| 1. |
- Gylling, Björn, et al.
(författare)
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SKB Task Force GWFTS : Lessons Learned from Modeling Field Tracer Experiments in Finland and Sweden
- 2022
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Konferensbidrag (refereegranskat)abstract
- SKB and several other waste management organizations have established the international SKB Task Force on Modeling of Groundwater Flow and Transport of Solutes (TF GWFTS) to support and interpret field experiments. Objectives of the task force are to develop, test and improve tools for conceptual understanding and simulating groundwater flow and transport of solutes in fractured rocks. Work is organized in collaborative modeling tasks. Task 9 focuses on realistic modeling of coupled matrix diffusion and sorption in heterogeneous crystalline rock matrix at depth, e.g. by inverse and predictive modeling of in-situ transport experiments. Posiva’s REPRO (rock matrix REtention PROperties) experimental campaign has been performed at the ONKALO rock characterization facility in Finland. The two REPRO experiments considered were the Water Phase Diffusion Experiment (WPDE), addressing matrix diffusion in gneiss around a single borehole interval (modeled in Task 9A), and the Through Diffusion Experiment, which is performed between sections of three boreholes and addressed by modeling in Task 9C. The Long-Term Diffusion and Sorption Experiment (LTDE-SD) was an in-situ radionuclide tracer test performed at the Swedish Äspö Hard Rock Laboratory at a depth of about 410 m below sea level. The experimental results indicated a possible deeper penetration of sorbing tracers into the rock matrix than expected. The shape of these tracer penetration profiles was difficult to reproduce. This experiment was modeled and interpreted in Task 9B. Task 9D is addressing the possible benefits of detailed models of the in-situ experiments in safety assessment calculations. The task is performed by upscaling of the WPDE models to conditions applicable for nuclear waste repositories. As Task 9 is now in a finalization process, a number of lessons learned from the 4 sub-tasks have been identified. These include: • field tracer experiments can provide surprises even when well designed and executed, • interaction between the experimentalists and modelers is important and mutually beneficial when investigating anomalous results, • differences in conceptual models have the greatest impact on model outcomes, • it is not trivial to go from modeling of field experiments to safety assessment modeling without making substantial simplifications.
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| 2. |
- Iraola, Aitor, et al.
(författare)
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Microtomography-based Inter-Granular Network for the simulation of radionuclide diffusion and sorption in a granitic rock
- 2017
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Ingår i: Journal of Contaminant Hydrology. - : Elsevier BV. - 0169-7722 .- 1873-6009. ; 207, s. 8-16
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Sanglas, Jordi, et al.
(författare)
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Significance of Low-Velocity Zones on Solute Retention in Rough Fractures
- 2024
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 60:4
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Tidskriftsartikel (refereegranskat)abstract
- Natural fractures are characterized by high internal heterogeneity. This internal variability is the cause of flow channeling, which in turn leads to contaminant transport taking place primarily along the high-velocity channels. Mass exchange between the high-velocity channels and the low-velocity zones has the potential to enhance contaminant retention, due to solute diffusion into the low-velocity zones and subsequent exposure to additional surface area for diffusion into the bordering rock matrix. Here, we derive a random walk particle tracking method for heterogeneous fractures, which includes an additional term to account for the aperture gradient. The method takes into account advection, diffusion in the fracture and matrix diffusion. The developed numerical framework is applied to assess the effect of low-velocity zones in rough self-affine fractures. The results show that diffusion into low-velocity zones has a visible but modest impact on contaminant retention. The magnitude of this impact does not change considerably, regardless of whether diffusion into the rock matrix is considered in the model, and increases for a decreasing average Peclet number of the fracture. Natural fractures are highly heterogeneous, comprising flowing channels and lower-velocity zones We study the effects that diffusion into low-velocity zones has in contaminant transport through rough fractures Accounting for diffusion in low-velocity zones has a relatively modest impact on contaminant retention
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| 4. |
- Soler, Josep M., et al.
(författare)
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Predictive and Inverse Modeling of a Radionuclide Diffusion Experiment in Crystalline Rock at ONKALO (Finland)
- 2023
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Ingår i: Nuclear Technology. - : Informa UK Limited. - 0029-5450 .- 1943-7471. ; 209:11, s. 1765-1784
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Tidskriftsartikel (refereegranskat)abstract
- The REPRO-TDE test was performed at a depth of about 400 m in the ONKALO underground research facility in Finland. Synthetic groundwater containing radionuclide tracers [tritiated water tracer (HTO), 36Cl, 22Na, 133Ba, and 134Cs] was circulated for about 4 years in a packed-off interval of the injection borehole. Tracer activities were additionally monitored in two observation boreholes. The test was the subject of a modeling exercise by the SKB GroundWater Flow and Transport of Solutes Task Force. Eleven teams participated in the exercise, using different model concepts and approaches. Predictive model calculations were based on laboratory-based information concerning porosities, diffusion coefficients, and sorption partition coefficients. After the experimental results were made available, the teams were able to revise their models to reproduce the observations. General conclusions from these back-analysis calculations include the need for reduced effective diffusion coefficients for 36Cl compared to those applicable to HTO (anion exclusion), the need to implement weaker sorption for 22Na compared to results from laboratory batch sorption experiments, and the observation of large differences between the theoretical initial concentrations for the strongly sorbing 133Ba and 134Cs, and the first measured values a few hours after tracer injection. Different teams applied different concepts, concerning mainly the implementation of isotropic versus anisotropic diffusion, or the possible existence of borehole disturbed zones around the different boreholes. The role of microstructure was also addressed in two of the models.
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| 5. |
- Soler, Josep, et al.
(författare)
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Modelling of the LTDE-SD radionuclide diffusion experiment in crystalline rock at the Aspo Hard Rock Laboratory (Sweden)
- 2022
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Ingår i: Geologica Acta. - : Universitat Autònoma de Barcelona. - 1695-6133 .- 1696-5728. ; 20, s. 1-32
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Tidskriftsartikel (refereegranskat)abstract
- This study shows a comparison and analysis of results from a modelling exercise concerning a field experiment involving the transport and retention of different radionuclide tracers in crystalline rock. This exercise was performed within the Swedish Nuclear Fuel and Waste Management Company (SKB) Task Force on Modelling of Groundwater Flow and Transport of Solutes (Task Force GWFTS). Task 9B of the Task Force GWFTS was the second subtask within Task 9 and focused on the modelling of experimental results from the Long Term Sorption Diffusion Experiment in situ tracer test. The test had been performed at a depth of about 410m in the Aspo Hard Rock Laboratory. Synthetic groundwater containing a cocktail of radionuclide tracers was circulated for 198 days on the natural surface of a fracture and in a narrow slim hole drilled in unaltered rock matrix. Overcoring of the rock after the end of the test allowed for the measurement of tracer distribution profiles in the rock from the fracture surface (A cores) and also from the slim hole (D cores). The measured tracer activities in the rock samples showed long profiles (several cm) for non-or weakly-sorbing tracers (Cl-36, Na-22), but also for many of the more strongly-sorbing radionuclides. The understanding of this unexpected feature was one of the main motivations for this modelling exercise. However, re-evaluation and revision of the data during the course of Task 9B provided evidence that the anomalous long tails at low activities for strongly sorbing tracers were artefacts due to cross-contamination during rock sample preparation. A few data points remained for Cs-137, Ba-133, Ni-63 and Cd-109, but most measurements at long distances from the tracer source (>10mm) were now below the reported detection limits. Ten different modelling teams provided results for this exercise, using different concepts and codes. The tracers that were finally considered were Na-22, Cl-36, Co-57, Ni-63, Ba-133, Cs-137, Cd-109, Ra-226 and Np-237. Three main types of models were used: i) analytical solutions to the transport-retention equations, ii) continuum -porous-medium numerical models, and iii) microstructure-based models accounting for small-scale heterogeneity (i.e. mineral grains, porosities and/or microfracture distributions) and potential centimetre-scale fractures. The modelling by the different teams led to some important conclusions, concerning for instance the presence of a disturbed zone (a few mm in thickness) next to the fracture surface and to the wall of the slim hole and the role of micro-fractures and cm-scale fractures in the transport of weakly sorbing tracers. These conclusions could be reached after the re-evaluation and revision of the experimental data (tracer profiles in the rock) and the analysis of the different sets of model results provided by the different teams.
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| 6. |
- Svensson, Urban, et al.
(författare)
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Grains, grids and mineral surfaces : approaches to grain-scale matrix modeling based on X-ray micro-computed tomography data
- 2019
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Ingår i: SN Applied Sciences. - : Springer Nature. - 2523-3963 .- 2523-3971. ; 1:10
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Tidskriftsartikel (refereegranskat)abstract
- X-ray micro-computed tomography (X-μCT) generates 3D mineral distribution maps currently with a resolution of about 10 μm. For tight crystalline rocks, this implies that the mineral grains are well resolved, while micro-fractures, having apertures of less than 10 μm, are not resolved. In this study, we propose a method to analyze the properties (size, volume, surface area) of the mineral grains based on X-μCT data. The numerical approach uses a resolution similar to that of the X-μCT data and hence shares the same limitations. For example, it is clear that a large fraction of the mineral surface area is due to so-called roughness, with scales below 10 μm. In the second part of the study, methods to generate the diffusion-available pore space are discussed. The inter-granular space (distance between grains) is often smaller than 10 μm, and we need to design methods to be able to perform diffusion simulations in the matrix. Three methods, all based on X-μCT, are discussed, and it is demonstrated that models with realistic global properties (mean porosity and effective diffusion coefficient) can be developed based on the suggested techniques.
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| 7. |
- Svensson, Urban, et al.
(författare)
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Modelling the diffusion-available pore space of an unaltered granitic rock matrix using a micro-DFN approach
- 2018
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 559, s. 182-191
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Tidskriftsartikel (refereegranskat)abstract
- In sparsely fractured rock, the ubiquitous heterogeneity of the matrix, which has been observed in different laboratory and in situ experiments, has been shown to have a significant influence on retardation mechanisms that are of importance for the safety of deep geological repositories for nuclear waste. Here, we propose a conceptualisation of a typical heterogeneous granitic rock matrix based on micro-Discrete Fracture Networks (micro-DFN). Different sets of fractures are used to represent grain-boundary pores as well as micro fractures that transect different mineral grains. The micro-DFN model offers a great flexibility in the way inter- and intra-granular space is represented as the different parameters that characterise each fracture set can be fine tuned to represent samples of different characteristics. Here, the parameters of the model have been calibrated against experimental observations from granitic rock samples taken at Forsmark (Sweden) and different variant cases have been used to illustrate how the model can be tied to rock samples with different attributes. Numerical through-diffusion simulations have been carried out to infer the bulk properties of the model as well as to compare the computed mass flux with the experimental data from an analogous laboratory experiment. The general good agreement between the model results and the experimental observations shows that the model presented here is a reliable tool for the understanding of retardation mechanisms occurring at the mm-scale in the matrix.
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| 8. |
- Trinchero, Paolo, et al.
(författare)
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Experimental and numerical analysis of flow through a natural rough fracture subject to normal loading
- 2024
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- Fractured crystalline rocks have been chosen or are under consideration by several countries as host rock formations for deep geological repositories for spent nuclear fuel. In such geological formations, flow and solute transport are mostly controlled by a network of connected natural fractures, each of them being characterised by internal heterogeneity, also denoted as roughness. Fractures are, in turn, subject to variable load caused by various factors, such as the presence of thick ice sheets formed during glaciation periods. Understanding how coupled hydro-mechanical (HM) processes affect flow and transport at the scale of a single natural fracture is crucial for a robust parameterisation of large-scale discrete fracture network models, which are not only used for nuclear waste disposal applications but are also of interest to problems related to geothermics, oil and gas production or groundwater remediation. In this work, we analyse and model an HM experiment carried out in a single natural fracture and use the results of both, the experimental and the modelling work, to get insights into fundamental questions such as the applicability of local cubic law or the effect of normal load on channeling. The initial fracture aperture was obtained from laser scanning of the two fracture surfaces and an equivalent initial aperture was then defined by moving the two fracture surfaces together and comparing the results obtained using a Navier–Stokes based computational fluid dynamics (CFD) model with the experimental flowrate obtained for unloaded conditions. The mechanical effect of the different loading stages was simulated using a high-resolution contact model. The different computed fracture apertures were then used to run groundwater flow simulations using a modified Reynolds equation. The results show that, without correction, local cubic law largely overestimates flowrates. Instead, we show that by explicitly acknowledging the difference between the mechanical aperture and the hydraulic aperture and setting the latter equal to 1/5 of the former, cubic law provides a very reasonable approximation of the experimental flowrates over the entire loading cycle. A positive correlation between fluid flow channeling and normal load is also found.
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| 9. |
- Trinchero, Paolo, et al.
(författare)
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FASTREACT : An efficient numerical framework for the solution of reactive transport problems
- 2014
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Ingår i: Applied Geochemistry. - : Elsevier BV. - 0883-2927 .- 1872-9134. ; 49, s. 159-167
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Tidskriftsartikel (refereegranskat)abstract
- In the framework of safety assessment studies for geological disposal, large scale reactive transport models are powerful inter-disciplinary tools aiming at supporting regulatory decision making as well as providing input to repository engineering activities. Important aspects of these kinds of models are their often very large temporal and spatial modelling scales and the need to integrate different non-linear processes (e.g., mineral dissolution and precipitation, adsorption and desorption, microbial reactions and redox transformations). It turns out that these types of models may be computationally highly demanding. In this work, we present a Lagrangian-based framework, denoted as FASTREACT, that aims at solving multi-component-reactive transport problems with a computationally efficient approach allowing complex modelling problems to be solved in large spatial and temporal scales. The tool has been applied to simulate radionuclide migration in a synthetic heterogeneous transmissivity field and the results have been successfully compared with those obtained using a standard Eulerian approach. Finally, the same geochemical model has been coupled to an ensemble of realistic three-dimensional transport pathways to simulate the migration of a set of radionuclides from a hypothetical repository for spent nuclear fuel to the surface. The results of this modelling exercise, which includes key processes such as the exchange of mass between the conductive fractures and the matrix, show that FASTREACT can efficiently solve large-scale reactive transport models.
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| 10. |
- Trinchero, Paolo, et al.
(författare)
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Modelling radionuclide transport in fractured media with a dynamic update of Kd values
- 2016
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Ingår i: Computers & Geosciences. - : Elsevier BV. - 0098-3004 .- 1873-7803. ; 86, s. 55-63
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Tidskriftsartikel (refereegranskat)abstract
- Radionuclide transport in fractured crystalline rocks is a process of interest in evaluating long term safety of potential disposal systems for radioactive wastes. Given their numerical efficiency and the absence of numerical dispersion, Lagrangian methods (e.g. particle tracking algorithms) are appealing approaches that are often used in safety assessment (SA) analyses. In these approaches, many complex geochemical retention processes are typically lumped into a single parameter: the distribution coefficient (Kd). Usually, the distribution coefficient is assumed to be constant over the time frame of interest. However, this assumption could be critical under long-term geochemical changes as it is demonstrated that the distribution coefficient depends on the background chemical conditions (e.g. pH, Eh, and major chemistry). In this work, we provide a computational framework that combines the efficiency of Lagrangian methods with a sound and explicit description of the geochemical changes of the site and their influence on the radionuclide retention properties.
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| 11. |
- Trinchero, Paolo, et al.
(författare)
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Modelling the water phase diffusion experiment at Onkalo (Finland) : Insights into the effect of channeling on radionuclide transport and retention
- 2020
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 590
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Tidskriftsartikel (refereegranskat)abstract
- In fractured crystalline rocks, contaminants are transported through open fractures by fluid flow and can access the connected pore space of the adjacent rock matrix by diffusion. The mass exchange term between the fracture and the matrix depends on in-plane groundwater flow patterns. Thus, channeling and preferential flow, due to heterogeneity in fracture aperture, have an impact on the overall rock retention capacity. Here, we have used experimental data from the recent Water Phase Diffusion Experiment, carried out at ONKALO (Finland), to assess the influence of channeling on contaminant transport and retention at increasingly larger scales. The upscaling is performed by extracting non-parametric Retention Time Distribution functions from the experimental data and using them to carry out transport simulations along a segmented pathway. The analysis shows that channeling leads to anomalous early breakthroughs at short scales, whereas at increasingly larger distances this effect is smeared out and the nuclide breakthrough curves become mostly controlled by mass exchange processes between the fracture and the matrix. This homogenisation occurs at shorter scales for sorbing nuclides. The influence of channelling on radionuclide retention is shown to be modest due to compensating effects between the shorter groundwater travel times and the larger specific fracture surface area made available through in-plane diffusion into stagnant water. Also, we have shown that the late-time behaviour of the experimental breakthrough curves can be used to infer equivalent parameters that, combined with existing homogeneous-based solutions, provide a good description of contaminant breakthrough curves at larger downstream distances.
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| 12. |
- Trinchero, Paolo, et al.
(författare)
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Upscaling of radionuclide transport and retention in crystalline rocks exhibiting micro-scale heterogeneity of the rock matrix
- 2020
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Ingår i: Advances in Water Resources. - : Elsevier BV. - 0309-1708 .- 1872-9657. ; 142
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Tidskriftsartikel (refereegranskat)abstract
- In different in-situ diffusion experiments carried out in fractured crystalline rocks, sorbing radionuclides have shown a behaviour that strongly differs from what is predicted by homogeneous-based models. Their breakthrough curves are in fact often characterised by a fast first-arrival and these radionuclides can penetrate surprisingly long distances deep into the matrix. The heterogeneous structure of mineral distribution and porosity geometry had been offered as an explanation for these discrepancies. Here, we use reactive transport simulations to investigate the effect of the sparse distribution of sorption sites on the breakthrough curves of sorbing radionuclides. At small scale, the computed breakthrough curves significantly differ from those predicted using homogeneous models. For instance, the early part of these curves does not show any clear separation with the corresponding part of the curve of a non-sorbing tracer and a long transition zone is observed, with a very smooth slope of the tailing. Two different upscaling strategies, aimed at propagating the signal of heterogeneous retention over larger scales, are proposed and demonstrated against independent solutions computed at intermediate scales. The upscaling strategies are also used to show that at large scales (e.g. the scale of interest in a safety assessment study for a deep geological repository for nuclear waste) the signature of mineralogical heterogeneity is smoothed out and the heterogeneous breakthrough curve is well approximated by a homogeneous solution where the radionuclide distribution coefficient for the pure mineral phase is scaled by the mineral volume fraction. However, the spatial persistence of the heterogeneous signature is significant when the sorbing mineral is present in a low amount.
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| 13. |
- Williams, Thomas, et al.
(författare)
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Recovering the Effects of Subgrid Heterogeneity in Simulations of Radionuclide Transport Through Fractured Media
- 2021
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Ingår i: Frontiers in Earth Science. - : Frontiers Media SA. - 2296-6463. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Groundwater flow and contaminant transport through fractured media can be simulated using Discrete Fracture Network (DFN) models which provide a natural description of structural heterogeneity. However, this approach is computationally expensive, with the large number of intersecting fractures necessitated by many real-world applications requiring modeling simplifications to be made for calculations to be tractable. Upscaling methods commonly used for this purpose can result in some loss of local-scale variability in the groundwater flow velocity field, resulting in underestimation of particle travel times, transport resistance and retention in transport calculations. In this paper, a transport downscaling algorithm to recover the transport effects of heterogeneity is tested on a synthetic Brittle Fault Zone model, motivated by the problem of large safety assessment calculations for geological repositories of spent nuclear fuel. We show that the variability in the local-scale velocity field which is lost by upscaling can be recovered by sampling from a library of DFN transport paths, accurately reproducing DFN transport statistic distributions and radionuclide breakthrough curves in an upscaled model.
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