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Natural Tracer Profiles Across Argillaceous Formations: The CLAYTRAC Project
OECD Publishing, March 2009, Pages: 362
This technical report describes the results of the Nuclear Energy Agency's CLAYTRAC project, in which natural tracer data from nine sites was evaluated to assess potential impacts of disposal of radiological waste in geological repositories. It shows scientific information from numerous sites and applies robust analytical methods to improve the understanding of radionuclide migration and evolution of sites for deep geological disposal. These results improve the understanding of sites, and thus the confidence in safety, for geological disposal of radioactive waste.
An important aspect of assessing the long-term safety of deep geological disposal of radioactive waste is developing a comprehensive understanding of the geological environment in order to define the initial conditions for the disposal system as well as to provide a sound scientific basis for constraining its future evolution. The NEA Working Group on the Characterisation, the Understanding and the Performance of Argillaceous Rocks as Repository Host Formations (the NEA Clay Club) is devoted to improving the scientific basis for clay host rocks in the context of geological disposal. The understanding of the transport pathways and mechanisms by which contaminants could migrate in the geosphere is a key element in any performance assessment and safety case. Relevant experiments in laboratories or underground test facilities can provide important information, but the challenge remains in being able to extrapolate the results to the spatial and temporal scales required for performance assessment, which are typically tens to hundreds of metres and from thousands to beyond a million years into the future. Profiles of natural tracers dissolved in pore water of argillaceous rock formations can be considered as large-scale and long-term natural experiments which enable the transport properties to be characterised. That is, the tracer profiles can be subjected to quantitative analysis and yield information on the dominant transport processes and pathways, as well as on key transport parameters such as the diffusion coefficient. Such situations can be conceived as natural analogues of solute transport experiments, offering the potential to bridge the gap in spatial and temporal scales between laboratory experiments and the needs for modelling and performance assessment.
The CLAYTRAC project on Natural Tracer Profiles Across Argillaceous Formations was established by the NEA Clay Club with the objective to evaluate the relevance of natural tracer data in constraining an understanding of past geological evolution and in confirming the dominant transport processes. An internally consistent methodology for data processing and evaluation was applied to nine argillaceous sites for which significant data was available regarding the spatial distribution of tracers in pore water. Emphasis was placed on the integrated understanding based on the whole suite of tracers available at any specific site. The results provide powerful evidence of non-sorbing solute transport and water movement in clay-rich rocks. Moreover, the interpretation of natural tracers is, overall, scientifically robust and consistent with established physical concepts. The relative advantages and disadvantages of various tracers have been evaluated in terms of sampling, analysis and interpretation.
The outcomes of the project show that, for the sites and clay-rich formations that were studied, there is strong evidence that solute transport is controlled mainly by diffusion; the results can improve site understanding and performance assessment in the context of deep geological disposal and have the potential to be applied to other sites and contexts.
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