insert link to glossary) under disposal/storage conditions
• Migration of contaminants towards man and environment

Apart from its obvious application in prediction, modelling has been increasingly used as a tool to integrate individual processes or results of laboratory research so that the sensitivity of each process to the behaviour of a coupled system can be evaluated. The modelling activities involve both the development of the modelling tools and the application of the tools to research projects related to geological and surface disposal of radioactive waste.

Development of modelling tools

• SCK•CEN is a member of the research consortium at the University of Illinois where the reactive transport computer code The Geochemist's Workbench® is being developed. The code involves a set of software tools for manipulating chemical reactions, calculating stability diagrams and the equilibrium states of natural waters, tracing reaction processes, and modelling reactive transport in 1 and 2 dimensions.
• A one-dimensional reactive transport code HP1 is developed in-house emphasising capabilities to simulate reactive transport for variably saturated water flow. The code combines two existing codes: HYDRUS-1D 2.0 and PHREEQC-2.4 .
• A self-consistent and state of the art thermochemical database is being compiled. The database is based on the EQ3/6 database with additions of new data or data of improved quality, e.g., the outcomes of the thermochemical database review at Nuclear Energy Agency

Applications of numerical reactive transport computer codes to research projects

• Reaction modelling is performed to establish the reference pore water geochemistry in Boom clay, a reference host formation for geological disposal of radioactive waste in Belgium. Pore water data collected at the underground laboratory are well described by a model taking mineral dissolution/precipitation, ion exchange and redox reactions into account.
• Reaction modelling also finds its application in describing radionuclide behaviours such as solubility, sorption, complex formation with organic and inorganic ligands. Established models such as surface complexation models, ion exchange models, and metal-natural organic matter binding models are used in simulating laboratory batch experiments.

• Diffusion- sorption modelling is carried out to represent migration profiles of radionuclides in a compact clay core. This modelling tests the sorption models and parameters extracted from batch experiments in a diffusion experiment.

• Reactive diffusion simulation is done to estimate the lifetime of a concrete engineered barrier designed for the high level waste disposal in Boom clay.
• Reactive diffusion modelling is also performed to predict the extent of an alkaline plume perturbation into the clay host formation due to the cement-clay interactions.
• Migration of alkaline plume through a compact clay core is simulated to mimic a column experiment where cement waters have been infiltrated a clay core for more than 10 years.

• Modelling is in progress to evaluate the degradation of cement waste forms in terms of contaminants retention under a surface disposal condition.

• The coupled code HP1 has been used to simulate a long-term transient flow and transport of major cations and heavy metals in a soil.

• Ongoing modelling is focused on describing geochemical processes originating from perturbations on the host geological formation as a result of waste disposal or storage. These processes include oxidation of pyrite containing clay, effects of heat generating waste on host clay geochemistry, e.g., CO₂ production, effects of NaNO₃ waste on host clay properties.

Contact: Wang Lian