Microtomography-based Inter-Granular Network for the simulation of radionuclide diffusion and sorption in a granitic rock
Field investigation studies, conducted in the context of safety analyses of deep geological repositories for nuclear waste, have pointed out that in fractured crystalline rocks sorbing radionuclides can diffuse surprisingly long distances deep into the intact rock matrix; i.e. much longer distances...
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Veröffentlicht in: | Journal of contaminant hydrology 2017-12, Vol.207, p.8-16 |
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Sprache: | eng |
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Zusammenfassung: | Field investigation studies, conducted in the context of safety analyses of deep geological repositories for nuclear waste, have pointed out that in fractured crystalline rocks sorbing radionuclides can diffuse surprisingly long distances deep into the intact rock matrix; i.e. much longer distances than those predicted by reactive transport models based on a homogeneous description of the properties of the rock matrix. Here, we focus on cesium diffusion and use detailed micro characterisation data, based on micro computed tomography, along with a grain-scale Inter-Granular Network model, to offer a plausible explanation for the anomalously long cesium penetration profiles observed in these in-situ experiments. The sparse distribution of chemically reactive grains (i.e. grains belonging to sorbing mineral phases) is shown to have a strong control on the diffusive patterns of sorbing radionuclides. The computed penetration profiles of cesium agree well with an analytical model based on two parallel diffusive pathways. This agreement, along with visual inspection of the spatial distribution of cesium concentration, indicates that for sorbing radionuclides the medium indeed behaves as a composite system, with most of the mass being retained close to the injection boundary and a non-negligible part diffusing faster along preferential diffusive pathways.
•X-ray micro-CT analyses are used to map the inter-granular porosity of a granitic rock.•The resulting Inter-Granular Network is used to run cesium diffusion experiments.•Biotite, which provides sorption sites, is sparsely distributed.•Anomalously long cesium penetration profiles are observed.•The model offers a possible explanation to observations from in-situ experiments. |
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ISSN: | 0169-7722 1873-6009 1873-6009 |
DOI: | 10.1016/j.jconhyd.2017.10.003 |