Modeling Transport of Cesium in Grimsel Granodiorite With Micrometer Scale Heterogeneities and Dynamic Update of K_d

Abstract

Transport and retardation of cesium in Grimsel granodiorite taking into account heterogeneity of mineral and pore structure was studied using rock samples overcored from an in situ diffusion test at the Grimsel Test Site. The field test was part of the Long‐Term Diffusion (LTD) project designed to characterize retardation properties (diffusion and distribution coefficients) under in situ conditions. Results of the LTD experiment for cesium showed that in‐diffusion profiles and spatial concentration distributions were strongly influenced by the heterogeneous pore structure and mineral distribution. In order to study the effect of heterogeneity on the in‐diffusion profile and spatial concentration distribution, a Time Domain Random Walk (TDRW) method was applied along with a feature for modeling chemical sorption in geological materials. A heterogeneous mineral structure of Grimsel granodiorite was constructed using X‐ray microcomputed tomography (X‐μCT) and the map was linked to previous results for mineral specific porosities and distribution coefficients (Kd) that were determined using C‐14‐PMMA autoradiography and batch sorption experiments, respectively. After this the heterogeneous structure contains information on local porosity and Kd in 3‐D. It was found that the heterogeneity of the mineral structure on the micrometer scale affects significantly the diffusion and sorption of cesium in Grimsel granodiorite at the centimeter scale. Furthermore, the modeled in‐diffusion profiles and spatial concentration distributions show similar shape and pattern to those from the LTD experiment. It was concluded that the use of detailed structure characterization and quantitative data on heterogeneity can significantly improve the interpretation and evaluation of transport experiments.

Imaging connected porosity of crystalline rock by contrast agent‐aided X‐ray microtomography and scanning electron microscopy

“…biotite) affect considerably the transport of radionuclides (Voutilainen et al. , ). The main minerals of Grimsel granodiorite, which was used in this work, are quartz (∼35%), plagioclase (∼30%), potassium feldspar (∼30%) and biotite (∼5%).…”

“…Furthermore, recent studies applying C-14-PMMA autoradiography have shown that mineral grains with intragranular nanometre scale pores (e.g. biotite) affect considerably the transport of radionuclides (Voutilainen et al, 2017). The main minerals of Grimsel granodiorite, which was used in this work, are quartz (ß35%), plagioclase (ß30%), potassium feldspar (ß30%) and biotite (ß5%).…”

“…The Grimsel in situ experiment and an in situ through diffusion experiment (TDE) in Olkiluoto have used caesium as a tracer in concentrations resembling the repository conditions. Results of caesium from the LTD experiment indicate that the mineral heterogeneity of the rock is important for retention of caesium (Jokelainen et al, 2013;Voutilainen et al, 2017). This factor combined with the high X-ray attenuation coefficient of caesium, and the possibility of facile intrusion via water solution, led to the decision to choose CsCl as the potential contrast agent.…”

On the long-term migration of uranyl in bentonite barrier for high-level radioactive waste repositories: The effect of different host rocks

“…Last but not least, the migration of radionuclides in an EBS and host rock has been the focus of numerous studies. Voutilainen et al (2017) simulated transport of cesium in Grimsel granodiorite with micrometer-scale heterogeneities; they concluded that the heterogeneity of the mineral structure at the micrometer scale could significantly affect the diffusion and adsorption of cesium in Grimsel granodiorite at the centimeter scale. Jin et al (2016) conducted a surface complexation modeling of U(VI) adsorption, using a Generalized Composite model on granite at ambient/elevated temperature; model results showed that the exchange reaction is not important for U(VI) adsorption on granite.…”

Effects of Fine‐Scale Surface Alterations on Tracer Retention in a Fractured Crystalline Rock From the Grimsel Test Site

“…The domain pore spaces between these mineral grains and intragranular secondary pores form water‐filled, heterogeneous networks with varying geometric factors such as tortuosity and constrictivity related to diffusion and accessible reactive surfaces related to sorption (e.g., EUR, ; NASEM, ). A number of experimental and modeling studies have been conducted to understand the effects of heterogeneous minerals and pores distributions on diffusion and sorption processes (e.g., Iraola et al, ; Sardini et al, ; Voutilainen et al, , ). On the other hand, in fractured parts of crystalline rock, RN transport occurs primarily in the fractures, where preferential, advective water flow takes place (Neretnieks, ).…”