| 1. |
- Klos, Ryk, et al.
(författare)
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Potential for high transient doses due to accumulation and chemical zonation of long-lived radionuclides across the geosphere-biosphere interface
- 2011
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Ingår i: Radioprotection - Revue de la Societé Francaise de Radioprotection. - : EDP Sciences. - 0033-8451 .- 1769-700X. ; 46:6, s. S453-S459
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Tidskriftsartikel (refereegranskat)abstract
- Planning for the disposal of spent nuclear fuel is at an advanced stage in several nations around the world. Licensing of the disposal facility requires correspondingly detailed assessment of the future performance of the facility. With increased site-specific detail available to the assessment, local characteristics play an increasingly important role in determining the potential radiological risk posed by releases to the biosphere. In this paper we go beyond existing reference biosphere models and investigate the potential for specific accumulation mechanisms. The implications for the modelling carried out in long timescale performance assessment are discussed.
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| 2. |
- Marklund, Lars, 1978-
(författare)
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Topographic Control of Groundwater Flow
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gravity is the main driving force for groundwater flow, and both landscape topography and geology distribute the effects of gravity on groundwater flow. The groundwater table defines the distribution of the potential energy of the water. In humid regions where the bedrock permeability is relatively low and the soil depth is sufficiently shallow, the groundwater table closely follows the landscape topography and, thus, the topography controls the groundwater circulation in these regions. In this thesis, I investigate multi-scale topography-controlled groundwater flow, with the goal of systematizing the spatial distribution of groundwater flow and assessing geological parameters of importance for groundwater circulation. Both exact solutions and numerical models are utilized for analyzing topography-controlled groundwater flow. The more complex numerical models are used to explore the importance of various simplifications of the exact solutions. The exact solutions are based on spectral representation of the topography and superpositioning of unit solutions to the groundwater flow field. This approach is an efficient way to analyze multi-scaled topography-controlled groundwater flow because the impact of individual topographic scales on the groundwater flow can be analyzed separately. The results presented here indicate that topography is fractal and affects groundwater flow cells at wide range of spatial scales. We show that the fractal nature of the land surface produces fractal distributions of the subsurface flow patterns. This underlying similarity in hydrological processes also yields a single scale-independent distribution of subsurface water residence times which have been found in distributions of solute efflux from watersheds. Geological trends modify the topographic control of the groundwater circulation pattern and this thesis presents exact solutions explaining the impact of geological layering, depth-decaying and anisotropic hydraulic conductivity on the groundwater flow field. For instance, layers of Quaternary deposits and decaying permeability with depth both increase the importance of smaller topographic scales and creates groundwater flow fields where a larger portion of the water occupies smaller and shallower circulation cells, in comparison to homogeneous systems.
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| 3. |
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| 4. |
- Wörman, Anders, et al.
(författare)
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Convergence of groundwater discharge through the hyporheic zone of streams
- 2023
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Ingår i: Ground Water. - : Wiley. - 0017-467X .- 1745-6584. ; 61:1, s. 66-85
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Tidskriftsartikel (refereegranskat)abstract
- Significant attention has been given to hyporheic water fluxes induced by hydromorphologic processes in streambeds and the effects they have on stream ecology. However, the impact of hyporheic fluxes on regional groundwater flow discharge zones as well as the interaction of these flows are much less investigated. The groundwater-hyporheic interactive flow not only governs solute mass and heat transport in streams but also controls the retention of solute and contamination following the discharge of deep groundwater, such as naturally occurring solutes and leakage from geological waste disposal facilities. Here, we applied a physically based modeling approach combined with extensive hydrologic, geologic and geographical data to investigate the effect of hyporheic flow on groundwater discharge in the Krycklan catchment, located in a boreal landscape in Sweden. Regional groundwater modeling was conducted using COMSOL Multiphysics by considering geologic heterogeneity and infiltration constraint of the groundwater circulation intensity. Moreover, the hyporheic flow was analyzed using an exact spectral solution accounting for the fluctuating streambed topography and superimposed with the regional groundwater flow. By comparing the discharge flow fields with and without consideration of hyporheic flows, we found that the divergence of the discharge was substantially enhanced and the distribution of the travel times of groundwater was significantly shifted toward shorter times due to the presence of hyporheic flow. Particularly important is that the groundwater flow paths contract near the streambed interface due to the hyporheic flow, which leads to a phenomenon that we name “fragmentation” of coherent areas of groundwater upwelling in pinhole-shaped stream tubes.
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| 5. |
- Wörman, Anders, 1961-, et al.
(författare)
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Field Study on Discharge of Natural Isotopes from the 238U and 232Th Series through Quaternary Deposit
- 2011
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Ingår i: Proceedings of the 13th International High-Level Radioactive Waste Management Conference (IHLRWMC). - 9780894480850 ; , s. 782-787
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Konferensbidrag (refereegranskat)abstract
- This paper describes tentative results of the transport of natural radionuclides of the Uranium and Thorium series through Quaternary deposits in Sweden. Three drill holes between 10-16 meters deep were made down to bedrock and several radionuclides were measured in the dissolved, adsorbed and mineralogical phases. The isotope composition of the pore water differs from that of the soil minerals, which indicates that the origin of the isotopes is from discharging groundwater from bedrock. Less than 1 % of the isotopes exist in dissolved or adsorbed phases and the remaining fraction in minerals.
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| 6. |
- Wörman, Anders, 1961-, et al.
(författare)
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Impact of repository depth on residence times for leaking radionuclides in land-based surface water
- 2006
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Ingår i: Proceedings of the 11th International High Level Radioactive Waste Management Conference, IHLRWM. - 9781604236293
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Konferensbidrag (refereegranskat)abstract
- The multiple scales of landscape topography produces a wide distribution of groundwater circulation cells that control the hydro-geological environments surrounding geological repositories for nuclear waste. The largest circulation cells tend to discharge water into major river reaches, large freshwater systems or the nearby Sea. We investigated numerically the release of radionuclides from repositories placed in bedrock with depths between 100 to 2,000 meters in a Swedish coastal area and found that, leakage from the deeper positions emerges primarily in the major aquatic systems. In effect, radionuclides from the deeper repositories are more rapidly transported towards the Sea by the stream system compared to leakage from more shallow repositories. The release from the shallower repositories is significantly retained in the initial stage of the transport in the (superficial) landscape because the discharge occurs in or near low-order streams with high retention characteristics. This retention and residence time for radioactivity in the landscape control radiological doses to biota and can, thus, be expected to constitute an essential part of an associated risk evaluation.
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| 7. |
- Wörman, Anders, et al.
(författare)
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Impact of repository depth on residence times for leaking radionuclides in land-based surface water
- 2007
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Ingår i: Acta Geophysica. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 55:1, s. 73-84
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Tidskriftsartikel (refereegranskat)abstract
- The multiple scales of landscape topography produce a wide distribution of groundwater circulation cells that control the hydro- geological environments surrounding geological repositories for nuclear waste. The largest circulation cells tend to discharge water into major river reaches, large freshwater systems or the nearby Baltic Sea. We investigated numerically the release of radionuclides from repositories placed in bedrock with depths between 100 to 2000 meters in a Swedish coastal area and found that leakage from the deeper positions emerges primarily in the major aquatic systems. In effect, radionuclides from the deeper repositories are more rapidly transported towards the Sea by the stream system compared to leakage from more shallow repositories. The release from the shallower repositories is significantly retained in the initial stage of the transport in the ( superficial) landscape because the discharge occurs in or near low- order streams with high retention characteristics. This retention and residence time for radioactivity in the landscape control radiological doses to biota and can, thus, be expected to constitute an essential part of an associated risk evaluation.
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| 8. |
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| 9. |
- Wörman, Anders, 1961-, et al.
(författare)
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Scaling of surface water-subsurface water interaction with implication to hydrogeological site evaluations
- 2008
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Ingår i: The 12th International High-Level Radioactive Waste Management Conference (IHLRWM). - : American Nuclear Society. - 9780894480621
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Konferensbidrag (refereegranskat)abstract
- Land surface topography governs the groundwater flow patterns at the regional-to-continental scale and on smaller scales such as in the hyporheic zone of streams. This produces a complicated groundwater circulation pattern that controls releases of radionuclides from a underground nuclear waste repository. Here we utilise a new spectral technique to analytically solve the groundwater flow with account taken to the spectrum of topographical scales on the continent, depth decaying permeability with depth and quaternary deposits. The proposed solution makes it possible to independently analyse the effects on the flow generated by topographical features on various scales. Basically, all topographical scales have the same influence on groundwater exchange with surface water, but the decaying permeability with depth in bedrock causes a predominance of local scales. Discharge of radionuclides from a deep repository placed in bedrock occurs in major rivers and streams where we find the thickest layers of quaternary deposits in post-glacial landscapes. The high sorption affinity in soil implies that radionuclides spend a very long time in quaternary deposits on their travel from an underground repository through bedrock and stream network to the sea.
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| 10. |
- Xu, Shulan, et al.
(författare)
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A regulatory modelling strategy for review of dose calculations
- 2008
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Ingår i: Proceedings of The 12th International High-Level Radioactive Waste managemenent Conference. - : American Nuclear Society. - 9780894480621 ; , s. 466-472
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Konferensbidrag (refereegranskat)abstract
- To be able to carry out review functions regulatory authorities must be able to make critical evaluations of proponent's performance assessment (PA) cases. In Sweden the Swedish Radiation Protection Authority (SSI) aims to have in place its own suite of PA tools. These will be used to address the forthcoming PAs submitted by the Swedish Nuclear Waste Management Company (SKB) for a geological repository for spent fuel (2010). This paper looks at the role and application of the regulator's models to important features of current modelling in SKB 's latest PAs. The SSI modelling tools are applied to demonstrate a possible size of the contaminant area due to leakage of radionuclides from a damaged canister. An improved representation of these processes will enhance our understanding of biosphere functions and provide a better basis for evaluating radiological consequences in the safety assessment.
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| 11. |
- Xu, Shulan, et al.
(författare)
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Application of a Regulatory Performance Assessment Approach for Comparison with the Proponent’s Calculations
- 2006
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Ingår i: Proceedings of the 11th International High Level Radioactive Waste Management Conference, IHLRWM. - 9781604236293 ; , s. 980-985
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Konferensbidrag (refereegranskat)abstract
- To be able to carry out review functions regulatory authorities must be able to make critical evaluations of proponents PA cases. In Sweden the Swedish Radiation Protection Authority (SSI) aims to have in place its own suite of performance assessment (PA) tools. These will be used to address the forthcoming PAs submitted by the Swedish Nuclear Waste Management Company (SKB)for an encapsulation plant (2006) and a geological repository for spent fuel (2008). This paper looks at the role and application of the regulator's models to important features of current modelling in SKB's latest PAs. The SSI modelling tools are applied to a mire system in which radionuclides are assumed to be released from a geological repository. Results are presented which illustrate the importance of a deeper understanding of the geosphere-biosphere interface zone than has been the case in assessments to date.
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| 12. |
- Xu, Shulan, et al.
(författare)
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Criteria for resolution-scales and parameterisation of compartmental models of hydrological and ecological mass flows
- 2007
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694 .- 1879-2707. ; 335:04-mar, s. 364-373
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Tidskriftsartikel (refereegranskat)abstract
- The effect of compartment resolution-scale, i.e. number of compartments and compartment size, on prediction of transport of water and solute mass in watersheds is analysed by using numerical discretisation and temporal moments methods. The one-dimensional advection-diffusion model is used as a theoretical frame of reference for representing transport along a single series of compartments along a pathway in the surface water or ecosystem. Criteria for parameterisation of compartment models and error estimates are derived for two cases: (1) requiring an exact match with the advection-diffusion model; and (2) requiring as good match as possible with a simplified compartment model using a small number of compartments. The matching criteria are based on a comparison of numerical discretisation and temporal moments of the compartmental backflow model with those of the one-dimensional advection-diffusion equation. We derived solutions in terms of temporal moments of both the advection-diffusion equation and compartmental backflow model. with arbitrary input concentration vs. time boundary condition. This approach can be used to translate parameters between the models and analyse the error caused by any arbitrary structure of the compartmental model as a function of boundary conditions and the physical parameters of the advective-diffusive equation. Data from tracer experiments were compared with model predictions to provide a practical frame of reference.
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| 13. |
- Xu, Shulan, et al.
(författare)
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Development of a model for radionuclide transport in streams for biosphere assessment purpose
- 2023
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Ingår i: Journal of Environmental Radioactivity. - : Elsevier BV. - 0265-931X .- 1879-1700. ; 264
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Tidskriftsartikel (refereegranskat)abstract
- As a part of the overall safety assessment for a geological disposal of radioactive waste, models for different ecosystems are used to evaluate doses to humans and biota from possible radionuclide discharges to the biosphere. In previous safety assessments, transport modelling of radionuclides in running waters such as streams has been much simplified to the extent that only dilution of the inflow of radionuclides has been considered with no regard of any other interactions.Hyporheic exchange flow (HEF) is the flow of surface water in streams that enters the subsurface zone and, after some time, returns to the surface. HEF has been studied for decades. Hyporheic exchange and the residence time in the hyporheic zone are key parameters controlling the transport of radionuclides in a stream. Further-more, recent studies have shown that HEF can reduce the groundwater upwelling area and increase the up -welling velocity in areas closest to the streambed water interface.In this paper, the development of an assessment model describing radionuclide transport with consideration of HEF and deep groundwater upwelling along streams is presented. An approach to parameterising the hyporheic exchange processes into an assessment model is based on a comprehensive study that has been performed in five different Swedish catchments. Sensitivity analyses are performed to explore the effect with consideration of the inflow of radionuclides with regard to HEF and deep groundwater upwelling in a safety assessment perspective. Finally, we include some suggestions for the application of the assessment model to long-term radiological safety assessments.
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| 14. |
- Xu, Shulan
(författare)
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Effect of Uncertainty of Rock Properties on Radionuclide Transport by Groundwater : Implications for Performance Assessments of the Repository of Spent Nuclear Fuel in Heterogeneous Bedrock
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The overall objective of the current study is to develop a quantitative understanding of the effects of spatial variability in physical and geochemical properties of crystalline rock on the migration of radionuclides along a single fracture in bedrock. A stochastic model was developed to describe the transport of solutes in fractured rock. The model describes the migration of radionuclides along a one-dimensional path and includes the transversal diffusion into the rock matrix and sorption kinetics. By using a Lagrangian method of description we can extend the model to the description of a two-dimensional transport problem in single fractures. This study presents the first analysis of the impact of heterogeneous mass transfer on the transport of radionuclides in rock fractures, where most of the relevant rock properties such as aperture, porosity, effective diffusivity, sorption capacity and maximum diffusion depth are defined as being spatially random. The stochastic analysis performed here reflects the uncertainty in our knowledge of the properties associated with a discrete sampling technique in site investigations.Geostatistics of the main parameters was determined experimentally on a large number of rock samples taken from the Swedish crystalline basement. The knowledge of the covariance functions of the main rock properties is then used as a basis for a stochastic analysis. By combining the small perturbation approach with the spectral method the problem could be solved in terms of closed form solutions for the central temporal moments of the residence time probability density function. In order to be able to distinguish between the effects of various mechanisms from the effects of heterogeneity on the migration of radionuclides, it was necessary to perform independent studies of the effect of the variation of the dispersion coefficient on the aspect ratio of a rectangular flow section and the effect of adsorption kinetics on the migration. Finally, the effect of the heterogeneous rock properties on the solute transport observed in a limited number of migration experiments corresponds fairly well to the theoretical effect expected on the basis of the experimentally determined auto-covariance functions.
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| 15. |
- Xu, Shulan, et al.
(författare)
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Effects of compartmental model structure and long-term inflow on model predictions
- 2005
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Ingår i: Radioprotection - Revue de la Societé Francaise de Radioprotection. - : EDP Sciences. - 0033-8451 .- 1769-700X. ; 40:1, s. 477-483
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Tidskriftsartikel (refereegranskat)abstract
- Model predictions of solute transport in a wetland were compared with tracer experimental data to illustrate that the number of compartments in a compartmental model should be selected according to certain rules to accurately describe the transport process. If the input pulse is short, the model structure affects the output significantly. The temporal moments of the residence time distribution was obtained from a general solution of the compartmental model in the Laplace domain derived with an arbitrary inflow pollutograph. The variance of the residence time can be used as a useful tool to analyse the effect of model structure and long-term inflow pollutograph on the response of the model predictions.
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| 16. |
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| 17. |
- Xu, Shulan, et al.
(författare)
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Prediction of concentration and model validation
- 2008
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Ingår i: Radioprotection - Revue de la Societé Francaise de Radioprotection. - : EDP Sciences. - 0033-8451 .- 1769-700X. ; 43:1, s. 701-706
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we examine some critical aspects concerning the justification of simplified radioecological models used in safety assessments for geological repositories. We propose a modelling approach for regulatory review of dose and risk calculations for a nuclear waste repository. The SSI modelling tools are applied to explore uncertainties in the size of the contaminant area due to leakage of radionuclides from a damaged nuclear waste canister. We demonstrate that an improved representation of geosphere transport processes will also enhance our understanding of radionuclide migration in the biosphere and provide a better basis for evaluating radiological consequences in the safety assessment.
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