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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

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

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

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

In situ changes in the moisture content of heated, welded tuff based on thermal neutron measurements

Description: Thermal neutron logs were collected to monitor changes in moisture content within a welded tuff rock mass heated from a borehole containing an electrical heater which remained energized for 195 days. Thermal neutron measurements were made in sampling boreholes before, during and after heating. The results generally corroborated our conceptual understanding of hydrothermal flow as well as most of the numerical modeling conducting for this study. Conceptual models have been developed in conjunction with the numerical model calculations to explain differences in the drying and re-wetting behavior above and below the heater. Numerical modeling indicated that the re-wetting of the dried-out zone was dominated by the binary diffusion of water vapor through fractures. Saturation gradients in the rock matrix resulted in relative humidity gradients which drove water vapor (primarily along fractures) back to the dried-out zone where it condensed along the fracture walls and was imbibed by the matrix. 4 refs., 28 figs.
Date: July 1, 1991
Creator: Ramirez, A.L.; Carlson, R.C. & Buscheck, T.A.
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

Evaluation of thermo-hydrological performance in support of the thermal loading systems study

Description: Heat generated as a result of emplacing spent nuclear fuel will significantly affect the pre- and post-closure performance of the Mined Geological Disposal System (MGDS) at the potential repository site in Yucca Mountain. Understanding thermo-hydrological behavior under repository thermal loads is essential in (a) planning and conducting the site characterization and testing program, (b) designing the repository and engineered barrier system, and (c) assessing performance. The greatest concern for hydrological performance is source of water that would contact a waste package, accelerate its failure rate, and eventually transport radionuclides to the water table. The primary sources of liquid water are: (1) natural infiltration, (2) condensate generated under boiling conditions, and (3) condensate generated under sub-boiling conditions. Buoyant vapor flow, occurring either on a sub-repository scale or on a mountain scale, any affect the generation of the second and third sources of liquid water. A system of connected fractures facilitates repository-heat-driven gas and liquid flow as well as natural infiltration. With the use of repository-scale and sub-repository-scale models, the authors analyze thermo-hydrological behavior for Areal Mass Loadings (AMLs) of 24.2, 35.9, 55.3, 83.4, and 110.5 MTU/acre for a wide range of bulk permeability. They examine the temporal and spatial extent of the temperature and saturation changes during the first 100,000 yr. They also examine the sensitivity of mountain scale moisture redistribution to a range of AMLs and bulk permeabilities. In addition, they investigate how boiling and buoyant, gas-phase convection influence thermo-hydrological behavior in the vicinity of emplacement drifts containing spent nuclear fuel.
Date: January 1, 1994
Creator: Buscheck, T.A.; Nitao, J.J. & Saterlie, S.F.
Partner: UNT Libraries Government Documents Department

Analysis of near-field thermal and psychometric waste package environment using ventilation

Description: The ultimate objective of the Civilian Radioactive Waste Management System (CRWMS) Program is to safely emplace and isolate the nations` spent nuclear fuel (SNF) and radioactive wastes in a geologic repository. Radioactive waste emplaced in a geologic repository will generate heat, increasing the temperature in the repository. The magnitude of this temperature increase depends upon (1) the heat source, i.e. the thermal loading of the repository, and (2) the geologic and engineered heat transport characteristics of the repository. Thermal management techniques currently under investigation include ventilation of the emplacement drifts during the preclosure period which could last as long as 100 years. Understanding the amount of heat and moisture removed from the emplacement drifts and near-field rock by ventilation, are important in determining performance of the engineered barrier system (EBS), as well as the corrosive environment of the waste packages, and the interaction of the EBS with the near-field host rock. Since radionuclide releases and repository system performance are significantly affected by the corrosion rate related to the psychometric environment, it is necessary to predict the amount of heat and moisture that are removed from the repository horizon using a realistic model for a wide range of thermal loading. This can be realized by coupling the hydrothermal model of the rock mass to a ventilation/climate model which includes the heat and moisture transport on the rock-air interface and the dilution of water vapor in the drift. This paper deals with the development of the coupled model concept, and determination of the boundary conditions for the calculations.
Date: March 1, 1995
Creator: Danko, G.; Buscheck, T. A.; Nitao, J. J. & Saterlie, S.
Partner: UNT Libraries Government Documents Department


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&amp;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

Thermohydeologic Behavior at the Potential Yucca Mountain Nuclear Waste Repository

Description: Radioactive decay of high-level nuclear waste emplaced in a Yucca Mountain repository will produce an initial heat flux on the order of 30 to 50 times the heat flux in the Geysers geothermal reservoir in California (Hardin et al., 1998). Even though the rate of heat production decreases rapidly with time after emplacement, this heat flux will change the thermal and hydrologic environment, affecting both the host rock and conditions within the drifts in ways significant to key repository performance variables.
Date: February 17, 2000
Creator: Buscheck, T.A.; Rosenburg, N.D.; Gansemer, J. & Sun, Y.
Partner: UNT Libraries Government Documents Department

The Role of Capillary Barrier in Reducing Moisture Content on Waste Packages

Description: Assessment of the performance of engineered capillary barriers at the potential Yucca Mountain nuclear waste repository site, in which 1.67-m-diameter waste packages are to be emplaced in 5-m-diameter tunnels according to current design, brings up aspects not commonly considered in more typical applications of capillary barriers (e.g., near-surface landfills). Engineered capillary barriers typically consist of two layers of granular materials with a sloping interface, in which the contrast in capillarity between the layers keeps infiltrating water in the upper layer. One issue is the effect of thermohydrologic processes that would occur at elevated repository temperatures (and temperature gradients). For example, backfill materials may be altered from that of the as-placed material by the hydrothermal regime imposed by the emplacement of waste in the repository, changing hydrologic properties in a way that degrades the performance of the barrier. A reduction of permeability in the upper layer might diminish the capacity of the upper layer to divert incoming seepage or to cause a ''vapor lid'' whereby buoyant vapor flow would be trapped, then condense and drain onto waste packages. Other concerns are the result of highly spatially and temporally variable seepage distribution and the very limited spatial scale available for flow attenuation and diversion.
Date: October 29, 1999
Creator: Rosenberg, N.D.; Buscheck, T.A.; Wildenschild, D. & Sun, Y.
Partner: UNT Libraries Government Documents Department

Prototype heater test of the environment around a simulated waste package

Description: This paper presents selected results obtained during the 301 day duration of the Prototype Engineered Barrier System Field Test (PEBSFT) performed in G-Tunnel within the Nevada Test Site. The test described is a precursor to the Engineered Barrier Systems Field Tests (EBSFT) planned for the Exploratory Shaft Facility in Yucca Mountain. The EBSFT will consist of in situ tests of the geohydrologic and geochemical environment in the near field (within a few meters) of heaters emplaced in welded tuff to simulate the thermal effects of waste packages. The paper discusses the evolution of hydrothermal behavior during the prototype test, including rock temperatures, changes in rock moisture content, air permeability of fractures and gas-phase humidity in the heater borehole.
Date: December 31, 1991
Creator: Ramirez, A.L.; Buscheck, T.A.; Carlson, R.; Daily, W.; Latorre, V.R.; Lee, K et al.
Partner: UNT Libraries Government Documents Department

Single-heater test final report

Description: The Single-Heater Test (SHT) was one phase of the field-scale thermal testing program of the Yucca Mountain Site Characterization Project. The primary purpose of the SHT was to study the thermomechanical (TM) behavior of the densely welded, non-lithophysal Topopah Spring tuff at the Exploratory Studies Facility. The SHT was also used as a shake-down for testing thermal-hydrologic-chemical-mechanical processes in situ, testing that will be conducted in the Drift-Scale Test. In the SHT, a line-heat source 5-m long was emplaced in a pillar and used to heat the pillar for approximately nine months. The thermal field was relatively cylindrical about the line-heat source. The heater was turned off after nine months of heating, and the rock mass was monitored during the cool-down for another nine months, until May 28, 1997, when the test was terminated.
Date: September 1, 1998
Creator: Blair, S. C.; Buscheck, T. A.; DeLoach, L. D.; Lin, W. & Ramirez, A. I.
Partner: UNT Libraries Government Documents Department

Analysis and evaluation of VOC removal technologies demonstrated at Savannah River

Description: Volatile Organic Compounds, or VOCs, are ubiquitous subsurface contaminants at industrial as well as DOE sites. At the Savannah River Plant, the principles VOCs contaminating the subsurface below A-Area and M-Area are Trichloroethylene (C{sub 2}HCl{sub 3}, or TCE) and Tetrachloroethylene (C{sub 2}Cl{sub 4}, or PCE). These compounds were used extensively as degreasing solvents from 1952 until 1979, and the waste solvent which did not evaporate (on the order of 2{times}10{sup 6} pounds) was discharged to a process sewer line leading to the M-Area Seepage Basin (Figure I.2). These compounds infiltrated into the soil and underlying sediments from leaks in the sewer line and elsewhere thereby contaminating the vadose zone between the surface and the water table as well as the aquifer.
Date: September 1, 1993
Creator: Chesnut, D. A.; Wagoner, J.; Nitao, J. J.; Boyd, S.; Shaffer, R. J.; Kansa, E. J. et al.
Partner: UNT Libraries Government Documents Department