Identification and Characterization of Potential Discharge Areas for Radionuclide Transport by Groundwater from a Nuclear Waste Repository in Sweden

This paper describes solute transport modeling carried out as a part of an assessment of the long-term radiological safety of a planned deep rock repository for spent nuclear fuel in Forsmark, Sweden. Specifically, it presents transport modeling performed to locate and describe discharge areas for groundwater potentially carrying radionuclides from the repository to the surface where man and the environment could be affected by the contamination. The modeling results show that topography to large extent determines the discharge locations. Present and future lake and wetland objects are central for the radionuclide transport and dose calculations in the safety assessment. Results of detailed transport modeling focusing on the regolith and the upper part of the rock indicate that the identification of discharge areas and objects considered in the safety assessment is robust in the sense that it does not change when a more detailed model representation is used.     

Long-term oxygen depletion from infiltrating groundwaters: model development and application to intra-glaciation and glaciation conditions

Processes that control the redox conditions in deep groundwaters have been studied. The understanding of such processes in a long-term perspective is important for the safety assessment of a deep geological repository for high-level nuclear waste. An oxidising environment at the depth of the repository would increase the solubility and mobility of many radionuclides, and increase the potential risk for radioactive contamination at the ground surface. Proposed repository concepts also include engineered barriers such as copper canisters, the corrosion of which increases considerably in an oxidising environment compared to prevailing reducing conditions. Swedish granitic rocks are typically relatively sparsely fractured and are best treated as a dual-porosity medium with fast flowing channels through fractures in the rock with a surrounding porous matrix, the pores of which are accessible from the fracture by diffusive transport. Highly simplified problems have been explored with the aim to gain understanding of the underlying transport processes, thermodynamics and chemical reaction kinetics. The degree of complexity is increased successively, and mechanisms and processes identified as of key importance are included in a model framework. For highly complex models, analytical expressions are not fully capable of describing the processes involved, and in such cases the solutions are obtained by numerical calculations. Deep in the rock the main source for reducing capacity is identified as reducing minerals. Such minerals are found inside the porous rock matrix and as infill particles or coatings in fractures in the rock. The model formulation also allows for different flow modes such as flow along discrete fractures in sparsely fractured rocks and along flowpaths in a fracture network. The scavenging of oxygen is exemplified for these cases as well as for more comprehensive applications, including glaciation considerations. Results show that chemical reaction kinetics control the scavenging of oxygen during a relatively short time with respect to the lifetime of the repository. For longer times the scavenging of oxygen is controlled by transport processes in the porous rock matrix. The penetration depth of oxygen along the flowpath depends largely on the hydraulic properties, which may vary significantly between different locations and situations. The results indicate that oxygen, in the absence of easily degradable organic matter, may reach long distances along a flow path during the life-time of the repository (hundreds to thousands of metres in a million years depending on e.g. hydraulic properties of the flow path and the availability of reducing capacity). However, large uncertainties regarding key input parameters exist leading to the conclusion that the results from the model must be treated with caution pending more accurate and validated data. Ongoing and planned experiments are expected to reduce these uncertainties, which are required in order to make more reliable predictions for a safety assessment of a nuclear waste repository.     

Description of hydrogeologic heterogeneity and evaluation of radionuclide transport at an underground nuclear test

Realistic models of contaminant transport in groundwater demand detailed characterization of the spatial distribution of subsurface hydraulic properties, while at the same time programmatic constraints may limit collection of pertinent hydraulic data. Fortunately, alternate forms of data can be used to improve characterization of spatial variability. We utilize a methodology that augments sparse hydraulic information (hard data) with more widely available hydrogeologic information to generate equiprobable maps of hydrogeologic properties that incorporate patterns of connected permeable zones. Geophysical and lithologic logs are used to identify hydrogeologic categories and to condition stochastic simulations using Sequential Indicator Simulation (SIS). The resulting maps are populated with hydraulic conductivity values using field data and Sequential Gaussian Simulation (SGS). Maps of subsurface hydrogeologic heterogeneity are generated for the purpose of examining groundwater flow and transport processes at the Faultless underground nuclear test, Central Nevada Test Area (CNTA), through large-scale, three-dimensional numerical modeling. The maps provide the basis for simulation of groundwater flow, while transport of radionuclides from the nuclear cavity is modeled using particle tracking methods. Sensitivity analyses focus on model parameters that are most likely to reduce the long travel times observed in the base case. The methods employed in this study have improved our understanding of the spatial distribution of preferential flowpaths at this site and provided the critical foundation on which to build models of groundwater flow and transport. The results emphasize that the impacts of uncertainty in hydraulic and chemical parameters are dependent on the radioactive decay of specific species, with rapid decay magnifying the effects of parameters that change travel time.     

Geochemical evaluation of different groundwater-host rock systems for radioactive waste disposal

The geochemical suitability of a deep bedrock repository for radioactive waste disposal is determined by the composition of geomatrix and groundwater. Both influence radionuclide solubility, chemical buffer capacity and radionuclide retention. They also determine the chemical compatibility of waste forms, containers and backfill materials. Evaluation of different groundwater-host rock systems is performed by modeling the geochemical environments and the resulting radionuclide concentrations. In order to demonstrate the evaluation method, model calculations are applied to data sets available for various geological formations such as granite, clay and rocksalt. The saturation state of the groundwater-geomatrix system is found to be fundamental for the evaluation process. Hence, calculations are performed to determine if groundwater is in equilibrium with mineral phases of the geological formation. In addition, corrosion of waste forms in different groundwater is examined by means of reaction path modeling. The corrosion reactions change the solution compositions and pH, resulting in significant changes of radionuclide solubilities. The results demonstrate that geochemical modeling of saturation state and compatibility of the host formation environment with the radioactive waste proves to be a feasible tool for evaluation of various sites considered as deep underground repositories.     

Geochemical evaluation of different groundwater–host rock systems for radioactive waste disposal

The geochemical suitability of a deep bedrock repository for radioactive waste disposal is determined by the composition of geomatrix and groundwater. Both influence radionuclide solubility, chemical buffer capacity and radionuclide retention. They also determine the chemical compatibility of waste forms, containers and backfill materials. Evaluation of different groundwater–host rock systems is performed by modeling the geochemical environments and the resulting radionuclide concentrations. In order to demonstrate the evaluation method, model calculations are applied to data sets available for various geological formations such as granite, clay and rocksalt.The saturation state of the groundwater–geomatrix system is found to be fundamental for the evaluation process. Hence, calculations are performed to determine if groundwater is in equilibrium with mineral phases of the geological formation. In addition, corrosion of waste forms in different groundwater is examined by means of reaction path modeling. The corrosion reactions change the solution compositions and pH, resulting in significant changes of radionuclide solubilities. The results demonstrate that geochemical modeling of saturation state and compatibility of the host formation environment with the radioactive waste proves to be a feasible tool for evaluation of various sites considered as deep underground repositories.     

Colloid-facilitated transport of radionuclides through fractured media

The sorption of radionuclides on natural colloids may significantly modify their transport behaviour through fractured media, since radionuclides bound to colloids may not be subject to the important retardation mechanisms of matrix diffusion and sorption onto pore surfaces. This paper reports on theoretical and experimental work aimed at assessing the relevance of colloid-facilitated transport to repository safety analyses, with specific reference to the Swiss case. Transport models are presented, developed in conjunction with field- and laboratory-based studies of deep groundwater in the crystalline basement of northern Switzerland, in which colloid size distributions, compositions and sorption properties have been measured. Various potential mechanisms giving rise to both reversible and irreversible sorption are discussed. In the first case, a simple approach is examined which is based on previously reported models of colloid transport and assumes reversible, linear sorption on colloids, for which experimental data have been obtained. It is shown that transport of radionuclides would not, in general, be significantly enhanced because of this process. A more recently developed and more complex model is then described incorporating irreversible sorption, in which case the transport of radionuclides tends to be strongly dependent on the extent of colloid-fracture wall interaction.     

Modeling the long-term transport and accumulation of radionuclides in the landscape for derivation of dose conversion factors

To evaluate the radiological impact of potential releases to the biosphere from a geological repository for spent nuclear fuel, it is necessary to assess the long-term dynamics of the distribution of radionuclides in the environment. In this paper, we propose an approach for making prognoses of the distribution and fluxes of radionuclides released from the geosphere, in discharges of contaminated groundwater, to an evolving landscape. The biosphere changes during the temperate part (spanning approximately 20,000 years) of an interglacial period are handled by building biosphere models for the projected succession of situations. Radionuclide transport in the landscape is modeled dynamically with a series of interconnected radioecological models of those ecosystem types (sea, lake, running water, mire, agricultural land and forest) that occur at present, and are projected to occur in the future, in a candidate area for a geological repository in Sweden. The transformation between ecosystems is modeled as discrete events occurring every thousand years by substituting one model by another. Examples of predictions of the radionuclide distribution in the landscape are presented for several scenarios with discharge locations varying in time and space. The article also outlines an approach for estimating the exposure of man resulting from all possible reasonable uses of a potentially contaminated landscape, which was used for derivation of Landscape Dose Factors.     

Modeling of non-reactive solute transport in fractured clayey till during variable flow rate and time

Fractures and biopores can act as preferential flow paths in clay aquitards and may rapidly transmit contaminants into underlying aquifers. Reliable numerical models for assessment of groundwater contamination from such aquitards are needed for planning, regulatory and remediation purposes. In this investigation, high resolution preferential water-saturated flow and bromide transport data were used to evaluate the suitability of equivalent porous medium (EPM), dual porosity (DP) and discrete fracture/matrix diffusion (DFMD) numerical modeling approaches for assessment of flow and non-reactive solute transport in clayey till. The experimental data were obtained from four large undisturbed soil columns (taken from 1.5 to 3.5 m depth) in which biopores and channels along fractures controlled 96-99% of water-saturated flow. Simulating the transport data with the EPM effective porosity model (FRACTRAN in EPM mode) was not successful because calibrated effective porosity for the same column had to be varied up to 1 order of magnitude in order to simulate solute breakthrough for the applied flow rates between 11 and 49 mm/day. Attempts to simulate the same data with the DP models CXTFIT and MODFLOW/MT3D were also unsuccessful because fitted values for dispersion, mobile zone porosity, and mass transfer coefficient between mobile and immobile zones varied several orders of magnitude for the different flow rates, and because dispersion values were furthermore not physically realistic. Only the DFMD modeling approach (FRACTRAN in DFMD mode) was capable to simulate the observed changes in solute transport behavior during alternating flow rate without changing values of calibrated fracture spacing and fracture aperture to represent the macropores.     

Influence of hydrological and geochemical processes on the transport of chelated metals and chromate in fractured shale bedrock

Field-scale processes governing the transport of chelated radionuclides in groundwater remain conceptually unclear for highly structured, heterogeneous environments. The objectives of this research were to provide an improved understanding and predictive capability of the hydrological and geochemical mechanisms that control the transport behavior of chelated radionuclides and metals in anoxic subsurface environments that are complicated by fracture flow and matrix diffusion. Our approach involved a long-term, steady-state natural gradient field experiment where nonreactive Br- and reactive 57Co(II)EDTA2- 109CdEDTA2-, and 51Cr(VI) were injected into a fracture zone of a contaminated fractured shale bedrock. The spatial and temporal distribution of the tracer and solutes was monitored for 500 days using an array of groundwater sampling wells instrumented within the fast-flowing fracture regime and a slower flowing matrix regime. The tracers were preferentially transported along strike-parallel fractures coupled with the slow diffusion of significant tracer mass into the bedrock matrix. The chelated radionuclides and metals were significantly retarded by the solid phase with the mechanisms of retardation largely due to redox reactions and sorption coupled with mineral-induced chelate-radionuclide dissociation. The formation of significant Fe(III)EDTA byproduct that accompanied the dissociation of the radionuclide-chelate complexes was believed to be the result of surface interactions with biotite which was the only Fe(III)-bearing mineral phase present in these Fe-reducing environments. These results counter current conceptual models that suggest chelated contaminants move conservatively through Fe-reducing environments since they are devoid of Fe-oxyhydroxides that are known to aggressively compete for chelates in oxic regimes. Modeling results further demonstrated that chelate-radionuclide dissociation reactions were most prevalent along fractures where accelerated weathering processes are expected to expose more primary minerals than the surrounding rock matrix. The findings of this study suggest that physical retardation mechanisms (i.e. diffusion) are dominant within the matrix regime, whereas geochemical retardation mechanisms are dominant within the fracture regime.     

Flow and transport in unsaturated fractured rock: effects of multiscale heterogeneity of hydrogeologic properties

The heterogeneity of hydrogeologic properties at different scales may have different effects on flow and transport processes in a subsurface system. A model for the unsaturated zone of Yucca Mountain, Nevada, is developed to represent complex heterogeneity at two different scales: (1) layer scale corresponding to geologic layering and (2) local scale. The layer-scale hydrogeologic properties are obtained using inverse modeling, based on the available measurements collected from the Yucca Mountain site. Calibration results show a significant lateral and vertical variability in matrix and fracture properties. Hydrogeologic property distributions in a two-dimensional, vertical cross-section of the site are generated by combining the average layer-scale matrix and fracture properties with local-scale perturbations generated using a stochastic simulation method. The unsaturated water flow and conservative (nonsorbing) tracer transport through the cross-section are simulated for different sets of matrix and fracture property fields. Comparison of simulation results indicates that the local-scale heterogeneity of matrix and fracture properties has a considerable effect on unsaturated flow processes, leading to fast flow paths in fractures and the matrix. These paths shorten the travel time of a conservative tracer from the source (repository) horizon in the unsaturated zone to the water table for small fractions of total released tracer mass. As a result, the local-scale heterogeneity also has a noticeable effect on global tracer transport processes, characterized by an average breakthrough curve at the water table, especially at the early arrival time of tracer mass. However, the effect is not significant at the later time after 20% tracer mass reaches the water table. The simulation results also verify that matrix diffusion plays an important role in overall solute transport processes in the unsaturated zone at Yucca Mountain.     

Organic carbon oxidation induced by large-scale shallow water intrusion into a vertical fracture zone at the Äspö Hard Rock Laboratory (Sweden)

Entrance tunnel construction at the Äspö Hard Rock Laboratory opened a conductive vertical fracture zone at a depth of 70 m on March 13, 1991. Three weeks later a sharp dilution front corresponding to 80% shallow water inflow to the originally saline fracture zone arrived at the entrance tunnel depth. In spite of this large inflow of shallow water, the fracture zone has remained persistently anoxic over a subsequent period. Results from gas sampling and ¹⁴C dating of dissolved organic and inorganic carbon conclusively show that recent organic carbon is being transported into the fracture zone and oxidized to carbon dioxide. These results are important when considering possible changes of redox status in the deep groundwater environment during construction and operation of a repository for spent nuclear fuel. Opening this fracture zone to large-scale surface water inflow adds reducing capacity in the form of organic carbon. This implies that the soil cover may provide important protection against input of dissolved oxygen to fractures being drained during the open phase of the repository.     

Study of the contaminant transport into granite microfractures using nuclear ion beam techniques

Hydrated bentonite is a very plastic material and it is expected to enter in the rock microfractures at the granite/bentonite boundary of a deep geological high-level waste repository. This process is enhanced by the high swelling pressure of the clay. Since bentonite has a very good sorption capability for many radionuclides, the displacement of the clay might lead to a "clay-mediated" contaminant transport into the rock. The aim of this work is to study the contaminant transport into granite microfractures using nuclear ion beam techniques, and to determine to what extent the clay can favour it. To do so, bentonite previously doped with uranium, cesium and europium was put in contact with the surface of granite sheets. Granite sheets contacted with non-doped bentonite and with radionuclide solutions were also prepared as references. This allowed analysing the differences in the diffusion behaviour of the three systems: clay, radionuclides and clay plus radionuclides. A combination of Rutherford backscattering spectrometry (RBS) and other nuclear ion-beam techniques such as particle-induced X-ray emission (PIXE) and microPIXE was used to study the depth and lateral distribution of clay and contaminants inside granite. It was also tried to evaluate not only the diffusion depth and diffusion coefficients but also the different areas of the granite where the diffusants have a preferential access.     

Model of the Long-Term Transport and Accumulation of Radionuclides in Future Landscapes

Assessments of radiological impacts on humans and other biota from potential releases to the biosphere from a deep geologic repository for spent nuclear fuel are associated with several challenges. Releases, if any, will likely occur in a far future and to an environment that will have experienced substantial transformations. Such releases would occur over very long periods during which environmental conditions will vary continuously due to climate change and ecosystem succession. Assessments of radiological impacts must therefore be based on simulations using models that can describe the transport and accumulation of radionuclides for a large variety of environmental conditions. In this paper we describe such a model and show examples of its application in a safety assessment, taking into account results from sensitivity and uncertainty analyses of the model predictions.     

Arsenic forensic modeling leads to decommission of downgradient/off-site monitoring wells

Arsenic was detected at concentrations exceeding the regulatory limit of 0.010 milligrams per liter (mg/L) in an off-site bedrock monitoring well downgradient of a former electroplating facility in Merrimack, New Hampshire. The bedrock underlying the site is associated with naturally occurring high concentrations of groundwater arsenic. Geochemical modeling was used to evaluate whether the arsenic in bedrock groundwater at the off-site monitoring location was site-related or naturally occurring. The hydrogeochemical signature of the off-site bedrock well did not resemble signatures of site-impacted bedrock wells. Multiple lines of evidence support that the arsenic observed in off-site bedrock groundwater was not a result of adverse impacts from site-related groundwater contamination.     

Scoping analyses of geochemical sealing of early cracks in a waste container and associated drip shield, Yucca Mountain, Nevada

Early after final emplacement of the nuclear waste containers at the proposed Yucca Mountain, Nevada, high-level-waste repository, tiny cracks (less than 200 microm wide, 1 to 2 cm deep, and a few centimeters long at most) could appear in the containers and in the drip shield protecting them. Modeling calculations were performed to understand how fast those cracks could be sealed. Under dripping conditions, they are expected to be bridged with water. If cracks are located in the drip shield, any further dripping on the waste containers located underneath will be limited. If cracks are located in a container, potentially harmful radionuclides could only travel by diffusion. In addition, water-bridged cracks will be sealed through at least two processes: precipitation of calcite with minor silica following evaporative concentration of the water residing in the cracks and continuous corrosion of the crack walls. The sealing rate is calculated as the intersection of the time of emergence of the cracks, the water dripping rate, and the decreasing evaporation rate. The evaporative driving force declines as short-lived radioactive elements, having given up much of the heat affecting the repository, are progressively depleted from the waste. Depending on the crack initiation time and environmental conditions, crack sealing varies from a few tens of years to a few thousand years. Because environmental conditions in the vicinity of the cracks and at the crack scale have not been produced, a parametric method scaling drift scale conditions is used.     

Characterization of flow and transport processes within the unsaturated zone of Yucca Mountain, Nevada, under current and future climates

This paper presents a large-scale modeling study characterizing fluid flow and tracer transport in the unsaturated zone of Yucca Mountain, Nevada, a potential repository site for storing high-level radioactive waste. The study has been conducted using a three-dimensional numerical model, which incorporates a wide variety of field data and takes into account the coupled processes of flow and transport in the highly heterogeneous, unsaturated fractured porous rock. The modeling approach is based on a dual-continuum formulation of coupled multiphase fluid and tracer transport through fractured porous rock. Various scenarios of current and future climate conditions and their effects on the unsaturated zone are evaluated to aid in the assessment of the proposed repository's system performance using different conceptual models. These models are calibrated against field-measured data. Model-predicted flow and transport processes under current and future climates are discussed.     

Uranium transport around the reactor zone at Bangombé and Okélobondo (Oklo): examples of hydrogeological and geochemical model integration and data evaluation

The sites at Bangombé and Okélobondo (Oklo) in Gabon provide a unique opportunity to study the behaviour of products from natural nuclear reactions in the vicinity of reactor zones which were active around two billion years ago. The Commission of the European Communities initiated the Oklo Natural Analogue Programme. One of the principal aims was to study indications of present time migration of elements from the reactor zones under ambient conditions. The hydrogeological and hydrochemical data from the Oklo sites were modelled in order to better understand the geochemical behaviour of radionuclides in the natural system, by using independent models and by comparing the modelling outcome. Two modelling approaches were used: M3 code (hydrochemical mixing and mass balance model), developed by the Swedish Nuclear Fuel and Waste Management Company (SKB) and HYTEC (reactive transport model) developed by Ecole des Mines de Paris.Two different reactor zones were studied: Bangombé, a shallow site, the reactor being at 11 m depth, and OK84 at Okélobondo, situated at about 450 m depth, more comparable with a real repository location. This allowed the validation of modelling tools in two different sedimentary environments: one shallow, with a more homogeneous layering situated in an area of meteoric alteration, and the other offering the opportunity to study radionuclide migration from the reaction zone over a distance of 450 m through very heterogeneous sedimentary layers.The modeling results indicate that the chemical reactions retarding radionuclide transport are very different at the two sites. At Bangombé, the decomposition of organic material consumes oxygen and at Okélobondo the oxygen is consumed by inorganic reactions resulting, in both cases, in uranium retardation. Both modelling approaches (statistic with M3 code and deterministic with HYTEC code) could describe this situation.The goal of this exercise is to test codes which can help to describe and understand the processes taking place at the sites, validate the models with in situ data, and thus build confidence in the tools used for future site characterization. Ultimately, this allows identifying and selecting processes and parameters that can be used as input into repository performance assessment calculations and modelling exercises.     

Uranium transport around the reactor zone at Bangombé and Okélobondo (Oklo): examples of hydrogeological and geochemical model integration and data evaluation

The sites at Bangombé and Okélobondo (Oklo) in Gabon provide a unique opportunity to study the behaviour of products from natural nuclear reactions in the vicinity of reactor zones which were active around two billion years ago. The Commission of the European Communities initiated the Oklo Natural Analogue Programme. One of the principal aims was to study indications of present time migration of elements from the reactor zones under ambient conditions. The hydrogeological and hydrochemical data from the Oklo sites were modelled in order to better understand the geochemical behaviour of radionuclides in the natural system, by using independent models and by comparing the modelling outcome. Two modelling approaches were used: M3 code (hydrochemical mixing and mass balance model), developed by the Swedish Nuclear Fuel and Waste Management Company (SKB) and HYTEC (reactive transport model) developed by Ecole des Mines de Paris. Two different reactor zones were studied: Bangombé, a shallow site, the reactor being at 11 m depth, and OK84 at Okélobondo, situated at about 450 m depth, more comparable with a real repository location. This allowed the validation of modelling tools in two different sedimentary environments: one shallow, with a more homogeneous layering situated in an area of meteoric alteration, and the other offering the opportunity to study radionuclide migration from the reaction zone over a distance of 450 m through very heterogeneous sedimentary layers. The modeling results indicate that the chemical reactions retarding radionuclide transport are very different at the two sites. At Bangombé, the decomposition of organic material consumes oxygen and at Okélobondo the oxygen is consumed by inorganic reactions resulting, in both cases, in uranium retardation. Both modelling approaches (statistic with M3 code and deterministic with HYTEC code) could describe this situation. The goal of this exercise is to test codes which can help to describe and understand the processes taking place at the sites, validate the models with in situ data, and thus build confidence in the tools used for future site characterization. Ultimately, this allows identifying and selecting processes and parameters that can be used as input into repository performance assessment calculations and modelling exercises.     

Flow and transport in fractured tuff at Yucca Mountain: numerical experiments on fast preferential flow mechanisms

Recent discovery of bomb-related ³⁶Cl at depth in fractured tuff in the unsaturated zone at the Yucca Mountain candidate high-level waste (HLW) repository site has called into question the usual modeling assumptions based on the equivalent continuum model (ECM). A dual continuum model (DCM) for simulating transient flow and transport at Yucca Mountain is developed. In order to ensure properly converged flow solutions, which are used in the transport simulation, a new flow solution convergence criteria is derived. An extensive series of simulation studies is presented which indicates that rapid movement of solute through the fractures will not occur unless there are intense episodic infiltration events. Movement of solute in the environs of the repository is enhanced if the properties of the tuff layer at the repository horizon are modified from current best-estimate values. Due to a large advective–dispersive coupling between the matrix and fractures, the matrix acts as a major buffer which inhibits rapid transport along the fractures. Consequently, fast movement of solutes through the fractures to the repository depth can only be explained if the matrix–fracture coupling term is significantly reduced from a value that would be calculated on the basis of data currently available.