Exploratory Studies Facility Research Articles

Uncertainty and variability of infiltration at Yucca Mountain: Part 2. Model results and corroboration

[2] The U.S. Nuclear Regulatory Commission actively investigated climate and infiltration at Yucca Mountain for two decades to (i) understand important controls and uncertainties influencing percolation through the unsaturated zone on multimillennial time scales and (ii) provide flux boundary conditions for up to 1 million years in performance assessment models of the proposed Yucca Mountain repository. This second part of a two-part series describes site-scale model results for present and potential future conditions and confirmatory analyses for present-day conditions. At both the grid-cell and site-average scale, the calculated uncertainty distribution for net infiltration is approximately lognormal, and the coefficient of variation decreases with increasing net infiltration. Smaller relative but larger absolute responses to climate change occur where net infiltration is large. Comparisons of distributed model estimates with temperature and geochemical observations from the unsaturated zone suggest that average estimates are generally consistent but exhibit significant variability. An observed seepage event in the South Ramp of the Exploratory Studies Facility, combined with related subsurface observations across the site, suggests that subsurface spreading from zones of high infiltration to zones of low infiltration may occur in stratabound fractures, laterally extensive discontinuities, or at transitions between welded and nonwelded tuff units. Two conceptual models for unsaturated-zone flow each explain the subsurface observations, collectively providing bounding estimates for net infiltration. Model-predicted uncertainty distribution for decadal-average site-scale net infiltration is generally consistent with estimated percolation fluxes using the bounding hypotheses, suggesting that the model-calculated uncertainty is reasonably consistent with the uncertainty in interpreting site observations.

Multiple component end-member mixing model of dilution: hydrochemical effects of construction water at Yucca Mountain, Nevada, USA

The standard dual-component and two-member linear mixing model is often used to quantify water mixing of different sources. However, it is no longer applicable whenever actual mixture concentrations are not exactly known because of dilution. For example, low-water-content (low-porosity) rock samples are leached for pore-water chemical compositions, which therefore are diluted in the leachates. A multicomponent, two-member mixing model of dilution has been developed to quantify mixing of water sources and multiple chemical components experiencing dilution in leaching. This extended mixing model was used to quantify fracture-matrix interaction in construction-water migration tests along the Exploratory Studies Facility (ESF) tunnel at Yucca Mountain, Nevada, USA. The model effectively recovers the spatial distribution of water and chemical compositions released from the construction water, and provides invaluable data on the matrix fracture interaction. The methodology and formulations described here are applicable to many sorts of mixing-dilution problems, including dilution in petroleum reservoirs, hydrospheres, chemical constituents in rocks and minerals, monitoring of drilling fluids, and leaching, as well as to environmental science studies.

Recent measurements of 36Cl in Yucca Mountain rock, soil and seepage

Samples of rock, soil and seepage were collected from Yucca Mountain, USA, and analyzed for 36Cl/Cl ratios by accelerator mass spectrometry (AMS). Rock excavated from the Drill Hole Wash fault at repository horizon depths produced a ratio suggesting that small amounts of water with “bomb-pulse” 36Cl had percolated to that site over the past 50 years. Rock from four other sites within the exploratory studies facility did not yield bomb-pulse ratios. Ratios in the soil varied depending on depth and location, with some samples producing bomb-pulse signatures. Ratios for seep water were slightly elevated above the present cosmogenic background value. This paper also discusses results from a column study mimicking the passage of 36Cl through volcanic rock and from an experiment using bromide instead of chloride as a carrier in sample preparation for AMS.

Chemical and colloidal analyses of natural seep water collected from the exploratory studies facility inside Yucca Mountain, Nevada, USA

Yucca Mountain is being considered as a geological repository for the USA's spent nuclear fuel and high-level nuclear waste. Numerous groundwater seeps appeared during March 2005 within the exploratory studies facility (ESF), a tunnel excavated in the mountain. Because of the relevance to radionuclide transport and unsaturated zone-modeling studies, we analyzed the seep samples for major anions and cations, rare earth elements, and colloids. Major ion species and elemental concentrations in seep samples reflect interaction of the water with the volcanic rock and secondary calcites. Elemental fractograms from flow-injection field-flow fractionation ICP-MS scans detected Br, Ca, Cl, Cu, Fe, I, Mg, Si, Sr, W, and U at void fractions, suggesting they may be present in the form of dissolved anions. Colloids approximately 10 nm in hydrodynamic diameter, possibly calcite, were also present in the seepage samples. Geochemical calculations indicate, however, these may be an artifact (not present in the groundwater) which arose because of loss of CO2 during sample collection and storage.

Mechanical degradation of emplacement drifts at Yucca Mountain—A modeling case study—Part I: Nonlithophysal rock

This paper outlines rock mechanics investigations associated with mechanical degradation of planned emplacement drifts at Yucca Mountain, which is the designated site for the proposed US high-level nuclear waste repository. The factors leading to drift degradation include stresses from the overburden, stresses induced by the heat released from the emplaced waste, stresses due to seismically related ground motions, and time-dependent strength degradation. The welded tuff emplacement horizon consists of two groups of rock with distinct engineering properties: nonlithophysal units and lithophysal units, based on the relative proportion of lithophysal cavities. The term ‘lithophysal’ refers to hollow, bubble like cavities in volcanic rock that are surrounded by a porous rim formed by fine-grained alkali feldspar, quartz, and other minerals. Lithophysae are typically a few centimeters to a few decimeters in diameter. Part I of the paper concentrates on the generally hard, strong, and fractured nonlithophysal rock. The degradation behavior of the tunnels in the nonlithophysal rock is controlled by the occurrence of keyblocks. A statistically equivalent fracture model was generated based on extensive underground fracture mapping data from the Exploratory Studies Facility at Yucca Mountain. Three-dimensional distinct block analyses, generated with the fracture patterns randomly selected from the fracture model, were developed with the consideration of in situ, thermal, and seismic loads. In this study, field data, laboratory data, and numerical analyses are well integrated to provide a solution for the unique problem of modeling drift degradation.

Dripping into unsaturated rock underground excavations: model testing at the Yucca Mountain exploratory studies facility

This paper presents a probabilistic formulation of unsaturated flow through rough multi-segmented fractures, with emphasis on Dripping Initiation Time (DIT), and attempts to explain the fast flow paths detected at the Yucca Mountain (YM) Exploratory Studies Facility (ESF). Three scenarios of parameters were used to test the validity of the model with these fast flow paths. The scenarios differed in the DITs and the corresponding parameter configurations were also tested for three different stratigraphic horizon depths. Each depth represented a different location where fast flow has been detected at YM. The first depth corresponds to the Bow Ridge Fault; the second represents a network of steep fractures and cooling joints with large variability in dip; and the third location, probably connected to the Diabolous Ridge Fault, has a flow path that is low dipping. Monte Carlo simulations for each configuration and depth produced probabilistic results from nine scenarios for DITs, which are quite consistent with field observations.

Dripping into unsaturated rock underground excavations—literature review and geologic and hydrogeologic setting description

A literature review of dripping in underground constructions in deep, unsaturated geologic media was performed as part of a phased study. The objective is to develop and test a formulation that can be used for studying unsaturated flow through rough multi-segmented fractures, with emphasis on predicting dripping initiation time. Geologic and hydrogeologic setting for the Yucca Mountain (YM) and its exploratory studies facility, which is the site where the model will be tested, are described.

Chemistry of Water Collected from an Unventilated Drift, Yucca Mountain, Nevada

AbstractWater samples (referred to as puddle water samples) were collected from the surfaces of a conveyor belt and plastic sheeting in the unventilated portion of the Enhanced Characterization of the Repository Block (ECRB) Cross Drift in 2003 and 2005 at Yucca Mountain, Nevada. The chemistry of these puddle water samples is very different than that of pore water samples from borehole cores in the same region of the Cross Drift or than seepage water samples collected from the Exploratory Studies Facility tunnel in 2005. The origin of the puddle water is condensation on surfaces of introduced materials and its chemistry is dominated by components of the introduced materials. Large CO2 concentrations may be indicative of localized chemical conditions induced by biologic activity.

Observations of preferential flow during a liquid release experiment in fractured welded tuffs

To better understand preferential flow in fractured rock, we carried out an in situ field experiment in the Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of ~22 m3 of ponded water (at a pressure head of ~0.04 m) over a period of 7 months, directly onto a 12 m2 infiltration plot located on a fractured welded tuff surface. As water was released, changes in moisture content were monitored along horizontal boreholes located in the formation ~19‐22 m below. Distinct flow zones, varying in flow velocity, wetted cross‐sectional area, and extent of lateral movement, intercepted the monitoring boreholes. There was also evidence of water being diverted above the ceiling of a cavity in the immediate vicinity of the monitoring boreholes. Observations from this field experiment suggest that isolated conduits, each encompassing a large number of fractures, develop within the fractured rock formation to form preferential flow paths that persist if there is a continuous supply of water. In addition, in fractured welded tuffs the propensity for fracture‐matrix interactions is significantly greater than that suggested by existing conceptual models, in which flow occurs along a section of fracture surfaces. An overriding conclusion is that field investigations at spatial scales of tens of meters provide data critical to the fundamental understanding of flow in fractured rock.

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.

Development of a Wet Plume Following Liquid Release along a Fault

To investigate unsaturated flow through a fault located within fractured welded tuff, we performed in situ field experiments in the Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of approximately 82000 L water for a period of 17 mo directly into a near-vertical fault under both constant positive head (at about 0.04 m) and decreasing fluxes. As water was released into the fault, changes in moisture content were monitored in the formation while a large cavity excavated below the test bed was visually inspected for seepage. We observed that water (introduced along the fault) maintained the fault as the primary vertical flowpath, while the adjacent fractured rock served to move water laterally and vertically. Further, unlike primary flowpaths along the fault, flow was not persistent along the secondary flowpaths. While this field experiment provided preliminary insights about the flow field associated with a fault, it also demonstrated the need to investigate the role of infill material and secondary fractures in diverting flow from gravity-driven fast flow toward flowpaths in which lateral flow may occur.

Vapor‐phase transport in the near‐drift environment at Yucca Mountain

A key issue regarding the performance of the potential high‐level radioactive waste repository at Yucca Mountain, Nevada, is the likelihood of precipitation percolating a vertical distance of ∼300 m through fractured unsaturated rock into drifts containing waste packages. Water enhances waste package corrosion and is required for transport of released radionuclides. To evaluate the propensity for seepage into tunnels at Yucca Mountain, a 5‐m‐diameter, 2.7‐km‐long tunnel, commonly referred to as the Cross Drift (CD), was excavated in 1998, branching off from the main Exploratory Studies Facility tunnel. Sections of this tunnel have been isolated from ventilation for extended periods over the last 4 years. We present continuous measurements of relative humidity and temperature and periodic observations of liquid water in the CD over two periods. During this observation duration the terminal section of the drift was partitioned into four sections by bulkheads, and ventilation to these sections was minimized to a few days. We compare these observations to results from analytical and numerical models to investigate processes associated with the movement of water vapor between the tunnel bore and the surrounding fractured rock formation. Observations from this effort indicate that fractures can be primary paths for unsaturated zone vapor flow in the immediate vicinity of emplacement drifts. Observations, measurements, and analysis indicate the need for a model that includes fracture‐dominated vapor transport as a significant contributor to total water flow into the drifts.

Progress in permanent geologic disposal of spent nuclear fuel and high-level radioactive waste in the United States

The mission of the Office of Civilian Radioactive Waste Management (OCRWM) of the US Department of Energy (DOE) is to manage and dispose of spent nuclear fuel and high-level radioactive waste in a manner that protects health, safety and the environment, enhances national and energy security and merits public confidence. Consolidation of spent nuclear fuel and high-level radioactive waste from 126 sites in 39 states and safe disposal at Yucca Mountain are vital to the US national interests. The US geologic repository programme's key objective remains to begin receiving spent nuclear fuel and high-level radioactive waste at the US Nuclear Regulatory Commission (NRC) licensed Yucca Mountain repository in 2010. To achieve this objective, the DOE must, in less than 7 years, seek and secure authorization to construct the repository from the NRC, begin constructing the repository and receive a license amendment allowing receipt of radioactive materials and operation of the repository. DOE must also develop a transportation system to ship spent nuclear fuel and high-level radioactive waste from civilian and defence storage sites to the repository. This paper describes near-term efforts in developing the license application and transportation system. Successfully licensing, constructing and operating a repository will rely on information gained from more than two decades of scientific investigations at the Yucca Mountain site, all of which contribute to the technical basis for understanding the repository system. This paper also summarizes ongoing and completed in situ testing in the exploratory studies facility (ESF) and cross-drift. The ESF, a U-shaped tunnel approximately 7.9 km long and about 300 m below the crest of Yucca Mountain, has been used extensively to conduct tests in 13 alcoves and niches and to access a smaller cross-drift, 5 m in diameter and 2.7 km long.

Transport of radon gas into a tunnel at Yucca Mountain—estimating large-scale fractured tuff hydraulic properties and implications for the operation of the ventilation system

Radon gas concentrations have been monitored as part of the operation of a tunnel (the Exploratory Studies Facility—ESF) at Yucca Mountain to ensure worker safety. The objective of this study was to examine the potential use of the radon data to estimate large-scale formation properties of fractured tuffs. This objective was examined by developing a numerical model, based upon the characteristics of the ESF and the Topopah Spring welded (TSw) tuff unit, capable of predicting radon concentrations for prescribed ventilation conditions. The model was used to address two specific issues. First, it was used to estimate the permeability and porosity of the fractures in the TSw at the length scale of the ESF and extending tens of meters into the TSw, which surrounds the ESF. Second, the model was used to understand the mechanism leading to radon concentrations exceeding a specified level within the ESF. The mechanism controlling radon concentrations in the ESF is a function of atmospheric barometric fluctuations being propagated down the ESF along with ventilated air flow and the slight suction induced by the ventilation exhaust fans at the South Portal of the ESF. These pressure fluctuations are dampened in the TSw fracture continuum according to its permeability and porosity. Consequently, as the barometric pressure in the ESF drops rapidly, formation gases from the TSw are pulled into the ESF, resulting in an increase in radon concentrations. Model calibration to both radon concentrations measured in the ESF and gas-phase pressure fluctuations in the TSw yielded concurrent estimates of TSw fracture permeability and porosity of 1×10 −11 m 2 and 0.00034, respectively. The calibrated model was then used as a design tool to predict the effect of adjusting the current ventilation-system operation strategy for reducing the probability of radon gas concentrations exceeding a specified level.

Unsaturated flow and transport through a fault embedded in fractured welded tuff

To evaluate the importance of matrix diffusion as a mechanism for retarding radionuclide transport in the vicinity of a fault located in unsaturated fractured rock, we carried out an in situ field experiment in the Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of ∼82,000 L of water over a period of 17 months directly into a near‐vertical fault under both constant positive head (at ∼0.04 m) and decreasing fluxes. A mix of conservative tracers (pentafluorobenzoic acid (PFBA) and bromide (applied in the form of lithium bromide)) was released along the fault over a period of 9 days, 7 months after the start of water release along the fault. As water was released into the fault, seepage rates were monitored in a large cavity excavated below the test bed. After the release of tracers, seepage water was continuously collected from three locations and analyzed for the injected tracers. Observations of bromide concentrations in seepage water during the early stages of the experiment and bromide and PFBA concentrations in the seepage water indicate the significant effects of matrix diffusion on transport through a fault embedded in fractured, welded rock.

Tracer migration experiments in unsaturated fractured tuff

Field tracer experiments were conducted at an underground tunnel—the Exploratory Studies Facility (ESF)—at Yucca Mountain, Nevada, to investigate the localized preferential flow and transport resulting from multiple episodic liquid-release tests in unsaturated fractured tuff. Liquid was released into an isolated test interval (0.30 m long) within a borehole drilled along ESF. Out of 60 liquid-release tests, 36 contained tracers (mostly food and fluorescent dyes) to help us to elucidate the flow paths in unsaturated tuff. After the tracer-migration test, an array of 12 boreholes was drilled into the test area to collect rock samples. These samples were then analyzed to delineate the extent of tracer migration. Complementary laboratory column transport studies using crushed rock showed that the dye tracers were—at most—very weakly sorbed to the tuff matrix. We found that the tracer-migration test, with a limited release volume of 1.5 L, resulted in a localized distribution of tracers, most likely confined to a 1.0 × 1.6-m area directly below the test interval (over a vertical experimental length of about 0.65 m). This conclusion was corroborated by detecting dyes that had been introduced during previous liquid-release seepage tests either before or after niche excavation; the spatial distribution of all dyes was near their individual release intervals. Limited lateral spreading of the dyes occurred, even when several non-dye-containing seepage tests were intermittently conducted following dye release. This work also demonstrates the utility of applying multiple tracers (including food and fluorescent dyes) to help delineate transport characteristics in unsaturated fractured rock.

Fault-matrix interactions in nonwelded tuff of the Paintbrush Group at Yucca Mountain

To investigate the potential for fast flow through altered tuff of the nonwelded unit of the Paintbrush Group (PTn) at Yucca Mountain, Nevada, we carried out in situ field experiments using water released directly into the matrix and along a minor subvertical normal fault at Alcove 4 in the Exploratory Studies Facility (ESF). During the experiments, changes in moisture content were monitored within the test bed, and a slot excavated below the test bed was visually inspected for seepage. Our field tests suggest that the dry porous PTn matrix is capable of attenuating episodic percolation fluxes in localized areas (such as around faults) where fast flow would be expected to dominate. Once wetted, the matrix is able to retain the moisture over a period of months. As saturation increases in the matrix, less water imbibes along the fault and more water travels farther along the fault. From this observation, we infer that a sequence of infiltration events separated by periods of up to a few months could convey water over increasing distances along the fault.

On the physics of unstable infiltration, seepage, and gravity drainage in partially saturated tuffs

To improve understanding of the physics of dynamic instabilities in unsaturated flow processes within the Paintbrush nonwelded unit (PTn) and the middle nonlithophysal portion of the Topopah Spring welded tuff unit (TSw) of Yucca Mountain, we analyzed data from a series of infiltration tests carried out at two sites (Alcove 4 and Alcove 6) in the Exploratory Studies Facility (ESF), using analytical and empirical functions. The analysis of infiltration rates measured at both sites showed three temporal scales of infiltration rate: (1) a macro-scale trend of overall decreasing flow, (2) a meso-scale trend of fast and slow motion exhibiting three-stage variations of the flow rate (decreasing, increasing, and [again] decreasing flow rate, as observed in soils in the presence of entrapped air), and (3) micro-scale (high frequency) fluctuations. Infiltration tests in the nonwelded unit at Alcove 4 indicate that this unit may effectively dampen episodic fast infiltration events; however, well-known Kostyakov, Horton, and Philip equations do not satisfactorily describe the observed trends of the infiltration rate. Instead, a Weibull distribution model can most accurately describe experimentally determined time trends of the infiltration rate. Infiltration tests in highly permeable, fractured, welded tuff at Alcove 6 indicate that the infiltration rate exhibits pulsation, which may have been caused by multiple threshold effects and water–air redistribution between fractures and matrix. The empirical relationships between the extrinsic seepage from fractures, matrix imbibition, and gravity drainage versus the infiltration rate, as well as scaling and self-similarity for the leading edge of the water front are the hallmark of the nonlinear dynamic processes in water flow under episodic infiltration through fractured tuff. Based on the analysis of experimental data, we propose a conceptual model of a dynamic fracture flow and fracture–matrix interaction in fractured tuff, incorporating the time-dependent processes of water redistribution in the fracture–matrix system.

Estimation of past seepage volumes from calcite distribution in the Topopah Spring Tuff, Yucca Mountain, Nevada

Low-temperature calcite and opal record the past seepage of water into open fractures and lithophysal cavities in the unsaturated zone at Yucca Mountain, Nevada, site of a proposed high-level radioactive waste repository. Systematic measurements of calcite and opal coatings in the Exploratory Studies Facility (ESF) tunnel at the proposed repository horizon are used to estimate the volume of calcite at each site of calcite and/or opal deposition. By estimating the volume of water required to precipitate the measured volumes of calcite in the unsaturated zone, seepage rates of 0.005 to 5 liters/year (l/year) are calculated at the median and 95th percentile of the measured volumes, respectively. These seepage rates are at the low end of the range of seepage rates from recent performance assessment (PA) calculations, confirming the conservative nature of the performance assessment. However, the distribution of the calcite and opal coatings indicate that a much larger fraction of the potential waste packages would be contacted by this seepage than is calculated in the performance assessment.

Chlorine-36 data at Yucca Mountain: statistical tests of conceptual models for unsaturated-zone flow

An extensive set of chlorine-36 ( 36Cl) data has been collected in the Exploratory Studies Facility (ESF), an 8-km-long tunnel at Yucca Mountain, Nevada, for the purpose of developing and testing conceptual models of flow and transport in the unsaturated zone (UZ) at this site. At several locations, the measured values of 36Cl/Cl ratios for salts leached from rock samples are high enough to provide strong evidence that at least a small component of bomb-pulse 36Cl, fallout from atmospheric testing of nuclear devices in the 1950s and 1960s, was measured, implying that some fraction of the water traveled from the ground surface through 200–300 m of unsaturated rock to the level of the ESF during the last 50 years. These data are analyzed here using a formal statistical approach based on log-linear models to evaluate alternative conceptual models for the distribution of such fast flow paths. The most significant determinant of the presence of bomb-pulse 36Cl in a sample from the welded Topopah Spring unit (TSw) is the structural setting from which the sample was collected. Our analysis generally supports the conceptual model that a fault that cuts through the nonwelded Paintbrush tuff unit (PTn) that overlies the TSw is required in order for bomb-pulse 36Cl to be transmitted to the sample depth in less than 50 years. Away from PTn-cutting faults, the ages of water samples at the ESF appear to be a strong function of the thickness of the nonwelded tuff between the ground surface and the ESF, due to slow matrix flow in that unit.