Hydrologic issues associated with nuclear waste repositories (original) (raw)
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MIST journal of science and technology, 2023
Water flow is an essential factor in the sealability of any underground cavern, including those for nuclear waste disposal, and is significantly affected by the permeability of the rock. The permeability of rocks is affected by various factors, including stress and temperature. The rock stress changes by excavating a cavern, and rock temperature changes by decay heat from nuclear waste, and the temperature change induces thermal stress. Therefore, water flow around such caverns must be evaluated considering the effects of stress and temperature. Numerical analyses of water migration around underground nuclear waste disposal caverns have been carried out. However, studies considering the stress and temperature-dependent permeability may not be published yet. To demonstrate the necessity to consider the stress and temperature-dependency in permeability, equations that represent the postfailure permeability as a function of average effective stress and temperature were proposed. The water inflow was numerically calculated for a simple underground nuclear waste disposal cavern with or without stress and temperature dependency which showed the significance of the dependency. Also, the importance of rock types was demonstrated by considering the three rocks of granite, sandstone, and tuff for a full-scale underground radioactive disposal site for the stress and temperature-dependent permeability. A high sealability could be expected for granite and sandstone but not for the tuff. Introducing the stress and temperature-dependent permeability could contribute to the thoughtful design of an underground repository for radioactive waste disposal considering rock types.
2008
As part of the ongoing international DECOVALEX project, four research teams used five different models to simulate coupled thermal, hydrological, and mechanical (THM) processes near waste emplacement drifts of geological nuclear waste repositories. The simulations were conducted for two generic repository types, one with open and the other with back-filled repository drifts, under higher and lower postclosure temperatures, respectively. In the completed first model inception phase of the project, a good agreement was achieved between the research teams in calculating THM responses for both repository types, although some disagreement in hydrological responses is currently being resolved. In particular, good agreement in the basic thermal-mechanical responses was achieved for both repository types, even though some teams used relatively simplified thermal-elastic heat-conduction models that neglected complex near-field thermal-hydrological processes. The good agreement between the complex and simplified process models indicates that the basic thermalmechanical responses can be predicted with a relatively high confidence level.
THM STUDY OF CLAY ROCKS FOR NUCLEAR WASTE STORAGE
2020
Argillaceous rocks, specifically stiff sedimentary clays, form the geological basis for numerous civil engineering projects. Recently, there has been heightened interest in these materials as potential host media for underground repositories of high-level radioactive waste (HLW). This possibility has led to the establishment of several underground laboratories. Among the various topics explored in these Underground Research Laboratories (URLs), the thermo-hydromechanical (THM) behavior of the host rock is of primary concern for the current research. In situ observations have identified numerous coupled THM processes involved in the operation of an HLW repository.
Impact of a high-level nuclear waste repository on the regional ground-water flow
International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1980
Of all the alternatives suggested for the disposal of the high level nuclear waste, that of the deep underground burial in geologic formations may prove to be the most attractive. Within the constraints of economy and feasibility, the primary concern in deep underground burial is the selection of a geologic formation that will contain and isolate the wastes so that radioactive nuclides from the waste do not enter the biosphere in amounts that will endanger public health and safety. It is recognized that the primary mechanism for the likely introduction of the radioactive elements into the biosphere is that of ground water transport. It is thus imperative that the effect of nuclear waste disposal repository on ground water flow be fully explored and understood. This paper examines: (1) the problem of defining reliable hydrogeological properties of a rock mass, (2) the prediction of thermally induced ground flow on a regional scale for a crystalline rock formation. It is concluded that in the absence of major geologic discontinuities the deep disposal of radioactive waste may be technically feasible. However, the need for further field and theoretical studies is highlighted by the paucity of reliable data and the inadequacies of existing methodologies.
Hydrogeologic Characterization of Fractured Rock Masses Intended for Disposal of Radioactive Waste
Radioactive Waste, 2012
A field site was developed in the foothills of the Sierra Nevada, California, to develop and test a multidisciplinary approach to the characterization of ground-water flow and transport in fractured rocks. Nine boreholes were drilled into the granitic bedrock, and a wide variety of instruments and methodologies were tested. Fracture properties were measured on outcrops and in boreholes using acoustic televiewer, digital borehole color scanner, and by down-hole camera logs. Conventional geophysical logs were collected. In addition, thermal-pulse and impeller flowmeter logging, fluid replacement and conductivity logging, packer-injection profiling tests, and ordinary open-hole pumping tests were conducted. Transmissive fractures were identified by integrating results from hydrologic and geophysical measurements, and the hydrogeologic structure of the formation was hypothesized. Cross-hole seismic surveys yielded tomograms of interborehole rock properties. Visualization software was used in combination with geophysical logs to interpolate interborehole properties, and a detailed 3-D model of the subsurface was constructed. Other referenced work at the site includes cross-hole hydrologic EPA/600/S-96/001
International Journal of Rock Mechanics and Mining Sciences, 2012
The present paper provides an overview of key coupled thermo-hydro-mechanical (THM) processes in clay formations that would result from the development of a high-level radioactive waste repository. Here, in this paper, clay formations include plastic clay such as the Boom Clay of Belgium, as well as more indurated clay such as the Callovo-Oxfordian and Upper Toarcian of France and Opalinus Clay of Switzerland. First, we briefly introduce and describe four major Underground Research Laboratories (URLs) that have been devoted to clay repository research over the last few decades. Much of the research results in this area have been gained through investigations in these URLs and their supporting laboratory and modeling research activities. Then, the basic elements in the development of a waste repository in clays are presented in terms of four distinct stages in repository development. For each of these four stages, key processes and outstanding issues are discussed. A summary of the important areas of research needs and some general remarks then conclude this paper.
Environmental Geology, 2009
This paper presents an international, multiple-code, simulation study of coupled thermal, hydrological, and mechanical (THM) processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement drifts. Simulations were conducted considering two types of repository settings: (a) open emplacement drifts in relatively shallow unsaturated volcanic rock, and (b) backfilled emplacement drifts in deeper saturated crystalline rock. The results showed that for the two assumed repository settings, the dominant mechanism of changes in rock permeability was thermalmechanically-induced closure (reduced aperture) of vertical fractures, caused by thermal stress resulting from repository-wide heating of the rock mass. The magnitude of thermal-mechanically-induced changes in permeability was more substantial in the case of an emplacement drift located in a relatively shallow, low-stress environment where the rock is more compliant, allowing more substantial fracture closure during thermal stressing. However, in both of the assumed repository settings in this study, the thermalmechanically induced changes in permeability caused relatively small changes in the flow field, with most changes occurring in the vicinity of the emplacement drifts.
Predicting Fluid Compositions and Mineral Alteration Around Nuclear Waste Emplacement Tunnels
2001
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 ''lowtemperature'' 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 wall rock 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 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