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The UFD Generic Disposal System Level Modeling group has been working on developing modeling tools for performance assessment (PA) of nuclear waste disposal in deep geological repositories. This development requires improved methodologies for quantifying radionuclide transport through the hosting geological formation. Because clay rocks, being considered potential host rocks in many countries, have very low permeability, diffusion is the main transport mechanism for radionuclide transport in these formations. Consequently, adequately modeling diffusive transport processes in clay rocks is critical for PA. The major focus of LBNL in this area is to support development of the system-level model for clay formation by investigating methodologies for modeling diffusion processes in clay rock.
Modeling of diffusive transport in clay rocks is complicated by the existence of heterogeneities at different scales and coupling between diffusive and electro-chemical processes (Appelo et al., 2010). At a local scale, different pore spaces co-exist within a representative elementary volume (REV), or a “point” within the context of continuum mechanics. They include pore spaces surrounded by grains other than clay, pore spaces surrounded by clay and other grains, pore spaces surrounded by clay grains only, and interlayer spaces within clay grains. Dominant transport processes of radionuclide can be quite different for different pore spaces. For example, the coupling between diffusive and electro-chemical processes, or interaction between diffusion in bulk fluid and electrical double diffusion layer near the clay surfaces, is negligible for pores surrounded by other grain particles, but critical for small pores surrounded by clay particles and inter-layer spaces. The last two pore-spaces are especially important for compacted clay systems (such as clay buffers in EBS). At large scales, diffusive transport may be subject to a considerable degree of spatial variability as a result of variability of physical and chemical properties of pore spaces. It is also well known that large-scale diffusive process is not isotropic in clay rocks. Diffusion coefficient is much larger along the bedding direction than that in the direction perpendicular to the bedding.
Keeping in mind that the modeling approach developed here will be used for PA purpose, our efforts are not focused on detailed process modeling of diffusion in clay, but phenomenological approaches based on Fick’s diffusion law and using semi-empirical constants to roughly incorporate the effects of electro-chemical processes. Our current research activities in this area includes (1) literature survey of effective parameter values for diffusion in clay, (2) incorporation of impacts of heterogeneity and electro-chemical processes into system-level models, and (3) validation of system-level models by collaboration with other participants within the UFD system-level modeling group.
Comparison between calculated (curves) and observed (rectangles) values for ration of diffusion coefficient as a function of charge number zi. The calucation is based on a newly developed approach with an assumption of a constant electronic potential in a clay formation.
|Liange Zheng||Staff Scientist||Hydrogeology Department||510-486-5502||510-486-5686||LZheng@lbl.gov|