Publications

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Deep boreholes have been proposed for many decades as an option for permanent disposal of high-level radioactive waste and spent nuclear fuel. Disposal concepts are straightforward, and generally call for drilling boreholes to a depth of three to five kilometers into crystalline basement rocks. Waste is placed in the lower portion of the hole, and the upper several kilometers of the hole are sealed to provide effective isolation from the biosphere. The potential for excellent long-term performance has been recognized in many previous studies. This paper reports updated results of what is believed to be the first quantitative analysis of releases from a hypothetical disposal borehole repository using the same performance assessment methodology applied to mined geologic repositories for high-level radioactive waste. Analyses begin with a preliminary consideration of a comprehensive list of potentially relevant features, events, and processes (FEPs) and the identification of those FEPs that appear to be most likely to affect long-term performance in deep boreholes. Performance assessment model estimates of releases from deep boreholes, and the annual radiation doses to hypothetical future humans associated with those releases, are extremely small, indicating that deep boreholes may be a viable alternative to mined repositories for disposal of both high-level radioactive waste and spent nuclear fuel.

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Disposal of high-level radioactive waste, including spent nuclear fuel, in deep (3 to 5 km) boreholes is a potential option for safely isolating these wastes from the surface and near-surface environment. Existing drilling technology permits reliable and cost-effective construction of such deep boreholes. Conditions favorable for deep borehole disposal in crystalline basement rocks, including low permeability, high salinity, and geochemically reducing conditions, exist at depth in many locations, particularly in geologically stable continental regions. Isolation of waste depends, in part, on the effectiveness of borehole seals and potential alteration of permeability in the disturbed host rock surrounding the borehole. Coupled thermal-mechanical-hydrologic processes induced by heat from the radioactive waste may impact the disturbed zone near the borehole and borehole wall stability. Numerical simulations of the coupled thermal-mechanical response in the host rock surrounding the borehole were conducted with three software codes or combinations of software codes. Software codes used in the simulations were FEHM, JAS3D, Aria, and Adagio. Simulations were conducted for disposal of spent nuclear fuel assemblies and for the higher heat output of vitrified waste from the reprocessing of fuel. Simulations were also conducted for both isotropic and anisotropic ambient horizontal stress in the host rock. Physical, thermal, and mechanical properties representative of granite host rock at a depth of 4 km were used in the models. Simulation results indicate peak temperature increases at the borehole wall of about 30 C and 180 C for disposal of fuel assemblies and vitrified waste, respectively. Peak temperatures near the borehole occur within about 10 years and decline rapidly within a few hundred years and with distance. The host rock near the borehole is placed under additional compression. Peak mechanical stress is increased by about 15 MPa (above the assumed ambient isotropic stress of 100 MPa) at the borehole wall for the disposal of fuel assemblies and by about 90 MPa for vitrified waste. Simulated peak volumetric strain at the borehole wall is about 420 and 2600 microstrain for the disposal of fuel assemblies and vitrified waste, respectively. Stress and volumetric strain decline rapidly with distance from the borehole and with time. Simulated peak stress at and parallel to the borehole wall for the disposal of vitrified waste with anisotropic ambient horizontal stress is about 440 MPa, which likely exceeds the compressive strength of granite if unconfined by fluid pressure within the borehole. The relatively small simulated displacements and volumetric strain near the borehole suggest that software codes using a nondeforming grid provide an adequate approximation of mechanical deformation in the coupled thermal-mechanical model. Additional modeling is planned to incorporate the effects of hydrologic processes coupled to thermal transport and mechanical deformation in the host rock near the heated borehole.

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Carbon capture and sequestration (CCS) is an option to mitigate impacts of atmospheric carbon emission. Numerous factors are important in determining the overall effectiveness of long-term geologic storage of carbon, including leakage rates, volume of storage available, and system costs. Recent efforts have been made to apply an existing probabilistic performance assessment (PA) methodology developed for deep nuclear waste geologic repositories to evaluate the effectiveness of subsurface carbon storage (Viswanathan et al., 2008; Stauffer et al., 2009). However, to address the most pressing management, regulatory, and scientific concerns with subsurface carbon storage (CS), the existing PA methodology and tools must be enhanced and upgraded. For example, in the evaluation of a nuclear waste repository, a PA model is essentially a forward model that samples input parameters and runs multiple realizations to estimate future consequences and determine important parameters driving the system performance. In the CS evaluation, however, a PA model must be able to run both forward and inverse calculations to support optimization of CO{sub 2} injection and real-time site monitoring as an integral part of the system design and operation. The monitoring data must be continually fused into the PA model through model inversion and parameter estimation. Model calculations will in turn guide the design of optimal monitoring and carbon-injection strategies (e.g., in terms of monitoring techniques, locations, and time intervals). Under the support of Laboratory-Directed Research & Development (LDRD), a late-start LDRD project was initiated in June of Fiscal Year 2010 to explore the concept of an enhanced performance assessment system (EPAS) for carbon sequestration and storage. In spite of the tight time constraints, significant progress has been made on the project: (1) Following the general PA methodology, a preliminary Feature, Event, and Process (FEP) analysis was performed for a hypothetical CS system. Through this FEP analysis, relevant scenarios for CO{sub 2} release were defined. (2) A prototype of EPAS was developed by wrapping an existing multi-phase, multi-component reservoir simulator (TOUGH2) with an uncertainty quantification and optimization code (DAKOTA). (3) For demonstration, a probabilistic PA analysis was successfully performed for a hypothetical CS system based on an existing project in a brine-bearing sandstone. The work lays the foundation for the development of a new generation of PA tools for effective management of CS activities. At a top-level, the work supports energy security and climate change/adaptation by furthering the capability to effectively manage proposed carbon capture and sequestration activities (both research and development as well as operational), and it greatly enhances the technical capability to address this national problem. The next phase of the work will include (1) full capability demonstration of the EPAS, especially for data fusion, carbon storage system optimization, and process optimization of CO{sub 2} injection, and (2) application of the EPAS to actual carbon storage systems.

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Simulation of potential radionuclide transport in the saturated zone from beneath the proposed repository at Yucca Mountain to the accessible environment is an important aspect of the total system performance assessment (TSPA) for disposal of high-level radioactive waste at the site. Analyses of uncertainty and sensitivity are integral components of the TSPA and have been conducted at both the sub-system and system levels to identify parameters and processes that contribute to the overall uncertainty in predictions of repository performance. Results of the sensitivity analyses indicate that uncertainty in groundwater specific discharge along the flow path in the saturated zone from beneath the repository is an important contributor to uncertainty in TSPA results and is the dominant source of uncertainty in transport times in the saturated zone for most radionuclides. Uncertainties in parameters related to matrix diffusion in the volcanic units, colloid-facilitated transport, and sorption are also important contributors to uncertainty in transport times to differing degrees for various radionuclides.

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Military test and training ranges operate with live-fire engagements to provide realism important to the maintenance of key tactical skills. Ordnance detonations during these operations typically produce minute residues of parent explosive chemical compounds. Occasional low-order detonations also disperse solid-phase energetic material onto the surface soil. These detonation remnants are implicated in chemical contamination impacts to groundwater on a limited set of ranges where environmental characterization projects have occurred. Key questions arise regarding how these residues and the environmental conditions (e.g., weather and geostratigraphy) contribute to groundwater pollution. This final report documents the results of experimental and simulation model development for evaluating mass transfer processes from solid-phase energetics to soil-pore water.