Potential Geologic Repository Research Articles

Quantitative Evaluation of CO2 Storage Potential in the Offshore Atlantic Lower Cretaceous Strata, Southeastern United States

The geological storage of CO2 in the Earth’s subsurface has the potential to significantly offset greenhouse gas emissions for safe, economical, and acceptable public use. Due to legal advantages and vast resource capacity, offshore CO2 storage provides an attractive alternative to onshore options. Although offshore Lower Cretaceous reservoirs have a vast expected storage capacity, there is a limited quantitative assessment of the offshore storage resource in the southeastern United States. This work is part of the Southeast Offshore Storage Resource Assessment (SOSRA) project, which presents a high-quality potential geological repository for CO2 in the Mid- and South Atlantic Planning Areas. This is the first comprehensive investigation and quantitative assessment of CO2 storage potential for the Lower Cretaceous section of the outer continental shelf that includes the Southeast Georgia Embayment and most of the Blake Plateau. An interpretation of 200,000 km of legacy industrial 2D seismic reflection profiles and geophysical well logs (i.e., TRANSCO 1005-1-1, COST GE-1, and EXXON 564-1) were utilized to create structure and thickness maps for the potential reservoirs and seals. We identified and assessed three target reservoirs isolated by seals based on their effective porosity values. The CO2 storage capacity of these reservoirs was theoretically calculated using the DOE-NETL equation for saline formations. The prospective storage resources are estimated between 450 and 4700 Mt of CO2, with an offshore geological efficiency factor of dolomite between 2% and 3.6% at the formation scale.

Preliminary investigation of14C migration from RBMK-1500 reactor graphite disposed of in a potential geological repository in crystalline rocks in Lithuania

ABSTRACTThere are two units with RBMK-1500 type reactors at the Ignalina Nuclear Power Plant (Ignalina NPP) in Lithuania where graphite was used as a neutron moderator and reflector. These reactors are now being decommissioned, and Lithuania has to find a solution for safe irradiated graphite disposal. It cannot be disposed of in a near surface repository due to large amounts of14C (radiocarbon, carbon-14); thus, a deep geological repository (DGR) is analyzed as an option. This study had the aim to evaluate14C migration from the RBMK-1500 irradiated graphite disposed of in a potential DGR in crystalline rocks taking into account the outcomes of the research performed under the collaborative European project CAST (CArbon-14 Source Term) and to identify the potential to reduce the conservatism in the assumptions that was introduced in the lack of data and led in the overestimated14C migration. The information gathered during the CAST project was used to model14C transport in the near field by the water pathway and to perform uncertainty analysis. The study demonstrated that more realistic assumptions could reduce the estimated14C flux from the near field by approximately one order of magnitude in comparison with the previous estimations based on very conservative assumptions.

Stochastic Seepage Model in Repository of Low and Intermediate Level Solid Radioactive Wastes

Repository of Low and Intermediate Level Solid Radioactive Wastes (ILRW) is accepted to deal with the ILRW produced from fast development of nuclear power industry and other fields. In performance assessment of the potential geologic repository, modeling plays a very important role in addition to experimental inverstigations. In addition to experimental inverstigations modeling has also to be used for these purposes. In the paper, stochastic mathematical theories were used for calculating the radionuclide transport in fractured rock masses at Repository of Low and Intermediate Level Solid Radioactive Wastes. To better understand about the dangers of public for radiation and the nuclides transport in fractured rock masses at low and intermediate level solid radioactive waste disposal, the paper builds three dimensional stochastic seepage grid model to describe radionuclide transport in fractured rock masses based on three-dimensional directional seepage theory that is built into percolation mechanics, sets up a rigorous computer simulating system to calculate the radionuclide transport and safety performance of the Repository of Low and Intermediate Level Solid Radioactive Wastes based on computing technology. Finally, To illustrate the precision and validity of this model that is built into the stochastic mathematical theories or computing technology, the simulation experiments are carried out to understand law of radionuclide transport.

Effects on groundwater flow of abandoned engineered structures for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark, Sweden

Effects on groundwater flow of abandoned engineered structures in relation to a potential geological repository for spent high-level nuclear fuel in fractured crystalline rock at the Forsmark site, Sweden, are studied by means of numerical modeling. The effects are analyzed by means of particle tracking, and transport-related performance measures are calculated. The impacts of abandoned, partially open repository tunnels are studied for two situations with different climate conditions: a “temperate” climate case with present-day boundary conditions, and a generic future “glacial” climate case with an ice sheet covering the repository. Then, the impact of abandoned open boreholes drilled through the repository is studied for present-day climate conditions. It is found that open repository tunnels and open boreholes can act as easy pathways from repository level to the ground surface; hence, they can attract a considerable proportion of particles released in the model at deposition hole positions within the repository. The changed flow field and flow paths cause some changes in the studied performance measures, i.e., increased flux at the deposition holes and decreased transport lengths and flow-related transport resistances. However, these effects are small and the transport resistance values are still high.

Hydrogeochemical characterisation and modelling of groundwaters in a potential geological repository for spent nuclear fuel in crystalline rocks (Laxemar, Sweden)

Two sites in the eastern coast of Sweden have been investigated by the Swedish Nuclear Fuel and Waste Management Company (SKB), within the framework of the site characterisation programme, as possible candidates for hosting the proposed repository for the long-term storage of spent nuclear fuel: Forsmark and Laxemar. This study presents the main results concerning the hydrogeochemical characterisation of the groundwaters in the second site, Laxemar. The distribution of the main chemical variables in groundwaters are shown and interpreted in combination with the results from speciation–solubility and reaction-path simulations, together with the available mineralogical information. The results indicate that the main processes determining the overall geochemical evolution of the Laxemar groundwaters are advective/diffusive mixing and water–rock interactions driven by past and present climatic changes inducing the input of different recharge waters over time (glacial meltwater, old marine water and modern meteoric water) and affecting the preexisting very old saline groundwaters in the bedrock. The superimposed effects of these mixing events, deduced from the behaviour of the conservative elements (Cl and δ18O), have generated a rather steep salinity gradient in the groundwater system, with diluted waters in the upper part, brackish waters in the middle, and saline waters in the lower part of the bedrock.The resultant successive disequilibrium states imposed by mixing have conditioned the water–rock interaction processes that have affected the non-conservative elements to different degrees. The main chemical reactions found to be important in controlling some of the variability of these elements and some important parameters like pH and alkalinity, are: aluminosilicate and carbonate dissolution/precipitation, quartz and fluorite equilibrium, cation exchange, and gypsum dissolution. These reactions and their importance in the system are presented in this paper.Once the main hydrogeochemical features of the Laxemar groundwaters and the potentially controlling water–rock interactions in the system have been identified and justified with the help of thermodynamic simulations, a general geochemical conceptual model has been proposed. This model will be used as the basis for predicting the future evolution of the groundwater chemistry as an essential part of the safety assessment of the future repository.

Characterisation and modelling of mixing processes in groundwaters of a potential geological repository for nuclear wastes in crystalline rocks of Sweden

This paper presents the mixing modelling results for the hydrogeochemical characterisation of groundwaters in the Laxemar area (Sweden). This area is one of the two sites that have been investigated, under the financial patronage of the Swedish Nuclear Waste and Management Co. (SKB), as possible candidates for hosting the proposed repository for the long-term storage of spent nuclear fuel. The classical geochemical modelling, interpreted in the light of the palaeohydrogeological history of the system, has shown that the driving process in the geochemical evolution of this groundwater system is the mixing between four end-member waters: a deep and old saline water, a glacial meltwater, an old marine water, and a meteoric water. In this paper we put the focus on mixing and its effects on the final chemical composition of the groundwaters using a comprehensive methodology that combines principal component analysis with mass balance calculations. This methodology allows us to test several combinations of end member waters and several combinations of compositional variables in order to find optimal solutions in terms of mixing proportions. We have applied this methodology to a dataset of 287 groundwater samples from the Laxemar area collected and analysed by SKB. The best model found uses four conservative elements (Cl, Br, oxygen-18 and deuterium), and computes mixing proportions with respect to three end member waters (saline, glacial and meteoric). Once the first order effect of mixing has been taken into account, water–rock interaction can be used to explain the remaining variability. In this way, the chemistry of each water sample can be obtained by using the mixing proportions for the conservative elements, only affected by mixing, or combining the mixing proportions and the chemical reactions for the non-conservative elements in the system, establishing the basis for predictive calculations.

Special Section on Mass Transport and Environmental Risk Assessment

Special Section on Mass Transport and Environmental Risk Assessment

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.

Understanding long-term corrosion of Alloy 22 container in the potential Yucca Mountain repository for high-level nuclear waste disposal

Alloy 22 (Ni–22Cr–13Mo–3W–4Fe) is the candidate material for the waste package outer container in a potential geologic repository for high-level nuclear waste disposal at Yucca Mountain, Nevada. This alloy exhibits very low corrosion rates in the absence of environmental conditions promoting crevice corrosion. However, there are uncertainties regarding Alloy 22’s corrosion performance when general corrosion rates and susceptibility to crevice corrosion are extrapolated to a geological time period (e.g. 10 5 years). This paper presents an analysis of available literature information relevant to the long-term extrapolation of general corrosion processes and the crevice corrosion behavior of Alloy 22, under potential repository environments. For assessment of general corrosion rates, potential degradation processes causing the loss of the long-term persistence of passive film formed are considered. For crevice corrosion, induction time, and the extent of susceptibility and opening area, are considered. Disclaimer: The US Nuclear Regulatory Commission (NRC) staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed nor of the acceptability of a license application for a geologic repository at Yucca Mountain. The paper describes work performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) for NRC under Contract Number NRC-02-02-012. The activities reported here were performed by CNWRA on behalf of the NRC office of Nuclear Material Safety and Safeguards, Division of High Level Waste Repository Safety. This paper is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC.

Determination of the effective diffusion coefficient for 125I− in Beishan granite

With the policy of active development of nuclear energy in China in the following 20 years or more, the high-level radioactive wastes and spent fuels will increase considerably. In order to dispose of these wastes safely, China plans to construct a geological repository in the middle of the 21st century. Through more than 20 years of research and discussion in China, granite has been regarded as one of the most favorable candidate host rocks for a potential geological repository and 129 I has been recognized as one of the most important nuclides for the safety of the potential repository. In this paper, the diffusion of 125 I (as a surrogate of 129 I) in deep granite taken from Beishan, Gansu Province is investigated with the help of a small scale diffusion apparatus. The granite core sample was taken at the depth of about 300 m and cut into sections of 5∼20 mm. The granite section samples were weighted and the pore volume was determined with water immersion method. The chemical and elemental compositions of the granite are presented. Experimental results were treated with different methods, producing an effective diffusion coefficient of 125 I − in the granite of (2.44–2.72)×10 −12 m 2 s −1 .

A coupled non-isothermal reactive transport model for long-term geochemical evolution of a HLW repository in clay

A numerical model of coupled saturated/unsaturated water flow, heat transfer and multi-component reactive solute transport is presented to evaluate the long-term geochemical evolution in bentonite, concrete and clay formation for a potential geological radioactive waste repository. Changes in formation porosity caused by mineral dissolution/precipitation reactions are taken into account. Simulations were carried out with a general-purpose multicomponent reactive transport code, CORE2D V4. Numerical results show that pH in the bentonite porewater can vary from neutral to up to 13 over a time scale of 1 Ma although dissolution of silica minerals and precipitation of secondary calcium silicate hydrate (CSH) minerals in bentonite buffer the effect of the hyperalkaline plume. Mineral precipitation reduces the volume of pore space in bentonite close to the bentonite–concrete interface due to the precipitation of CSH minerals. Model results indicate that bentonite porosity decreases less than 25%. The hyperalkaline plume from the concrete only extends to a distance of 0.7 m in the clay formation over the time range of 1 Ma.

Analysis of Water Flow Paths: Methodology and Example Calculations for a Potential Geological Repository in Sweden

Safety assessment related to the siting of a geological repository for spent nuclear fuel deep in the bedrock requires identification of potential flow paths and the associated travel times for radionuclides originating at repository depth. Using the Laxemar candidate site in Sweden as a case study, this paper describes modeling methodology, data integration, and the resulting water flow models, focusing on the Quaternary deposits and the upper 150 m of the bedrock. Example simulations identify flow paths to groundwater discharge areas and flow paths in the surface system. The majority of the simulated groundwater flow paths end up in the main surface waters and along the coastline, even though the particles used to trace the flow paths are introduced with a uniform spatial distribution at a relatively shallow depth. The calculated groundwater travel time, determining the time available for decay and retention of radionuclides, is on average longer to the coastal bays than to other biosphere objects at the site. Further, it is demonstrated how GIS-based modeling can be used to limit the number of surface flow paths that need to be characterized for safety assessment. Based on the results, the paper discusses an approach for coupling the present models to a model for groundwater flow in the deep bedrock.

Regulatory Perspective on Implementation of a Dose Standard for a One-Million Year Compliance Period

AbstractThe disposal of high-level radioactive wastes in a potential geologic repository at Yucca Mountain, Nevada is governed by the U.S. Environmental Protection Agency (EPA) standards and U.S. Nuclear Regulatory Commission (NRC) regulations. The EPA has the responsibility for setting public health and safety and environmental standards for radioactive waste disposal at Yucca Mountain and it is the NRC's responsibility to implement those standards in its regulations to ensure public health and safety and the environment are protected. The U.S. Department of Energy (DOE), as the developer of the potential repository, must submit a license application to the NRC to seek approval to construct the repository. DOE must comply with NRC's regulations for NRC to authorize construction and license operation of a potential repository at Yucca Mountain. In 2005, EPA issued proposed revised standards and NRC issued proposed revised regulations for conducting performance assessment beyond 10,000 years up to 1 million years. The challenge for the EPA and NRC is to develop standards and regulations that provide an appropriate method for evaluating the safety of the potential repository given the unprecedented time period to be analyzed and the inherent uncertainties in estimating the future evolution of the Yucca Mountain site and the containment of the waste in the waste packages. A fundamental aspect of the proposed EPA standard is the specified approach for limiting undue speculation on future behavior of the site by constraining the features, events, and processes that need to be considered in the performance assessment. EPA proposed to limit the assessment of specific features, events, and processes in the period after 10,000 years to effects on the repository system that are most relevant (i.e., ignoring lesser or secondary effects that may add to speculation and uncertainties but would not be expected to have a significant effect on peak dose over a 1 million year period). For example, DOE's performance assessment may (1) limit the analysis of seismic activity to the effects caused by damage to the drifts and the waste package; (2) limit analysis of igneous activity to effects on the waste package that result in release of radionuclides to the atmosphere or ground water; (3) require DOE to include general corrosion in its analysis of engineered barrier performance, and (4) limit the effect of climate variation to those resulting from increased water flowing to the repository. NRC has been reviewing its performance assessment models and techniques to assure they are consistent with EPA's proposed requirements for the period after 10,000 years and adequate to assist the review of a potential license application from the DOE. Currently, only slight modifications to the models and approaches used in the performance assessment for the initial 10,000 years are expected to be needed to accommodate calculations for longer times (e.g., modifications to implement a steady-state value to represent the proposed climate change). NRC staff will be able to use the modified performance assessment to identify additional sensitivities associated with estimating doses over very long time periods and improve its understanding of the performance of a potential repository at Yucca Mountain.

Numerical simulation of thermal–mechanical processes observed at the Drift-Scale Heater Test at Yucca Mountain, Nevada, USA

Abstract Results from the 4-year-long heating phase of the Drift-Scale Heater Test at the Exploratory Studies Facility at Yucca Mountain, Nevada, USA, provide a basis to evaluate conceptual and numerical models used to simulate thermal–mechanical coupled processes expected to occur at the potential geologic repository at Yucca Mountain. The objectives of the evaluation were to investigate coupled processes associated with (i) temperature effects on mechanical deformation and (ii) effect of thermal–mechanical processes on rock-mass permeability. Two-dimensional numerical models were built to perform the thermal–mechanical analyses. Thermal–mechanical simulations were predicated on a continuum representation of a deformation-permeability relationship based on fracture normal stress. The estimated trend of permeability responses using a normal stress-based deformation-permeability relationship compared reasonably to that measured in the coupled thermal–mechanical analyses.

The evaluation of corrosion processes for engineered barrier systems

The U.S. Nuclear Regulatory Commission (NRC) established a corrosion program to evaluate the predicted performance of the U.S. Department of Energy’s proposed engineered barrier system for the potential high-level waste geologic repository at Yucca Mountain, Nevada. The risk-informed, performance-based regulation established by the NRC for the potential licensing of the proposed repository is introduced in this article with a discussion of resolution methods for various technical issues. These resolution methods include probability or consequence screening, deterministic confirmatory research investigations, and the use of the Total-System Performance Assessment Code to determine the risk significance of specific materials degradation processes on the total system performance. An example of the deterministic analysis method is the evaluation of the susceptibility of mill-annealed and gas-tungsten-arc welded alloy 22 to lead-assisted stress corrosion cracking in deaerated PbCl2 and Pb(NO3)2 solutions at 95°C. Also in this article is an evaluation of the risk significance of high-temperature deliquescence-induced localized corrosion of the alloy 22 waste package outer barrier on the total system performance.

Testing claims about volcanic disruption of a potential geologic repository at Yucca Mountain, Nevada

Yucca Mountain is a potential repository site for high‐level radioactive waste. We examine claims that the frequency of future basaltic dike intersection could be 1 × 10−6/yr or higher (i.e., on average, one penetration every million yrs). These claims fail simple tests at four time scales. Spatial‐temporal models that predict intrusion frequencies &gt;2 × 10−7/yr are overly pessimistic because no dikes have been found in the 13 Myr‐old potential repository footprint. More realistic models are developed by considering this non‐detection of dikes and by applying probabilistic hazard assessments based on known patterns of Pleistocene volcanism. Using a hazard assessment code, we estimate the frequency of dike intersection at 5.4 × 10−8/yr with an upper 95% confidence bound of 9.8 × 10−8/yr.

Site Investigations of Potential Repository Sites in Sweden

ABSTRACTThe Swedish nuclear waste management programme has entered the site investigation phase. Early 2002 SKB received permission from the municipalities of Östhammar and Oskarshamn to perform site investigations for a potential deep geologic repository for spent nuclear fuel. The goal of the site investigation phase is to obtain a permit to build the deep repository for spent nuclear fuel. In parallel with the investigations, consultations will be held with county administrative boards, regulatory authorities and municipalities, as well as with members of the public.

Probabilistic Seismic Hazard Analyses for Ground Motions and Fault Displacement at Yucca Mountain, Nevada

Probabilistic seismic hazard analyses were conducted to estimate both ground motion and fault displacement hazards at the potential geologic repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. The study is believed to be the largest and most comprehensive analyses ever conducted for ground-shaking hazard and is a first-of-a-kind assessment of probabilistic fault displacement hazard. The major emphasis of the study was on the quantification of epistemic uncertainty. Six teams of three experts performed seismic source and fault displacement evaluations, and seven individual experts provided ground motion evaluations. State-of-the-practice expert elicitation processes involving structured workshops, consensus identification of parameters and issues to be evaluated, common sharing of data and information, and open exchanges about the basis for preliminary interpretations were implemented. Ground-shaking hazard was computed for a hypothetical rock outcrop at -300 m, the depth of the potential waste emplacement drifts, at the designated design annual exceedance probabilities of 10-3 and 10-4. The fault displacement hazard was calculated at the design annual exceedance probabilities of 10-4 and 10-5.

7.5. Development of the Reference Design Description for a Geologic Repository

ABSTRACTAs the Management and Operating Contractor for the Department of Energy (DOE), TRW developed the “Reference Design Description (RDD) for a Geologic Repository.” The document describes the current design expectations for a potential geologic repository that could be located at Yucca Mountain in Nevada. The Reference Design Description conveys the surface and subsurface repository and the disposal container design. Additionally, it presents the expected long‐term performance of the potential repository. This paper presents the systems approach used to develop the document. In addition, the role of the Reference Design Description in the overall systems design approach to the Mined Geologic Disposal Systems (MGDS) is discussed. Finally, the overall benefits provided to the Civilian Radioactive Waste Management System (CRWMS) and Yucca Mountain Project (YMP) by the Reference Design Description are discussed.

Post-test Comparison of Thermalhyrologic Measurements and Numerical Predictions for theIn-Situ Single Heater Test, Yucca Mountain, Nevada

The Single Heater Test (SHT) is a sixteen-month-long heating and cooling experiment begun in August, 1996, located underground within the unsaturated zone near the potential geologic repository at Yucca Mountain, Nevada. During the 9 month heating phase of the test, roughly 15 m{sup 3} of rock were raised to temperatures exceeding 100 C. In this paper, temperatures measured in sealed boreholes surrounding the heater are compared to temperatures predicted by 3D thermal-hydrologic calculations performed with a finite difference code. Three separate model runs using different values of bulk rock permeability (4 microdarcy to 5.2 darcy) yielded significantly different predicted temperatures and temperature distributions. All the models differ from the data, suggesting that to accurately model the thermal-hydrologic behavior of the SHT, the Equivalent Continuum Model (ECM), the conceptual basis for dealing with the fractured porous medium in the numerical predictions, should be discarded in favor of more sophisticated approaches.