Potential Repository Research Articles

Large-scale numerical simulation of groundwater flow and solute transport in discretely-fractured crystalline bedrock

A large-scale fluid flow and solute transport model was developed for the crystalline bedrock at Olkiluoto Island, Finland, which is considered as potential deep geological repository for spent nuclear fuel. Site characterization showed that the main flow pathways in the low-permeability crystalline bedrock on the island are 13 subhorizontal fracture zones. Compared to other sites investigated in the context of deep disposal of spent nuclear fuel, most deep boreholes drilled at Olkiluoto are not packed-off but are instead left open. These open boreholes intersect the main fracture zones and create hydraulic connections between them, thus modifying groundwater flow. The combined impact of fracture zones and open boreholes on groundwater flow is simulated at the scale of the island. The modeling approach couples a geomodel that represents the fracture zones and boreholes with a numerical model that simulates fluid flow and solute transport. The geometry of the fracture zones that are intersected by boreholes is complex, and the 3D geomodel was therefore constructed with a tetrahedral mesh. The geomodel was imported into the numerical model to simulate a pumping test conducted on Olkiluoto Island. The pumping test simulation demonstrates that fracture–borehole intersections must be accurately discretized, because they strongly control groundwater flow. The tetrahedral mesh provides an accurate representation of these intersections. The calibrated flow model was then used for illustrative scenarios of radionuclide migration to show the impact of fracture zones on solute transport once the boreholes were backfilled. These mass transport simulations constitute base cases for future predictive analyses and sensitivity studies, since they represent key processes to take into consideration for repository performance assessment.

Special Section on Mass Transport and Environmental Risk Assessment

Special Section on Mass Transport and Environmental Risk Assessment

Evolutionary applications summer 2011.

Evolutionary applications summer 2011.

Analytical model for screening potential CO2 repositories

Assessing potential repositories for geologic sequestration of carbon dioxide using numerical models can be complicated, costly, and time-consuming, especially when faced with the challenge of selecting a repository from a multitude of potential repositories. This paper presents a set of simple analytical equations (model), based on the work of previous researchers, that could be used to evaluate the suitability of candidate repositories for subsurface sequestration of carbon dioxide. We considered the injection of carbon dioxide at a constant rate into a confined saline aquifer via a fully perforated vertical injection well. The validity of the analytical model was assessed via comparison with the TOUGH2 numerical model. The metrics used in comparing the two models include (1) spatial variations in formation pressure and (2) vertically integrated brine saturation profile. The analytical model and TOUGH2 show excellent agreement in their results when similar input conditions and assumptions are applied in both. The analytical model neglects capillary pressure and the pressure dependence of fluid properties. However, simulations in TOUGH2 indicate that little error is introduced by these simplifications. Sensitivity studies indicate that the agreement between the analytical model and TOUGH2 depends strongly on (1) the residual brine saturation, (2) the difference in density between carbon dioxide and resident brine (buoyancy), and (3) the relationship between relative permeability and brine saturation. The results achieved suggest that the analytical model is valid when the relationship between relative permeability and brine saturation is linear or quasi-linear and when the irreducible saturation of brine is zero or very small.

CO 2 mineral sequestration mechanisms and capacity of saline aquifers of the Upper Silesian Coal Basin (Central Europe) - Modeling and experimental verification

Significant part of CO 2 emissions from industrial sources in Central Europe originates from the territory of the Upper Silesian Coal Basin (USCB). This paper presents a study of the suitability of saline aquifers within the USCB, as potential greenhouse gas repositories. Evaluation of mineral-trapping mechanisms and assessment of storage capacity of the aquifers is based on hydrochemical modeling and experimental tests of rock–water–gas interactions. Two stages of modeling enabled prediction of the immediate changes in the aquifer and insulating rocks impacted by the beginning of CO 2 injection, and the assessment of long-term effects of sequestration. In the analyzed sandstone aquifers the minerals able to trap CO 2 are dawsonite and dolomite, while siderite or calcite are able to degrade. The phases capable of mineral CO 2 trapping in the cap rocks are: dawsonite, dolomite, and siderite. Mineral-trapping capacity, for the sandstone aquifers is relatively low: 1.2 to 1.9 kgCO 2/m 3, with the exception of the Upper Silesian Sandstone Series – over 6.6 kgCO 2/m 3. The solubility trapping capacity does not exceed 4.07 kgCO 2/m 3.

Bentonite Alteration Due to Thermal-Hydro-Chemical Processes During the Early Thermal Period in a Nuclear Waste Repository

After closure of an underground nuclear waste repository, the decay of radionuclides will raise the temperature in the repository, and the bentonite buffer will resaturate by water inflow from the surrounding host rock. The perturbations from these thermal and hydrological processes are expected to dissipate within hundreds to a few thousand years. Here, we investigate coupled thermal-hydro-chemical processes and their effects on the short-term performance of a potential nuclear waste repository located in a clay formation. Using a simplified geometric configuration and abstracted hydraulic parameters of the clayey formation, we examine geochemical processes, coupled with thermohydrologic phenomena, and potential changes in porosity near the waste container during the early thermal period. The developed models were used for evaluating the mineral alterations and potential changes in porosity of the buffer, which can affect the repository performance. The results indicate that mineral alteration and associated changes in porosity induced by early thermal and hydrological processes are relatively small and are expected to not significantly affect flow and transport properties. Chlorite precipitation was obtained in all simulation cases. A maximum of one percent volume fraction of chlorite could be formed, whose process may reduce swelling and sorption capacity of bentonite clay, affecting the performance of the repository. The illitization process was not obtained from the present simulations.

Reactive transport modeling of the interaction between water and a cementitious grout in a fractured rock. Application to ONKALO (Finland)

► It is planned to seal conductive fractures near a repository with cementitious grout. ► Modeling includes simultaneous hydration and leaching of the grout. ► Modeling results show a very limited formation of the high-pH plume. ► Results are in qualitative agreement with borehole monitoring data. Grouting of water-conducting fractures with low-alkali cement is foreseen for the potential future repository for spent nuclear fuel in Finland (ONKALO). A possible consequence of the interaction between groundwater and grout is the formation of high-pH solutions which will be able to react with the host rock (gneisses) and alter its mineralogy and porosity. A reactive transport modeling study of this possible alteration has been conducted. First, the hydration of the low-alkali cementitious grout has been modeled, using results from the literature as a guide. The hydrated cement is characterized by the absence of portlandite and the presence of a C–S–H gel with a Ca/Si ratio about 0.8 after tens of years (Ca/Si is about 1.7 in Ordinary Portland Cement). Second, calculations have simulated the interaction between flowing water and grout and the formation of an alkalinity plume, which flows beyond the grouted section of the fracture. The calculations include the hydration and simultaneous leaching of the grout through diffusive exchange between the porewater in the grout and the flowing water in the fracture. The formation of an alkaline plume is extremely limited when the low-pH grout is used. Even when using a grout with a lower silica fume content, the extent and magnitude of the alkaline plume is quite minor. These results are in qualitative agreement with monitoring at ONKALO.

Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group

The Mars Science Laboratory (MSL) has an instrument package capable of making measurements of past and present environmental conditions. The data generated may tell us if Mars is, or ever was, able to support life. However, the knowledge of Mars' past history and the geological processes most likely to preserve a record of that history remain sparse and, in some instances, ambiguous. Physical, chemical, and geological processes relevant to biosignature preservation on Earth, especially under conditions early in its history when microbial life predominated, are also imperfectly known. Here, we present the report of a working group chartered by the Co-Chairs of NASA's MSL Project Science Group, John P. Grotzinger and Michael A. Meyer, to review and evaluate potential for biosignature formation and preservation on Mars. Orbital images confirm that layered rocks achieved kilometer-scale thicknesses in some regions of ancient Mars. Clearly, interplays of sedimentation and erosional processes govern present-day exposures, and our understanding of these processes is incomplete. MSL can document and evaluate patterns of stratigraphic development as well as the sources of layered materials and their subsequent diagenesis. It can also document other potential biosignature repositories such as hydrothermal environments. These capabilities offer an unprecedented opportunity to decipher key aspects of the environmental evolution of Mars' early surface and aspects of the diagenetic processes that have operated since that time. Considering the MSL instrument payload package, we identified the following classes of biosignatures as within the MSL detection window: organism morphologies (cells, body fossils, casts), biofabrics (including microbial mats), diagnostic organic molecules, isotopic signatures, evidence of biomineralization and bioalteration, spatial patterns in chemistry, and biogenic gases. Of these, biogenic organic molecules and biogenic atmospheric gases are considered the most definitive and most readily detectable by MSL.

Landscape development in the safety assessment of a potential repository in Forsmark, Sweden

One challenge in the analysis of far future radiological effects of a deep geological repository is to handle changes in time on ecosystems potentially affected. Under Swedish regulations, the time span that needs to be assessed is 100,000 yrs or more. This has historically been treated by using a reference biosphere that, in short, takes into account the total data range of the site properties that may change with different climates and sites. One problem with this strategy has always been to show that these reference biospheres have relevance for a specific site. To overcome this problem, SKB has adopted a strategy that uses site specific information together with scientific literature to build a narrative. This narrative describes the site development during a 100,000-yrs period (time frame of the present global glacial cycle). The information is used to compile a number of models that describe how the succession of abiotic and biotic features and their properties will develop at the site. The resulting Landscape Development Model is finally used in two ways: to tell the narrative of the site development based on scientific understanding and interdisciplinary consensus, and to populate the Radionuclide Model that is used to describe the effects of released radionuclides in a changing environment.

Effect of Well Orientation (Vertical vs. Horizontal) and Well Length on the Injection of CO2 in Deep Saline Aquifers

Simulations of CO2 injection into confined saline aquifers were conducted for both vertical and horizontal injection wells. The metrics used in quantifying the performances of different injection scenarios included changes in pressure near the injection well, mass of CO2 dissolved into brine (solubility trapping), and storage efficiency, all evaluated with an assumed injection period of 50 years. Metrics were quantified as functions of well length, well orientation, CO2 injection rate, and formation anisotropy (ratio of vertical to horizontal conductivity). When equal well lengths are compared, there is not a significant difference between the predicted performances of horizontal and vertical wells. However, the length of a horizontal well may exceed the length of a vertical well because the length of the horizontal well is not constrained to the vertical thickness of the geologic formation. Simulations show that, as the length of the horizontal well is allowed to increase, the geologic formation can receive a significantly higher CO2 injection rate without exceeding a maximum allowable pressure. This result is observed in both isotropic and anisotropic formations, and suggests that horizontal wells may be advantageous under pressure-limited conditions. However, the use of horizontal wells does not significantly improve the storage efficiency, and under strongly anisotropic conditions, a vertical well provides higher storage efficiency than a horizontal well. We conclude that horizontal wells may be preferable if the goal is to sequester a large amount of CO2 in a short period of time, but do not offer a significant advantage in terms of long-term capacity of a potential repository.

Data Repositories for Medical Education Research: Issues and Recommendations

The authors explore issues surrounding digital repositories with the twofold intention of clarifying their creation, structure, content, and use, and considering the implementation of a global digital repository for medical education research data sets-an online site where medical education researchers would be encouraged to deposit their data in order to facilitate the reuse and reanalysis of the data by other researchers. By motivating data sharing and reuse, investigators, medical schools, and other stakeholders might see substantial benefits to their own endeavors and to the progress of the field of medical education.The authors review digital repositories in medicine, social sciences, and education, describe the contents and scope of repositories, and present extant examples. The authors describe the potential benefits of a medical education data repository and report results of a survey of the Society for Directors of Research in Medicine Education, in which participants responded to questions about data sharing and a potential data repository. Respondents strongly endorsed data sharing, with the caveat that principal investigators should choose whether or not to share data they collect. A large majority believed that a repository would benefit their unit and the field of medical education. Few reported using existing repositories. Finally, the authors consider challenges to the establishment of such a repository, including taxonomic organization, intellectual property concerns, human subjects protection, technological infrastructure, and evaluation standards. The authors conclude with recommendations for how a medical education data repository could be successfully developed.

Partitioning of Histone H3-H4 Tetramers During DNA Replication–Dependent Chromatin Assembly

Semiconservative DNA replication ensures the faithful duplication of genetic information during cell divisions. However, how epigenetic information carried by histone modifications propagates through mitotic divisions remains elusive. To address this question, the DNA replication-dependent nucleosome partition pattern must be clarified. Here, we report significant amounts of H3.3-H4 tetramers split in vivo, whereas most H3.1-H4 tetramers remained intact. Inhibiting DNA replication-dependent deposition greatly reduced the level of splitting events, which suggests that (i) the replication-independent H3.3 deposition pathway proceeds largely by cooperatively incorporating two new H3.3-H4 dimers and (ii) the majority of splitting events occurred during replication-dependent deposition. Our results support the idea that "silent" histone modifications within large heterochromatic regions are maintained by copying modifications from neighboring preexisting histones without the need for H3-H4 splitting events.

Translational research queries, fascia research congress sequels, and beyond....

Translational research queries, fascia research congress sequels, and beyond....

Stochastic Simulation of Non-Gaussian 3D Conductivity Fields in a Fractured Medium with Multiple Statistical Populations: Case Study

This paper applies a stochastic inverse method, named as gradual conditioning (GC) method, to the fractured site of Aspo, Sweden, which is an underground hard rock laboratory initially designed as a potential future deep geological repository for spent nuclear fuel. The aim of this paper is (1) the verification of GC method in a real three-dimensional (3D) fractured rock medium, showing that the GC method is a competitive stochastic tool in highly heterogeneous aquifers, furthermore, it gathers a set of capabilities so far not included in any existing method; (2) to characterize the site as adequately as possible, experimental data are reproduced closely; (3) to provide measures on the uncertainty of the estimates by means of using multiple equally likely realizations, thus showing the importance of conditioning to as much information as possible in order to reduce the uncertainty; (4) to prove that this fractured media can be adequately modeled by assuming a (pseudo-) continuum media, or equivalent porou...

Comparison of Experimental and Numerical Two-Phase Flows in a Porous Micro-Model

Characterizing two-phase flow in porous media is important to provide estimates of sweep efficiency in enhanced oil recovery and storage estimates in potential geological CO2 sequestration repositories. To further the current understanding of two-phase flow in porous media a micro-model of interconnected channels was designed and fabricated using stereolithography to experimentally study gas-liquid flows. This flowcell was created with a wide variability of throat dimensions to represent naturally occurring porous media. Low flow rate experiments of immiscible two-phase drainage were performed within this cell. Additionally, a computational model for analyzing two-phase flows in the same flowcell was developed and used to simulate conditions not possible with our laboratory settings. The computational model was first tested for the identical conditions used in the experimental studies, and was shown to be in good agreement with the experimentally determined fractal dimension of the invading gas structure, time until breakthrough, and fluid saturation. The numerical model was then used to study two-phase air-water flows in flowcells with the same geometry and different gas-liquid-solid contact angles. The percent saturation of air and the motion of the fluids through the cell were found to vary with changes in these parameters. Finally, to simulate flows expected during geologic carbon sequestration, the fluid properties and interface conditions were set to model the flow of CO2 into a brine-saturated porous medium at representative subsurface conditions. The CO2 flows were shown to have larger gas saturations than the previous air into water studies. Thus the accuracy of the computational model was supported by the flowcell experiments, and the computational model extended the laboratory results to conditions not possible with the apparatus used in the experiments.

Deformation analysis of tuffaceous sediments in the Volcanic Tableland near Bishop, California

Small-scale brittle faults and fractures that cut bedded tuffaceous sediments of variable textures and grain sizes were studied in a 110-m-long cutbank exposure of poorly consolidated sediments at the southern erosional boundary of the Volcanic Tableland, Owens Valley, California. This study was motivated by the need to evaluate potential length scales for lateral flow in nonwelded bedded tuffs and tuffaceous sediments at Yucca Mountain, Nevada—the site of a potential high-level radioactive waste repository. Small-displacement ( 20 cm). Vertical fractures are present throughout the exposure, but fracture frequency is generally highest in the vicinity of larger faults. Fault zones are characterized by grain-size reduction and discrete slip surfaces, the number of which increases with increasing displacement. A semiquantitative stress field interpretation, based on tectonic constraints and reconstruction of overburden thicknesses, yields a simple history of burial, deformation, and exhumation under continuous tectonic extension. We interpret the deformation to include shear (faults), hybrid (faults, nonvertical fractures), and tensile (vertical fractures) failure of the tuffaceous sediments under conditions of low overburden stress (<2.5 MPa). The intersecting network of faults and fractures is characterized by grain comminution, cementation, and fracture dilation. These features, in conjunction with stratigraphic layering, likely produce anisotropic permeability, where maximum permeability is parallel to fault and fracture strike.

Effects of Test Methods on Crevice Corrosion Repassivation Potential Measurements of Alloy 22

Abstract Alloy 22 (UNS N06022) may be used as the waste package outer container material for disposal of high-level waste at the potential repository at Yucca Mountain, Nevada. Crevice corrosion is one of the corrosion processes that may affect the performance of the waste package outer container. The relative susceptibility of Alloy 22 to crevice corrosion is evaluated through the measurement of crevice corrosion repassivation potential (Ercrev) using electrochemical test methods. The main purpose of the work described in this paper is to verify some Ercrev values reported in the literature and investigate the effects of different electrochemical test methods on the measurement of Ercrev. This work also provides risk insights and reduces the measurement uncertainties associated with evaluating the effects of the fabrication process on crevice corrosion susceptibility. In this work, Ercrev was measured in various chloride solutions at 90°C (194°F) using three methods: ASTM G61-86, the potentiodynamic pola...

Episodic fluid migration in the Fennoscandian Shield recorded by stable isotopes, rare earth elements and fluid inclusions in fracture minerals at Forsmark, Sweden

Drilling through the Palaeoproterozoic bedrock at Forsmark, central Sweden, during the site investigation for a potential geological repository of highly radioactive nuclear waste has provided high quality drill-core material from the upper 1 km of the Fennoscandian Shield. Analyses of stable isotopes (δ 13C, δ 18O, δ 34S, 87Sr/ 86Sr), rare earth elements and fluid inclusions in fracture filling calcite and pyrite from these drill cores have resulted in the discrimination of several episodes of fracture mineralisations. These events represent migration of fluids during a wide range of conditions, ranging from high-temperature hydrothermal to present-day groundwater circulation. Four major events have been distinguished: 1) Precipitation of epidote, chlorite and quartz under hydrothermal conditions ( T > 150–200 °C) during the Proterozoic, sometime between 1.8 and 1.1 Ga. 2) Hydrothermal circulation at temperatures close to 200 °C with precipitation of adularia, albite, prehnite, laumontite, calcite and chlorite. Most of these minerals precipitated during a tectonothermal event between 1.1 and 1.0 Ga, possibly in response to far-field effects of the Sveconorwegian orogeny. 3) Precipitation of mainly quartz, calcite, pyrite and asphaltite occurred during the Palaeozoic, at temperatures between 60 and 190 °C (mainly at < 100 °C). Mixing of a fluid emanating from an organic rich overlying sedimentary cover and a deep basinal fluid from the crystalline bedrock is suggested to have caused this precipitation, possibly as a far-field response to the Caledonian orogeny and/or the development of the Caledonian foreland basin. 4) The youngest generation of fracture minerals is associated with formation of clay minerals and calcite with minor occurrences of pyrite and goethite. These minerals have probably precipitated episodically during a long time period (possibly from the Late Palaeozoic to the present) from various fluids at low temperature conditions (< 50 °C). Few calcites in equilibrium with the present groundwater suggest that the ongoing precipitation of calcite is very limited.

Detecting the near-surface redox front in crystalline bedrock using fracture mineral distribution, geochemistry and U-series disequilibrium

Oxidizing conditions normally prevail in surface waters and near-surface groundwaters, but there is usually a change to reducing conditions in groundwater at greater depth. Dissolved O2 originally present is consumed through biogenic and inorganic reactions along the flow paths. Fracture minerals participate in these reactions and the fracture mineralogy and geochemistry can be used to trace the redox front. An important task in the safety assessment of a potential repository for the disposal of nuclear waste in crystalline bedrock, at an approximate depth of 500 m in Sweden, is to demonstrate that reducing conditions can be maintained for a long period of time. Oxygen may damage the Cu canisters that host nuclear waste; additionally, in the event of a canister failure, oxidizing conditions may increase the mobility of some radionuclides. The present study of the near-surface redox front is based on mineralogical (redox-sensitive minerals), geochemical (redox-sensitive elements) and U-series disequilibrium investigations of mineral coatings along open fractures. The fractures have been sampled along drill cores from closely spaced, 100 m deep boreholes, which were drilled during the site investigation work in the Laxemar area, south-eastern Sweden, carried out by the Swedish Nuclear Fuel and Waste Management Co. (SKB). The distribution of the redox-sensitive minerals pyrite and goethite in open fractures shows that the redox front (switch from mainly goethite to mainly pyrite in the fractures) generally occurs at about 15–20 m depth. Calcite leaching by recharging water is indicated in the upper 20–30 m and positive Ce-anomalies suggest oxidation of Ce down to 20 m depth. The U-series radionuclides show disequilibrium in most of the samples, indicating mobility of U during the last 1 Ma. In the upper 20 m, U is mainly removed (due to oxidation) or has experienced complex removal and/or deposition. At depths of 35–55 m, both deposition and removal of U are indicated. Below 55 m, recent deposition of U is generally indicated which suggests removal of U near surface (oxidation) and deposition of U below the redox front. Scattered goethite occurrences below the general redox front (down to ca 80 m) and signs of U removal at 35–55 m mostly correlate with sections of high transmissivity (and/or high fracture frequencies). This shows that highly transmissive fractures are generally required to allow oxygenated groundwaters at depth greater than ca 30 m. Removal of U (oxidation) below 55 m within the last 300 ka is not observed. Although penetration of glacial waters to great depths has been confirmed in the study area, their potential O2 load seems to have been reduced near the surface.

Environmental mobility of cobalt—Influence of solid phase characteristics and groundwater chemistry

The adsorption of cobalt on samples from a potential waste repository site in an arid region was investigated in batch experiments, as a function of various solution phase parameters including the pH and ionic strength. The samples were characterized using a range of techniques, including BET surface area measurements, total clay content and quantitative X-ray diffraction. The statistical relationships between the measured cobalt distribution coefficients ( K d values) and the solid and liquid phase characteristics were assessed. The sorption of cobalt increased with the pH of the aqueous phase. In experiments with a fixed pH value, the measured K d values were strongly correlated to the BET surface area, but not to the amount of individual clay minerals (illite, kaolinite or smectite). A further set of sorption experiments was undertaken with two samples of distinctive mineralogy and surface area, and consequently different sorption properties. A simple surface complexation model (SCM) that conceptualized the surface sites as having equivalent sorption properties to amorphous Fe-oxide was moderately successful in explaining the pH dependence of the sorption data on these samples. Two different methods of quantifying the input parameters for the SCM were assessed. While a full SCM for cobalt sorption on these complex environmental substrates is not yet possible, the basic applicability and predictive capability of this type of modeling is demonstrated. A principal requirement to further develop the modeling approach is adequate models for cobalt sorption on component mineral phases of complex environmental sorbents.