Rock Mechanical Analyses in Deep Underground Tunneling Projects for High Level Nuclear Waste Disposals
Sarah Bonasegale - Politecnico di Torino - [1998]
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  • Abstract
    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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