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On the infiltration of a liquid front in an unsaturated, fractured porous medium

Description: The unsaturated zone at Yucca Mountain, Nevada, is currently under scientific investigation as a proposed site for the permanent storage of high-level nuclear waste. A deeper understanding of fracture-matrix interaction needed for the prediction of water movement around an in the repository. We show that the liquid front movement can be classified into physically interpretable, distinctive flow regimes. Asymptotic solutions for the front movement are given for each flow period and comparisons with numerical solutions are made. In addition to applications in nuclear waste storage, the results of our study is relevant to hazardous waste disposal, petroleum recovery, and flow in soil macropores. 17 refs., 13 figs., 6 tabs.
Date: August 1, 1989
Creator: Nitao, J. & Buscheck, T.
Partner: UNT Libraries Government Documents Department

MULTISCALE THERMOHYDROLOGIC MODEL

Description: The intended purpose of the multiscale thermohydrologic model (MSTHM) is to predict the possible range of thermal-hydrologic conditions, resulting from uncertainty and variability, in the repository emplacement drifts, including the invert, and in the adjoining host rock for the repository at Yucca Mountain. The goal of the MSTHM is to predict a reasonable range of possible thermal-hydrologic conditions within the emplacement drift. To be reasonable, this range includes the influence of waste-package-to-waste-package heat output variability relevant to the license application design, as well as the influence of uncertainty and variability in the geologic and hydrologic conditions relevant to predicting the thermal-hydrologic response in emplacement drifts. This goal is quite different from the goal of a model to predict a single expected thermal-hydrologic response. As a result, the development and validation of the MSTHM and the associated analyses using this model are focused on the goal of predicting a reasonable range of thermal-hydrologic conditions resulting from parametric uncertainty and waste-package-to-waste-package heat-output variability. Thermal-hydrologic conditions within emplacement drifts depend primarily on thermal-hydrologic conditions in the host rock at the drift wall and on the temperature difference between the drift wall and the drip-shield and waste-package surfaces. Thus, the ability to predict a reasonable range of relevant in-drift MSTHM output parameters (e.g., temperature and relative humidity) is based on valid predictions of thermal-hydrologic processes in the host rock, as well as valid predictions of heat-transfer processes between the drift wall and the drip-shield and waste-package surfaces. Because the invert contains crushed gravel derived from the host rock, the invert is, in effect, an extension of the host rock, with thermal and hydrologic properties that have been modified by virtue of the crushing (and the resulting geometry of the gravel grains). Thus, given that reasonable invert properties are applied, the ability to predict a ...
Date: July 7, 2005
Creator: Buscheck, T.
Partner: UNT Libraries Government Documents Department

Estimates of the width of the wetting zone along a fracture subjected to an episodic infiltration event in variably saturated, densely welded tuff

Description: A central issue to be addressed within the Nevada Nuclear Waste Storage Investigations (NNWSI) is the role which fractures will play as the variably saturated, fractured rock mass surrounding the waste package responds to heating, cooling, and episodic infiltration events. Understanding the role of fractures during such events will, in part, depend on our ability to make geophysical measurements of perturbations in the moisture distribution in the vicinity of fractures. In this study we first examine the details of the perturbation in the moisture distribution in and around a fracture subjected to an episodic infiltration event, and then integrate that behavior over the scale at which moisture measurements are likely to be made during the Engineered Barrier Design Test of the NNWSI project. To model this system we use the TOUGH hydrothermal code and fracture and matrix properties considered relevant to the welded ash flow tuff found in the Topopah Spring member at Yucca Mountain as well as in the Grouse Canyon member within G-Tunnel at the Nevada Test Site. Our calculations provide insight into the anticipated spatial and temporal resolution obtainable through the use of the geophysical techniques being considered. These calculations should prove useful both in planning the implementation of these methods as well as in the interpretation of their results. 41 refs., 28 figs.
Date: May 31, 1988
Creator: Buscheck, T.A. & Nitao, J.J.
Partner: UNT Libraries Government Documents Department

Thermal-hydrological models

Description: This chapter describes the physical processes and natural and engineered system conditions that affect thermal-hydrological (T-H) behavior in the unsaturated zone (UZ) at Yucca Mountain and how these effects are represented in mathematical and numerical models that are used to predict T-H conditions in the near field, altered zone, and engineered barrier system (EBS), and on waste package (WP) surfaces.
Date: April 29, 1998
Creator: Buscheck, T., LLNL
Partner: UNT Libraries Government Documents Department

Estimates of the hydrologic impact of drilling water on core samples taken from partially saturated densely welded tuff

Description: The purpose of this work is to determine the extent to which drill water might be expected to be imbibed by core samples taken from densely welded tuff. In a related experimental study conducted in G-Tunnel, drill water imbibition by the core samples was observed to be minimal. Calculations were carried out with the TOUGH code with the intent of corroborating the imbibition observations. Due to the absence of hydrologic data pertaining directly to G-Tunnel welded tuff, it was necessary to apply data from a similar formation. Because the moisture retention curve was not available for imbibition conditions, the drainage curve was applied to the model. The poor agreement between the observed and calculated imbibition data is attributed primarily to the inappropriateness of the drainage curve. Also significant is the value of absolute permeability (k) assumed in the model. Provided that the semi-log plot of the drainage and imbibition moisture retention curves are parallel within the saturation range of interest, a simple relationship exists between the moisture retention curve, k, and porosity ({phi}) which are assumed in the model and their actual values. If k and {phi} are known, we define the hysteresis factor {lambda} to be the ratio of the imbibition and drainage suction pressures for any saturation within the range of interest. If k and {phi} are unknown, {lambda} also accounts for the uncertainties in their values. Both the experimental and modeling studies show that drill water imbibition by the core has a minimal effect on its saturation state. 22 refs., 6 figs., 2 tabs.
Date: September 1, 1987
Creator: Buscheck, T.A. & Nitao, J.J.
Partner: UNT Libraries Government Documents Department

On the movement of a liquid front in an unsaturated, fractured porous medium, Part 1

Description: The primary aim of this paper is to present approximate analytical solutions of the fracture flow which gives the position of the liquid fracture front as a function of time. These solutions demonstrate that the liquid movement in the fracture can be classified into distinctive time periods, or flow regimes. It is also shown that when plotted versus time using a log-log scale, the liquid fracture front position asymptotically approaches a series of line segments. Two-dimensional numerical simulations were run utilizing input data applicable to the densely welded, fractured tuff found at Yucca Mountain in order to confirm these observations. 19 refs., 15 figs., 8 tabs.
Date: June 1, 1989
Creator: Nitao, J.J. & Buscheck, T.A.
Partner: UNT Libraries Government Documents Department

Multiscale Thermohydrologic Model

Description: The purpose of the multiscale thermohydrologic model (MSTHM) is to predict the possible range of thermal-hydrologic conditions, resulting from uncertainty and variability, in the repository emplacement drifts, including the invert, and in the adjoining host rock for the repository at Yucca Mountain. Thus, the goal is to predict the range of possible thermal-hydrologic conditions across the repository; this is quite different from predicting a single expected thermal-hydrologic response. The MSTHM calculates the following thermal-hydrologic parameters: temperature, relative humidity, liquid-phase saturation, evaporation rate, air-mass fraction, gas-phase pressure, capillary pressure, and liquid- and gas-phase fluxes (Table 1-1). These thermal-hydrologic parameters are required to support ''Total System Performance Assessment (TSPA) Model/Analysis for the License Application'' (BSC 2004 [DIRS 168504]). The thermal-hydrologic parameters are determined as a function of position along each of the emplacement drifts and as a function of waste package type. These parameters are determined at various reference locations within the emplacement drifts, including the waste package and drip-shield surfaces and in the invert. The parameters are also determined at various defined locations in the adjoining host rock. The MSTHM uses data obtained from the data tracking numbers (DTNs) listed in Table 4.1-1. The majority of those DTNs were generated from the following analyses and model reports: (1) ''UZ Flow Model and Submodels'' (BSC 2004 [DIRS 169861]); (2) ''Development of Numerical Grids for UZ Flow and Transport Modeling'' (BSC 2004); (3) ''Calibrated Properties Model'' (BSC 2004 [DIRS 169857]); (4) ''Thermal Conductivity of the Potential Repository Horizon'' (BSC 2004 [DIRS 169854]); (5) ''Thermal Conductivity of the Non-Repository Lithostratigraphic Layers'' (BSC 2004 [DIRS 170033]); (6) ''Ventilation Model and Analysis Report'' (BSC 2004 [DIRS 169862]); (7) ''Heat Capacity Analysis Report'' (BSC 2004 [DIRS 170003]).
Date: October 12, 2004
Creator: Buscheck, T.
Partner: UNT Libraries Government Documents Department

MULTISCALE THERMOHYDROLOGIC MODEL

Description: The purpose of the Multiscale Thermohydrologic Model (MSTHM) is to describe the thermohydrologic evolution of the near-field environment (NFE) and engineered barrier system (EBS) throughout the potential high-level nuclear waste repository at Yucca Mountain for a particular engineering design (CRWMS M&O 2000c). The process-level model will provide thermohydrologic (TH) information and data (such as in-drift temperature, relative humidity, liquid saturation, etc.) for use in other technical products. This data is provided throughout the entire repository area as a function of time. The MSTHM couples the Smeared-heat-source Drift-scale Thermal-conduction (SDT), Line-average-heat-source Drift-scale Thermohydrologic (LDTH), Discrete-heat-source Drift-scale Thermal-conduction (DDT), and Smeared-heat-source Mountain-scale Thermal-conduction (SMT) submodels such that the flow of water and water vapor through partially-saturated fractured rock is considered. The MSTHM accounts for 3-D drift-scale and mountain-scale heat flow, repository-scale variability of stratigraphy and infiltration flux, and waste package (WP)-to-WP variability in heat output from WPs. All submodels use the nonisothermal unsaturated-saturated flow and transport (NUFT) simulation code. The MSTHM is implemented in several data-processing steps. The four major steps are: (1) submodel input-file preparation, (2) execution of the four submodel families with the use of the NUFT code, (3) execution of the multiscale thermohydrologic abstraction code (MSTHAC), and (4) binning and post-processing (i.e., graphics preparation) of the output from MSTHAC. Section 6 describes the MSTHM in detail. The objectives of this Analyses and Model Report (AMR) are to investigate near field (NF) and EBS thermohydrologic environments throughout the repository area at various evolution periods, and to provide TH data that may be used in other process model reports.
Date: December 21, 2001
Creator: Buscheck, T.A.
Partner: UNT Libraries Government Documents Department

The impact of thermal loading on repository performance at Yucca Mountain

Description: In the unsaturated zone at Yucca Mountain, liquid flow along preferential fracture pathways is the only credible mechanism capable of bringing water to waste packages and transporting radionuclide to the water table. Three categories of features or mechanisms will mitigate the impact of flow along preferential fracture pathways: (1) discontinuity in fracture pathways, (2) liquid-phase dispersion in fracture networks, and (3) fracture-matrix interaction. For repository areal power densities (APDs) that are too low to result in significant boiling or rock dry-out effects, the primary mode of fracture-matrix interaction is matrix imbibition. For high APDs, boiling and enhanced matrix imbibition due to rock dry-out significantly add to the capacity of the unsaturated zone to retard fracture-dominated flow. With the use of V-TOUGH code, hydrothermal flow calculations are made for a range of APDs and spent fuel ages. For APD > 20 kW/acre, repository-heat-generated flow of vapor and liquid in fractures is found to dominate the ambient hydrological system. For high APDs, boiling conditions can persist for 10,000 yr or longer and rock-dry benefits for at least 100,000 yr.
Date: January 15, 1992
Creator: Buscheck, T.A. & Nitao, J.J.
Partner: UNT Libraries Government Documents Department

Scoping analysis of in situ thermal-hydrological testing at Yucca Mountain

Description: In situ thermal tests, which are to be conducted in the Exploratory Studies Facility (ESF) in the unsaturated zone (UZ) at Yucca Mountain, are required to test coupled thermal-hydrological-geomechanical-geochemical (T-H-M-C) process models that support total system performance assessment. The ESF thermal tests must provide an understanding of coupled T-H-M-C processes that are relevant to expected repository conditions. Current planning includes the possibility of two large-scale tests: (1) the first ESF (drift-scale) thermal test, which will be conducted under an accelerated heatup and cooldown schedule, and (2) a second ESF (multi-drift) test, which will be larger-scale, longer-duration test, conducted under a less accelerated heatup and cooldown schedule. With the V-TOUGH (vectorized transport of unsaturated groundwater and heat) code, the authors modeled and evaluated a range of heater test sizes, heating rates, and heating durations under a range of plausible hydrological conditions to develop a test design that provides sufficient (and timely) information to determine the following: the dominant mode(s) of heat flow; the major T-H regime(s) and the T-H-M-C processes that determine the magnitude and direction of vapor and condensate flow; and the influence of heterogeneous conditions on the flow of heat, vapor, and condensate. A major purpose of the ESF thermal tests is to determine which major decay-heat-driven T-H flow regime(s) will govern the magnitude and direction of vapor and condensate flow in the UZ. Another major purpose of the thermal tests is to determine the degree of vapor diffusion enhancement.
Date: February 5, 1996
Creator: Buscheck, T.A. & Nitao, J.J.
Partner: UNT Libraries Government Documents Department

Discrete-fracture modeling of thermal-hydrological processes at Yucca Mountain and the LLNL G-Tunnel heater test

Description: An in situ heater test was performed at G-Tunnel, Nevada Nuclear Test Site, to investigate the thermal-hydrological response of unsaturated, fractured volcanic tuff under conditions similar to those at Yucca Mountain. The NUFT flow and transport code was used to model the test using discrete-fracture and equivalent-continuum approaches. Nonequilibrium fracture flow and thermal buoyant gas-phase convection were found to be the likely causes for observed lack of condensate imbibition into the matrix. The potential repository at Yucca Mountain was also modeled. Disequilibrium fracture flow is predicted to occur for less than a hundred years after emplacement followed by a period of fracture-matrix equilibrium, during which the equivalent-continuum and discrete-fracture models give almost identical results.
Date: November 9, 1995
Creator: Nitao, J.J. & Buscheck, T.A.
Partner: UNT Libraries Government Documents Department

The importance of thermal loading conditions to waste package performance at Yucca Mountain

Description: Temperature and relative humidity are primary environmental factors affecting waste package corrosion rates for the potential repository in the unsaturated zone at Yucca Mountain, Nevada. Under ambient conditions, the repository environment is quite humid. If relative humidity is low enough (<70%), corrosion will be minimal. Under humid conditions, corrosion is reduced if the temperature is low (<60 C). Using the V-TOUGH code, the authors model thermo-hydrological flow to investigate the effect of repository heat on temperature and relative humidity in the repository for a wide range of thermal loads. These calculations indicate that repository heat may substantially reduce relative humidity on the waste package, over hundreds of years for low thermal loads and over tens of thousands of year for high thermal loads. Temperatures associated with a given relative humidity decrease with increasing thermal load. Thermal load distributions can be optimized to yield a more uniform reduction in relative humidity during the boiling period.
Date: October 1, 1994
Creator: Buscheck, T.A. & Nitao, J.J.
Partner: UNT Libraries Government Documents Department

The impact of buoyant gas-phase flow and heterogeneity on thermo-hydrological behavior at Yucca Mountain

Description: To safety and permanently store high-level nuclear waste, the potential Yucca Mountain repository system must mitigate the release and transport of radionuclides for tens of thousands of years. In the failure scenario of greatest concern, water would contact a waste package, accelerate its failure rate, and eventually transport radionuclides to the water table. Our analyses have demonstrated that the only significant source of liquid water is fracture flow from: (1) natural infiltration, (2) condensate drainage generated under boiling conditions, and (3) condensate drainage generated under sub-boiling conditions. The first source of liquid water arises from the ambient system; the second and third sources are generated by repository heat. Buoyant, gas-phase flow, occurring either on a sub-repository scale or on a mountain scale, may play an important role in generating the second and third sources of liquid water. By considering a wide range in bulk permeability of the fractured rock, we identify a threshold bulk permeability at which buoyant, gas-phase convection begins to dominate hydrological behavior. At 10 times this threshold, convection begins to dominate thermal behavior. These effects can dominate moisture movement in the unsaturated zone on the order of 100,000 yr. We find that the development of a large above-boiling zone suppresses the effects of buoyant vapor flow. Zones of sharply contrasting bulk permeability also influence condensate generation and drainage. Of particular concern are conditions that focus vapor flow and condensate drainage, which could result in persistent refluxing at the repository, causing water to drip onto waste packages. These effects can occur under both sub-boiling and boiling conditions Long-term in situ heater tests are required to diagnose the potential for major repository-heat- driven sources of fractures flow.
Date: January 1, 1994
Creator: Buscheck, T.A. & Nitao, J.J.
Partner: UNT Libraries Government Documents Department

Thermal-hydrological analysis of large-scale thermal tests in the exploratory studies facility at Yucca Mountain

Description: In situ thermal tests, which are to be conducted in the Exploratory Studies Facility (ESF) at Yucca Mountain, will provide a major portion of the experimental basis supporting the validation of coupled thermal-hydrological-geomechanical-geochemicaI (T-H-M-C) process models required to assess the total system performance at the site. With respect to advective rock dryout, we have identified three major T-H flow regimes: (1) throttled, nonbuoyant, advective rock dryout; (2) unthrottled, nonbuoyant, advective rock dryout; and (3) unthrottled, buoyant, advective rock dryout. With the V-TOUGH code, we modeled a range of heater test sizes, heating rates, and heating durations under a range of plausible hydrological conditions to help optimize an in situ thermal test design that provides sufficient information for determining (a) the dominant mode(s) of heat flow, (b) the major T-H regime(s) and processes (such as vapor diffusion) that govern the magnitude and direction of vapor and condensate flow, and (c) the influence of heterogeneous properties and conditions on the flow of heat, vapor, and condensate. For the plate thermal test, which uniformly heats a disk-shaped area, we evaluated a wide range of test areas, ranging from 50 to 5077 m{sup 2}. We evaluated the single-drift thermal test, which consists of a row of large-waste-package-sized heaters sitting on the floor of the heater drift, and then developed an optimized thermal test configuration, called the single-drift, winged thermal test, in which the heater drift is flanked by wing heater arrays. For this configuration, we considered three heating schedules (with 1-, 2-, and 4-yr full-power heating periods) and three heating rates (122, 177, and 236 W/m{sup 2}). For determining the dominant T-H regime(s) and dominant heat-flow mode(s), the most important diagnostic measurements are vertical temperature and gas-phase pressure profiles and gas-phase pressure and relative humidity RH histories in the drift.
Date: February 20, 1996
Creator: Buscheck, T.A. & Nitao, J.J.
Partner: UNT Libraries Government Documents Department

The analysis of repository-heat-driven hydrothermal flow at Yucca Mountain

Description: To safely and permanently store high-level nuclear waste, the potential Yucca Mountain repository site must mitigate the release and transport of radionuclides for tens of thousands of years. In the failure scenario of greatest concern, water would contact the waste package (WP), accelerate its failure rate, and eventually transport radionuclides to the water table. In a concept called the ``extended-dry repository,`` decay heat arising from radioactive waste extends the time before liquid water can contact a WP. Recent modeling and theoretical advances in nonisothermal, multiphase fracture-matrix flow have demonstrated (1) the critical importance of capillary pressure disequilibrium between fracture and matrix flow, and (2) that radioactive decay heat plays a dominant role in the ability of the engineered and natural barriers to contain and isolate radionuclides. Our analyses indicate that the thermo-hydrological performance of both the unsaturated zone (UZ) and saturated zone (SZ) will be dominated by repository-heat-driven hydrothermal flow for tens of thousands of years. For thermal loads resulting in extended-dry repository conditions, UZ performance is primarily sensitive to the thermal properties and thermal loading conditions and much less sensitive to the highly spatially and temporally variable ambient hydrologic properties and conditions. The magnitude of repository-heat-driven buoyancy flow in the SZ is far more dependent on the total mass of emplaced spent nuclear fuel (SNF) than on the details of SNF emplacement, such as the Areal Power Density [(APD) expressed in kill/acre] or SNF age.
Date: January 1, 1993
Creator: Buscheck, T.A. & Nitao, J.J.
Partner: UNT Libraries Government Documents Department

The impact of repository heat on thermo-hydrological performance at Yucca Mountain

Description: To safely and permanently store high-level nuclear waste, the potential Yucca Mountain repository site must mitigate the release and transport of radionuclides for tens of thousands of years. In the failure scenario of greatest concern, water would contact a waste package (WP), accelerate its failure rate, and eventually transport radionuclides to the water table. These analyses have demonstrated that the only significant source of liquid water is nonequilibrium fracture flow from: (1) meteoric sources, (2) condensate drainage generated under boiling conditions, and (3) condensate drainage generated under sub-boiling conditions. The first source of liquid water arises from the ambient system; the second and third sources are generated by repository heat. Buoyant vapor flow, occurring either on a sub-repository scale or on a mountain scale, may play an important role in the generation of the second and third sources of liquid water. By considering a wide range in bulk permeability, k{sub b}, the authors identify the threshold k{sub b} (called k{sub b}{sup hyd}) at which buoyant, vapor convection begins to dominate hydrological behavior, and the threshold k{sub b} (called k{sub b}{sup th}) at which this convection begins to dominate thermal behavior. They find that k{sub b}{sup th} is generally an order of magnitude larger than k{sub b}{sup hyd} and that the development of a large above-boiling zone suppresses the effects of buoyant vapor flow. Of particular concern are conditions that promote the focusing of vapor flow and condensate drainage, which could result in persistent two-phase conditions (often referred to as the heat-pipe effect) in the vicinity of WPs. The results of this study underscore the need for in situ heater tests to help diagnose the potential for the major repository-heat-driven sources of fracture flow.
Date: September 1, 1993
Creator: Buscheck, T.A. & Nitao, J.J.
Partner: UNT Libraries Government Documents Department

Single-hole in situ thermal probe for hydrothermal characterization at Yucca Mountain

Description: The REKA thermal probe method, which uses a single borehole to measure in situ rock thermophysical properties and provides for efficient and low-cost site characterization, is analyzed for its application to hydrothermal system characterization. It is demonstrated throughout the evaluation of several temperature fields obtained for different thermal zones that the REKA method can be applied to simultaneously determine (1) two independent thermophysical properties, i.e., heat conductivity and thermal diffusivity and (2) a set of heat transport parameters, which can be used to characterize the behavior of a hydrothermal system. Based on the direct physical meaning of these transport parameters, the components of the heat transport mechanism in a given time and location of the hydrothermal system can be described. This evaluation can be applied to characterizing and quantifying in situ rock dry-out and condensate shedding at the proposed repository site.
Date: January 1, 1993
Creator: Danko, G. & Buscheck, T.A.
Partner: UNT Libraries Government Documents Department

Influence of Hydrologic Heterogencity on Thermal-Hydrologic Behavior in Emplacement Drifts

Description: Fracture networks have been characterized as highly permeable continuum within the porous rock matrix in thermal-hydrologic models used to support performance assessments of the proposed nuclear-waste repository at Yucca Mountain. Uncertainty and spatial variability of the fracture permeability are important considerations for understanding thermal-hydrologic behavior within the host rock surrounding an emplacement drift. In this paper, we conducted numerical experiments with a number of realizations of intrinsic fracture permeability and examine thermal conditions around an emplacement drift. Peak temperature and boiling duration on the drift wall are used as indices to quantify, the influence of fracture permeability. The variability of peak temperature and boiling duration resulting from small-scale fracture-permeability heterogeneity is compared with the variability resulting from variability of host-rock thermal conductivity and infiltration flux. An examination of rock dryout and condensate drainage shows that small-scale heterogeneity in fracture permeability results in a relatively small range in dryout volume and does not prevent the shedding of condensate through the pillar-separating emplacement drifts.
Date: March 28, 2006
Creator: Sun, Y.; Buscheck, T.A. & Hao, Y.
Partner: UNT Libraries Government Documents Department

Double Diffusive Natural Convection in a Nuclear Waste Repository

Description: In this study, we conduct a two-dimensional numerical analysis of double diffusive natural convection in an emplacement drift for a nuclear waste repository. In-drift heat and moisture transport is driven by combined thermal- and compositional-induced buoyancy forces. Numerical results demonstrate buoyancy-driven convective flow patterns and configurations during both repository heat-up and cool-down phases. It is also shown that boundary conditions, particularly on the drip-shield surface, have strong impacts on the in-drift convective flow and transport.
Date: March 28, 2006
Creator: Hao, Y.; Nitao, J.; Buscheck, T.A. & Sun, Y.
Partner: UNT Libraries Government Documents Department

Multiscale Thermohydrologic Model Supporting the Licence Application for the Yucca Mountain Repository

Description: The MultiScale ThermoHydrologic Model (MSTHM) predicts thermal-hydrologic (TH) conditions within emplacement tunnels (drifts) and in the adjoining host rock at Yucca Mountain, Nevada, which is the proposed site for a radioactive waste repository in the US. Because these predictions are used in the performance assessment of the Yucca Mountain repository, they must address the influence of variability and uncertainty of the engineered- and natural-system parameters that significantly influence those predictions. Parameter-sensitivity studies show that the MSTHM predictions adequately propagate the influence of parametric variability and uncertainty. Model-validation studies show that the influence of conceptual-model uncertainty on the MSTHM predictions is insignificant compared to that of parametric uncertainty, which is propagated through the MSTHM.
Date: March 28, 2006
Creator: Buscheck, T.A>; Sun, Y. & Hao, Y.
Partner: UNT Libraries Government Documents Department

The impact of episodic nonequilibrium fracture-matrix flow on repository performance at the potential Yucca Mountain site

Description: Adequate representation of fracture-matrix interaction during episodic infiltration events is crucial in making valid hydrological predictions of repository performance at Yucca Mountain. Approximations have been applied to represent fracture-matrix flow interaction, including the Equivalent Continuum Model, which assumes capillary equilibrium between fractures and matrix, and the Fracture-Matrix Model, which accounts for nonequilibrium fracture-matrix flow and transport for the eight major hydrostratigraphic units in the unsaturated zone at Yucca Mountain. 18 refs., 6 figs., 3 tabs.
Date: November 1, 1991
Creator: Buscheck, T.A.; Nitao, J.J. & Chesnut, D.A.
Partner: UNT Libraries Government Documents Department

Localized dryout: An approach for managing the thermal hydrologi-cal effects of decay heat at Yucca Mountain

Description: For a nuclear waste repository in the unsaturated zone at Yucca Mountain, there are two thermal loading approaches to using decay heat constructively -- that is, to substantially reduce relative humidity and liquid flow near waste packages for a considerable time, and thereby limit waste package degradation and radionuclide dissolution and release. ``Extended dryout`` achieves these effects with a thermal load high enough to generate large-scale (coalesced) rock dryout. ``Localized dryout``(which uses wide drift spacing and a thermal load too low for coalesced dryout) achieves them by maintaining a large temperature difference between the waste package and drift wall; this is done with close waste package spacing (generating a high line-heat load) and/or low-thermal-conductivity backfill in the drift. Backfill can greatly reduce relative humidity on the waste package in both the localized and extended dryout approaches. Besides using decay heat constructively, localized dryout reduces the possibility that far-field temperature rise and condensate buildup above the drifts might adversely affect waste isolation.
Date: November 1, 1995
Creator: Buscheck, T. A.; Nitao, J.J. & Ramspott, L.D.
Partner: UNT Libraries Government Documents Department