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ASSOCIATES, INC.Mining & Civil Engineers & Geologists
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AGAPITO
ASSOCIATES, INC. Mining & Civil Engineers & Geologists |
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Project Summaries - Mineral |
Thermomechanical models are being developed to support the design of an Exploratory Studies Facility and a potential high-level nuclear waste repository at Yucca Mountain, Nevada. These models are used for preclosure design of underground openings such as access drifts, emplacement drifts, and waste emplacement boreholes; and in support of post-closure issues such as performance of the waste canister, disturbance of the hydro-geological properties of the host rock, and performance of the overall system. Therefore, validation is an important process that must be pursued in conjunction with the development and application of models. This paper proposes a process for thermomechanical model validations; however, the process is general and could be extended to models for other phenomena. Both empirical models used in rock engineering design and more complex numerical models used for design and performance assessment are addressed. The validation process itself is viewed as having three main components: evaluation relative to experimental data obtained from in situ and laboratory tests, evaluation relative to empirical evidence and case histories (including natural analogues), and evaluation by peer review.
Application of Analytical Methods for Jointed Rock as Part of a Drift Design
Methodology for the Yucca Mountain Project The Yucca Mountain Project, managed by the Nevada Operations Office of the U.S. Department of Energy (DOE), is examining the feasibility of siting a repository for high-level nuclear waste at Yucca Mountain, on and adjacent to the Nevada Test Site. Excavation stability will be required during construction, waste emplacement, retrieval (if required), and closure, covering a period of approximately 100 years. In order to incorporate a means of evaluating excavation stability in the design process, a drift design methodology has been developed. This methodology uses both empirical and analytical methods in conjunction with detailed descriptions of site conditions to evaluate a proposed design. At present, the emphasis is on analytical numerical methods because of the limited experience in tuff at elevated temperatures. Proposed methods for analysis of systematically jointed, isotropically jointed, and widely spaced, discretely jointed rock masses are described. |
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Project Summaries - Capability |
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Design of Underground Repository Openings in Hard Rock to Accommodate
Vibratory Ground Motions A
methodology and preliminary application for designing repository openings to
withstand vibratory ground motion is presented. The methodology first
establishes the design basis ground motion based on the usage category,
performance goals, and hazard exceedance guidelines and incorporates those
motions into the more general drift design methodology. The repository drift
design methodology includes the effects of the in situ stresses and the
thermal loads generated from the heat released from the nuclear waste
packages. Empirical and analytical methods for design for seismic loading
are reviewed, and it is concluded that analytical methods provide the only
means method to adequately incorporate the effects of seismic and thermal
loads on design. Quasi-static and dynamic analysis methods are discussed.
The example application to the potential repository at the Yucca Mountain
site illustrates, by using the quasi-static method, that the seismic loads
in combination with the thermal loads can be significant in the design of
the ground support/reinforcement system in some locations. Under present
expected conditions, the seismic loads themselves are not excessive and can
be accommodated in the design by available ground support/reinforcement
systems. However, in the waste emplacement drift where the thermal loads are
high, the potential and additional seismic loads could require novel ground
support/reinforcement designs and maintenance.
Fault Stress Analysis for the Yucca Mountain Site Characterization Project
An understanding of the state of stress on faults is important for pre- and post-closure performance considerations for the potential high-level radioactive waste repository at Yucca Mountain. This paper presents the results of three-dimensional numerical analyses that provide estimates of the state of stress through time (10,000 years) along three major faults in the vicinity of the potential repository due to thermal stresses resulting from waste emplacement. It was found that the safety factor for slip close to the potential repository increases with time after waste emplacement. Possible fault slip is predicted above and below the potential repository for certain loading conditions and times. In general, thermal loading reduces the potential for slip in the vicinity of the potential repository.
Geotechnical Monitoring of High-Level Nuclear Waste Repository Performance
The paper discusses an approach to geotechnical monitoring of a geological repository constructed for the purpose of isolation of high-level nuclear waste, and describes the conceptual framework for development of an integrated monitoring scheme that is initiated during the earliest phases of site investigation and continues through facility decommissioning. The approach that is proposed recognizes the need to provide specific data to support the licensing process for the repository while avoiding adverse impact upon the system performance. Further, it recognizes present and inherent limitations in both the instrumentation and techniques that might be used within a monitoring scheme.
Numerical Modeling of the Geomechanical Response of a Rock Mass to a
Radioactive Waste Repository Geotechnical numerical models capable of predicting the thermomechanical response and groundwater movements around an underground radioactive waste repository are vital to the success of the nuclear waste disposal program. In the absence of directly related engineering experience, the design, risk assessment and licensing procedures of a repository will be reliant on predictions made using such models. This paper reviews models being used to assist in repository design and summarizes the results of a recent parametric study of underground disposal in basaltic rocks. On the basis of preliminary site data, it is concluded that the allowable areal density of heat generating waste will be controlled by the stability of placement rooms and the boreholes in which waste canisters are placed. Regional effects, including thermally induced upward groundwater flow, appear to present less severe problems.
Predicted Thermal and Stress Environments in the Vicinity of Repository
Openings An understanding of the thermal and stress environment in the vicinity of repository openings is important for pre-closure performance considerations and worker health and safety considerations for the proposed high-level radioactive waste repository at Yucca Mountain. This paper presents the results of two and three dimensional numerical analyses which have determined the thermal and stress environments for typical repository openings. In general, it is predicted that openings close to heat sources attain high temperatures and experience a significant stress increase. Openings away from heat sources experience more uniform temperature changes and experience a stress change which results in part from a far-field thermal loading.
Preliminary Drift Design Analyses for Nuclear Waste Repository in Tuff
The Yucca Mountain Project (YMP), managed by the Nevada Operations Office of the U.S. Department of Energy (DOE), is examining the feasibility of siting a repository for high-level nuclear waste at Yucca Mountain, on and adjacent to the Nevada Test Site (NTS). The proposed repository will be excavated in the Topopah Spring Member, which is a moderately fractured, unsaturated, welded tuff. Excavation stability will be required during construction, waste emplacement, retrieval (if required), and closure to ensure worker safety. The subsurface excavations will be subject to stress changes resulting from thermal expansion of the rock mass and seismic events associated with regional tectonic activity and underground nuclear explosions. Analyses of drift stability are required to assess the acceptable waste emplacement density, to design the drift shapes and ground support systems, and to establish schedules and cost of construction. This paper outlines the proposed methodology to assess drift stability and then focuses on an example of its application to the YMP repository drifts based on preliminary site data. Because site characterization activities have not begun, the database currently lacks the extensive site-specific field and laboratory data needed to form conclusions as to the final ground support requirements. This drift design methodology will be applied and refined as more site-specific data are generated and as analytical techniques and methodologies are verified during the site characterization process. The work presented in this paper is part of an ongoing effort to improve the interface between analysis and design, to identify weaknesses in the available data, and to assist the site geomechanics characterization effort in identifying critical parameter needs. The numerical values used in the example, although based on data in the Reference Information Base (DOE 1989), are not necessarily representative of the site nor useful for design purposes.
Rock Mass Mechanical Property Estimation Strategy for the Yucca Mountain
Site Characterization Project
Rock mass mechanical properties are required in the design of repository drifts and ramps to assess the impact of thermal loads from the heat-generating wastes on excavation performance and long-term structural stability. The ramps, exploratory drifts, and repository excavations will intersect both welded and nonwelded tuffs with varying abundances of fractures, degree of welding, degree of welding, and lithophysal content. Rock mass mechanical properties are dependent on both intact rock properties and joint characteristics, and were derived from empirically based relationships of laboratory determined intact rock properties and rock mass quality indices (Q and RMR). Definition and verification of these empirical relationships is considered critical to the drift design methodology because planned thermomechanical testing in the Exploratory Studies Facility (ESF) is limited. Rock mass quality determinations may provide the only practical vehicle for assessment of the effects of variability in drift performance under thermal loads. This paper presents a method of estimating the rock mass properties for the welded and nonwelded tuffs based on currently available information of intact rock and joint characteristics at the Yucca Mountain site. Variability of the expected ground conditions at the potential repository horizon (the TSw2 thermomechanical unit) and in the Calico Hills nonwelded tuffs is accommodated by defining five rock mass quality categories in each unit based upon assumed and observed distributions of the data.
Rock Mechanics Considerations in Designing a Nuclear Waste Repository in
Hard Rock A design methodology is presented for assessing drift stability for a potential high-level radioactive waste repository. Excavation stability is required during construction, emplacement, retrieval (if required), and closure phases to ensure worker health and safety, and to prevent development of potential pathways for radionuclide migration in the post-closure period. Requirements for the design, site conditions, and stresses are considered in the methodology. Methods for evaluating empirical and analytical results in order to estimate ground support requirements are outlined.
Rock Mechanics Design Criteria for Repository Design in Hard Rock
Rock mechanics design criteria for temperatures, opening stability, nuclide transport, and deformation of rock masses are essential for investigations into the feasibility and eventual design of high-level nuclear waste repositories. The development of rock mechanics design criteria relies on an understanding of the rock mass thermomechanical and hydrological behavior specifically in determining parameters such as strength, diffusivity, deformability, thermal expansion, and permeability. These properties are dependent on the spatial distribution of joints and their strength, deformational, and hydrological characteristics. In repository design, many of these parameters need to be quantified from a limited database determined predominately from drill hole data and some full-scale in situ experiments. A design methodology used for a repository site at depth in hard rock is presented. Preliminary criteria have been presented for temperature limits on geological materials and for operational constraints, room stability criteria in terms of rock mass properties and factors of safety, permeability changes, and rock mass deformational constraints. Results from thermomechanical and contaminant transport analyses are presented for a repository in basalt.
Seismic Design of Circular-Section Concrete-Lined Underground Openings;
Preclosure Performance Considerations for the Yucca Mountain Site
Yucca Mountain, the potential site of a repository for high-level radioactive waste, is situated in a region of natural and man-made seismicity. Underground openings excavated at this site must be designed for worker safety in the seismic environment anticipated for the pre-closure period. This includes accesses developed for site characterization, regardless of the ultimate outcome of the repository siting process. Experience with both civil and mining structures has shown that underground openings are much more resistant to seismic effects than surface structures, and that even severe dynamic strains can usually be accommodated with proper design. This paper discusses the design and performance of lined openings in the seismic environment of Yucca Mountain. The types and ranges of possible ground motions (seismic loads) are briefly discussed. Relevant historical records of underground opening performance during seismic loading are reviewed. Simple analytical methods of predicting liner performance under combined in situ, thermal, and seismic loading are presented, and results of calculations are discussed in the context of realistic performance requirements for concrete-lined openings for the pre-closure period. Design features that will enhance liner stability and mitigate the impact of the potential seismic load are reviewed. The paper is limited to pre-closure performance concerns involving worker safety because present decommissioning plans specify maintaining the option for liner removal at seal locations, thus, decoupling liner design from repository post-closure performance issues.
Thermal and Seismic Impacts on the North Ramp at Yucca Mountain The impacts of thermal and seismic loads on the stability of the Exploratory Studies Facility North Ramp at Yucca Mountain were assessed using both empirical and analytical approaches. This paper presents the methods and results of the analyses. Thermal loads were first calculated using the computer code STRES3D. This code calculates the conductive heat transfer through a semi-infinite elastic, isotropic, homogeneous solid and the resulting thermally-induced stresses. The calculated thermal loads, combined with simulated earthquake motion, were then modeled using UDEC and DYNA3D, numerical codes with dynamic simulation capabilities. The thermal- and seismic-induced yield zones were post-processed and presented for assessment of damage. Uncoupled bolt stress analysis was also conducted to evaluate the seismic impact on the ground support components. |
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