Prediction of water seepage into a geologic repository for high-level radioactive waste (original) (raw)
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Analysis of the vaporization barrier above waste emplacement drifts
2003
Prediction of the amount of water that may seep into the waste emplacement drifts is an important aspect of assessing the performance of the proposed geologic nuclear waste repository at Yucca Mountain, Nevada. The repository is to 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 water percolating down towards the repository 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, and we evaluate the impact of such flow behavior on the effectiveness of the vaporization barrier.
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.
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.
Transport in Porous Media, 2011
The current design of a deep geological repository for high-and intermediatelevel radioactive waste in France consists of a complex system of different underground structures (ANDRA, Dossier 2005 Argile, les recherches de l'Andra sur le stockage géologique des déchets radioactifs à haute activité et à vie longue, collection les Rapports. Châtenay-Malabry, France, 2005). For a comprehensive understanding of the long-term hydraulic evolution of the entire repository, numerical non-isothermal two-phase flow and transport simulation, taking into consideration the generation, accumulation, and release of hydrogen gas and decay heat, are compulsory. However, a detailed numerical model of the entire repository system would require a tremendous computational effort and pose a laborious task with respect to the operation of the model. To handle these difficulties, we have developed an efficient method for the numerical modeling of a complete repository system and its geologic environment. The method consists of the following steps: (i) subdivision of the repository plane into a large number of "sectors" based on the position of hydraulic seals and on other geometrical considerations, (ii) exploitation of existing symmetries (inside or between sectors), (iii) adoption of the "multiplying concept", and (iv) connection of the individual sectors at the drift interfaces to form the entire repository model. Each sector is modeled as a three-dimensional (3D) block, and the entire model is computed with TOUGH2-MP. The Electronic supplementary material The online version of this article (123 78 A. Poller et al.
Water, Vapor, and Salt Dynamics in a Hot Repository
MRS Proceedings, 2006
The purpose of this paper is to report the results of a new model study critically examining the high temperature nuclear waste disposal concept at Yucca Mountain using MULTIFLUX, an integrated in-drift- and mountain-scale thermal-hydrologic model. In addition to new results the paper summarizes results of a previous study. The results show that a large amount of vapor flow into the drift is expected during the period of above-boiling temperatures in the emplacement drift. This phenomenon makes the emplacement drift a water/moisture attractor for thousands of years during the above-boiling temperature operation.The evaporation of the percolation water into the drift gives rise to salt accumulation in the rock wall, especially in the crown of the drift for about 1500 years in the example. The deposited salts over the drift footprint, almost entirely present in the fractures, may enter the drift either by rock fall or by water drippage. During the high temperature operation mode the b...
Physics and Chemistry of the Earth, Parts A/B/C, 2006
An understanding of processes affecting seepage into emplacement tunnels is needed for correctly predicting the performance of underground radioactive waste repositories. It has been previously estimated that the capillary and vaporization barriers in the unsaturated fractured rock of Yucca Mountain are enough to prevent seepage under present day infiltration conditions. It has also been thought that a substantially elevated infiltration flux will be required to cause seepage after the thermal period is over. While coupled thermal-hydrological-chemical (THC) changes in Yucca Mountain host rock due to repository heating has been previously investigated, those THC models did not incorporate elements of the seepage model. In this paper, we combine the THC processes in unsaturated fractured rock with the processes affecting seepage. We observe that the THC processes alter the hydrological properties of the fractured rock through mineral precipitation and dissolution. We show that such alteration in the hydrological properties of the rock often leads to local flow channeling. We conclude that such local flow channeling may result in seepage under certain conditions, even with nonelevated infiltration fluxes.