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Rock Mechanical Analyses in Deep Underground Tunneling Projects for High Level Nuclear Waste Disposals

There are presently over 430 nuclear power plants in operating throughout the world. They produce nuclear fuel waste containing long-lived radioactive elements that need to be isolated from man and environment for thousand of years in underground repositories. The present report deals with the most relevant problems connected with the deep storing of nuclear wastes. In order to prevent as much as possible the risk of spreading highly radioactive nuclear wastes, the storage in crystalline rocks is planned to be several hundred meters below the ground level. But this is just possible where the crystalline rocks show to have a rather good quality.
High orizontal in situ stresses in crystalline bedrock is one of the most important parameters affecting the planning and understanding of the behavior of the rock mass around an underground nuclear waste repository. By opening the disposal tunnels in highly stressed rocks the stress state greatly redistributes and high secondary (boundary) stresses are likely to occur around and in the vicinity of excavated rooms in the deposition holes. The magnitudes of the secondary stresses might be close to the strength of the intact rock. Under these conditions, the occurrence of the rock failure (spalling, rock burst, etc.) might be possible. The objective of the first section is to understand the problems that may be caused by excavating underground structures in high stress field. The exceeding of rock strength and intensive failure might exist around the tunnels based on high stress/strength relation. Besides, the methods to estimate and prevent failures around the excavations are studied.
The behavior of the rockmass around the excavated rooms depends mostly on the stress/strength ratio at depths. When designing underground openings, the orientation of the tunnel play an important role in high stress regime with large stress differences and deviatoric stresses. If it is not possible to locate the tunnels with favorable orientation they can be stabilized by the use of rockbolts and meshes. The type of support is dependent on the strength and structure of the rock mass. The containment of rockbursts energy with appropriate tunnel support and reinforcement is the subject of the second chapter.
Canada was the first country to construct an underground facility dedicated to geotechnical research. Now, after 15 years of research, other countries have recognized the importance of in situ research. Today Sweden, Finland, Switzerland, Germany and United States have dedicated underground facilities, with United Kingdom and Japan planning such facilities. The last section of this work is dedicated to a worldwide overview on underground labs. Conceptually, the development of a vault for the disposal of nuclear waste would be undertaken in separate stages. These stages would include siting, construction, operation, decommissioning and closure. The siting stage would include site screening, surface-based evaluation and underground evaluation. Underground laboratories have an essential role in the research and development for, and implementation of, nuclear waste disposal.

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Since the consumption of energy can be expected to rise as population increases, the world has a constant increased demand of energy. Moreover, the need of a clean and powerful source of energy is even a bigger requirement due to the potential impact that the burning of fossil fuels has on the environment. Nowadays, many scientists consider atomic energy an alternative source of energy, both to supply essential energy as well as to be of low impact on the environment. However, major environmental problems are related to atomic energy. Some of these problems concern the storage of nuclear wastes. In fact the spent fuel from the nuclear fission is anyhow still radioactive, and therefore hazardous for the environment. There is a classification of nuclear wastes; low-level, intermediate-level and high level. The division is an imprecise one. The low-level and intermediate level wastes are relatively low radioactivity wastes, not requiring extraordinary disposal precautions. Volumetrically they account for 90% of radioactive wastes generated. High-level wasted are more dangerous and various more or less elaborate disposal schemes must be used. The basic disposal of high- level wastes is to isolate them from the biosphere with the confidence of providing any contact of them with the biosphere for many thousand of years. It is therefore necessary to depose this kind of radioactive wastes in a safety place, where contact from the biosphere, atmosphere and hydrosphere is avoided. Nowadays, many ideas have been raised on the disposal of high-level radioactive wastes. However, the proposal that has received the most serious attention is the deposition of the radioactive wastes underground, on which this report is about. Caverns hollowed out of low-permeability, not fractured igneous rocks such as basalt or granite are used in the high latitude countries, such as Canada and Scandinavia. Other disposals are constructed in salt formations, such as in Germany and USA, as well as in deep cemented sedimentary rocks. The principle is anyhow the same that is to depose the wastes in a safe and deep place, away as much as possible from the contact with the biosphere. There is general agreement among geologists that the ideal underground storage site for radioactive wastes should possess the following characteristics • The enclosing rock should have few fractures and low permeability • The enclosing rock should be effective at dissipating heat and chemicals absorbing any potential leaks • The enclosing rock should contain no mineral with present or future economic potential • Local groundwater flow should be as slow as possible and in a direction away from the biosphere • Only very long paths of groundwater flows should be directed toward places accessible to humans • The area should have low rainfall • The zone of aeration should be thick • The rate of erosion should be very low • The probability of earthquakes or volcanic activity should be very low • Future changes of climate in the region should be unlikely to affect groundwater conditions substantially The types of reservoir rock that are being investigated for long-based geologic isolation of high-level radioactive wastes are:

Tesi di Master

Autore: Sarah Bonasegale Contatta »

Composta da 43 pagine.

 

Questa tesi ha raggiunto 1092 click dal 20/03/2004.

 

Consultata integralmente 2 volte.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.