Evaluating the effectiveness of liquid diversion around an underground opening when evaporation is non-negligible (original) (raw)
Related papers
Modeling Coupled Evaporation and Seepage in Ventilated Cavities
Vadose Zone Journal, 2004
conditions that trigger seepage into various idealized cavity geometries excavated in homogeneous forma-Cavities excavated in unsaturated geological formations are importions. Detailed numerical models have been used to tant to activities such as nuclear waste disposal and mining. Such study unsaturated flow in heterogeneous fractured mecavities provide a unique setting for simultaneous occurrence of seepdia and seepage into cavities of various geometries unage and evaporation. Previously, inverse numerical modeling of field der transient conditions (e.g., Birkholzer et al., 1999; liquid-release tests and associated seepage into cavities were used to provide seepage-related large-scale formation properties, ignoring the Finsterle, 2000; Finsterle and Trautz, 2001; Li and Tsang, impact of evaporation. The applicability of such models was limited 2003). Site-specific seepage models for the proposed to the narrow range of ventilation conditions under which the models nuclear waste repository at YM were developed by caliwere calibrated. The objective of this study was to alleviate this limitabrating the effective seepage-related parameters against tion by incorporating evaporation into the seepage models. We modfield seepage test data (Finsterle et al., 2003). eled evaporation as an isothermal vapor diffusion process. The semi-Most of the previous numerical models assumed that physical model accounts for the relative humidity (RH), temperature, liquid water leaking into a cavity drips (seeps) immediand ventilation conditions of the cavities. The evaporation boundary ately from the place of entry. The potential for evaporalayer thickness (BLT) over which diffusion occurs was estimated by tion to compete with seepage has been generally igcalibration against free-water evaporation data collected inside the nored, and its effect was lumped with the effective flow experimental cavities. The estimated values of BLT were 5 to 7 mm parameters of the unsaturated medium (Finsterle et al., for the open underground drifts and 20 mm for niches closed off by 2003). In calibration of the analytical model of Philip bulkheads. Compared with previous models that neglected the effect et al. (1989b) against field seepage data, Trautz and of evaporation, this new approach showed significant improvement Wang (2002) accounted for the effect of evaporation by in capturing seepage fluctuations into open cavities of low RH. At adjusting the field seepage data for evaporation. Behigh relative-humidity values (ΟΎ85%), the effect of evaporation on cause the data were obtained from tests conducted in seepage was very small.
Inverse and predictive modeling of seepage into underground openings
Journal of Contaminant Hydrology, 2003
We discuss the development and calibration of a model for predicting seepage into underground openings. Seepage is a key factor affecting the performance of the potential nuclear-waste repository at Yucca Mountain, Nevada. Three-dimensional numerical models were developed to simulate field tests in which water was released from boreholes above excavated niches. Data from air-injection tests were geostatistically analyzed to infer the heterogeneous structure of the fracture permeability field. The heterogeneous continuum model was then calibrated against the measured amount of water that seeped into the opening. This approach resulted in the estimation of model-related, seepage-specific parameters on the scale of interest. The ability of the calibrated model to predict seepage was examined by comparing calculated with measured seepage rates from additional experiments conducted in different portions of the fracture network. We conclude that an effective capillary-strength parameter is suitable to characterize seepage-related features and processes for use in a prediction model of average seepage into potential waste emplacement drifts.
Prediction of water seepage into a geologic repository for high-level radioactive waste
Lawrence Berkeley National Laboratory, 2003
Predicting the amount of water that may seep into waste emplacement drifts is important for assessing the performance of the proposed geologic repository for high-level radioactive waste at Yucca Mountain, Nevada. The repository would be located in thick, partially saturated fractured tuff that will be heated to above-boiling temperatures as a result of heat generation from the decay of nuclear waste. Since infiltrating water will be subject to vigorous boiling for a significant time period, the superheated rock zone (i.e., rock temperature above the boiling point of water) can form an effective vaporization barrier that reduces the possibility of water arrival at emplacement drifts. In this paper, we analyze the behavior of episodic preferential flow events that penetrate the hot fractured rock, evaluate the impact of such flow behavior on the effectiveness of the vaporization barrier, and discuss the implications for the performance assessment of the repository. A semi-analytical ...
Modeling Seepage into Heated Waste Emplacement Tunnels in Unsaturated Fractured Rock
Vadose Zone Journal, 2004
Predicting the amount of water that may seep into waste emplacement tunnels (drifts) is important for assessing the performance of the proposed geologic repository for high-level radioactive waste at Yucca Mountain, Nevada. The repository will be located in thick, partially saturated fractured tuff that-for the first several hundred years after emplacement-will be heated to above-boiling temperatures as a result of heat generation from the decay of radioactive waste. Heating of rock water to above-boiling conditions induces water saturation changes and perturbs water fluxes that affect the potential for water seepage into drifts. In this paper, we describe numerical analyses of the coupled thermal-hydrological (TH) processes in the vicinity of waste emplacement drifts, evaluate the potential of seepage during the heating phase of the repository, and discuss the implications for the performance of the site. In addition to the capillary barrier at the rock-drift interface-independent of the thermal conditions-a second barrier exists to downward percolation at above-boiling conditions. This barrier is caused by vaporization of water in the fractured rock overlying the repository. A TOUGH2 dual-permeability simulation model was developed to analyze the combined effect of these two barriers; it accounts for all relevant TH processes in response to heating, while incorporating the capillary barrier condition at the drift wall. Model results are presented for a variety of simulation cases that cover the expected variability and uncertainty of relevant rock properties and boundary conditions.
Journal of Contaminant Hydrology, 2003
The evolution of fluid chemistry and mineral alteration around a potential waste emplacement tunnel (drift) is evaluated using numerical modeling. The model considers the flow of water, gas, and heat, plus reactions between minerals, CO 2 gas, and aqueous species, and porositypermeability-capillary pressure coupling for a dual permeability (fractures and matrix) medium. Two possible operating temperature modes are investigated: a "high-temperature" case with temperatures exceeding the boiling point of water for several hundred years, and a "low-temperature" case with temperatures remaining below boiling for the entire life of the repository. In both cases, possible seepage waters are characterized by dilute to moderate salinities and mildly alkaline pH values. These trends in fluid composition and mineral alteration are controlled by various coupled mechanisms. For example, upon heating and boiling, CO 2 exsolution from pore waters raises pH and causes calcite precipitation. In condensation zones, this CO 2 redissolves, resulting in a decrease in pH that causes calcite dissolution and enhances feldspar alteration to clays. Heat also enhances dissolution of wallrock minerals leading to elevated silica concentrations. Amorphous silica precipitates through evaporative concentration caused by boiling in the high-temperature case, but does not precipitate in the low-temperature case. Some alteration of feldspars to clays and zeolites is predicted in the high-temperature case. In both cases, calcite precipitates when percolating waters are heated near the drift. The predicted porosity decrease 2 around drifts in the high-temperature case (several percent of the fracture volume) is larger by at least one order of magnitude than in the low temperature case. Although there are important differences between the two investigated temperature modes in the predicted evolution of fluid compositions and mineral alteration around drifts, these differences are small relative to the model uncertainty and the variability of water compositions at Yucca Mountain.
Development and testing of a method for efficient simulation of evaporation from a seepage face
2003
Evaporation from the surface of a porous medium is a complex process, governed by interplay between (1) coupled liquid and vapor flow in the porous medium, and (2) relative humidity, temperature, and aerodynamic conditions in the surrounding air. In order to avoid the computational expense of explicitly simulating liquid, gas, and heat flow in the porous medium (and the possible further expense of simulating the flow of water vapor in the atmosphere), evaporative potentials can be treated in a simplified manner within a model where liquid is the only active phase. In the case of limited air mixing, evaporation can be approximated as a diffusion process with a linear vapor-concentration gradient. We have incorporated a simplified scheme into the EOS9 module of iTOUGH2 to represent evaporation as isothermal Fickian diffusion. This is notable because the EOS9 module solves a single equation describing saturated and unsaturated flow, i.e., phase transitions and vapor flow are not explicitly simulated. The new approach was applied to three simple problems and the results were compared to those obtained with analytical solutions or the EOS4 module, which explicitly considers advective and diffusive vapor flow. Where vapor flow within the porous medium can be neglected, this new scheme represents significant improvement over the computational expense of explicitly simulating liquid, gas, and heat flow, while providing an adequate reproduction of the overall hydrologic system. The scheme is set up to allow parallel flow of liquid and vapor, so that evaporation from an actively seeping face can be simulated. In addition, dynamic relative humidity boundary conditions can be simulated using standard iTOUGH2 features.
Modeling studies and analysis of seepage into drifts at Yucca mountain
Journal of Contaminant Hydrology, 1999
An important issue for the long-term performance of underground nuclear waste repositories is the rate of water seepage into the waste emplacement drifts. A prediction of the seepage rate is particularly complicated for the potential repository site at Yucca Mountain, NV, which is located in a thick sequence of unsaturated, fractured tuffs. Underground openings in unsaturated media might act as capillary barriers, diverting water around them. In the present work, we study the potential rates of seepage into drifts as a function of predicted percolation flux at Yucca Mountain, based on a stochastic model of the fractured rock mass in the drift vicinity. A variety of flow scenarios are considered, assuming estimated present-day and predicted future climate conditions. We show that the heterogeneity in the flow domain is a key factor controlling seepage rates, since it causes channelized flow and local ponding in the unsaturated flow field. The rates of seepage are related in a complex non-linear manner to the rock properties, the size and shape of the drift, the degree of heterogeneity, and the assumed percolation scenario.
Journal of Porous Media, 2009
When hot radioactive waste is placed in subsurface tunnels, a series of complex changes occurs in the surrounding medium. The water in the pore space of the medium undergoes vaporization and boiling. Subsequently, vapor migrates out of the matrix pore space, moving away from the tunnel through the permeable fracture network. This migration is propelled by buoyancy, by the increased vapor pressure caused by heating and boiling, and through local convection. In cooler regions, the vapor condenses on fracture walls, where it drains through the fracture network. Slow imbibition of water thereafter leads to gradual rewetting of the rock matrix. These thermal and hydrological processes also bring about chemical changes in the medium. Amorphous silica precipitates from boiling and evaporation, and calcite from heating and CO 2 volatilization. The precipitation of amorphous silica, and to a much lesser extent calcite, results in long-term permeability reduction. Evaporative concentration also results in the precipitation of gypsum (or anhydrite), halite, fluorite and other salts. These evaporative minerals eventually redissolve after the boiling period is over, however, their precipitation results in a significant temporary decrease in permeability. Reduction of permeability is also associated with changes in fracture capillary characteristics. In short, the coupled thermal-hydrological-chemical (THC) processes dynamically alter the hydrological properties of the rock. A model based on the TOUGHREACT reactive transport software is presented here to investigate the impact of THC processes on flow near an emplacement tunnel at Yucca Mountain, Nevada. We show how transient changes in hydrological properties caused by THC processes often lead to local flow channeling and saturation increases above the tunnel. For models that include only permeability changes to fractures, such local flow channeling may lead to seepage relative to models where THC effects are ignored. However, coupled THC seepage models that include both permeability and capillary changes to fractures may not show this additional seepage.
for the discussions they generated, the information they provided regarding the performance assessment use of this type of process model, and their detailed reviews of this work. James Craig provided exceptional programmatic support and budget tracking, for which we are grateful. Finally, we thank the U.S. Department of Energy (DOE) and the DOE Waste Management project manager, Jhon Carilli, for providing funding to accomplish this work, which was performed under contract number W-7405-ENG-36.