ABSTRACT: For the safe design of an underground nuclear waste repository, it is necessary to investigate the influence of the major parameters on the tunnel stability. In this study, sensitivity analysis was carried out to find the major parameters on the mechanical stability point of view. Fourteen parameters consisted of 10 site parameters and 4 design parameters were included in the FLAC3D. From the numerical analyses employing single parameter variation, it was possible to determine important parameters. In order to investigate the interaction between the parameters, fractional factorial design for the parameters, such as in situ stress ratio, depth, tunnel dimensions, joint spacing, joint stiffness, friction angle, and rock strength, was carried out.
I. INSTRUCTIONS Since the deep geological repository is generally accepted as the most promising method for the permanent disposal of high-level nuclear waste from the extensive researches during the last decades, the accurate understanding of the behavior of rock mass in deep location is became crucial for the safe design and construction of the underground repository. In order to investigate the fundamental deformational mechanism of rock mass, various researches had been carried out in many countries. In Korea deep underground repository is considered as the way for the disposal of spent fuel generated from the sixteen nuclear power plants and started a R&D program in 1997. The research plan for the disposal system in Korea is divided into 3 stages:the first stage for setting up methodology for analysis for three years from Jan. 1997 to Dec. 1999;
the second stage for the next three years will focus on the optimization of design concept; and
the third stage to validate the design concept through underground in situ tests and finalize the repository concept from Jan. 2003 to Dec. 2006. According to the preliminary design concept developed during the last three years, the high-level waste repository is located at several hundred meters below surface in a massive crystalline rock such as granite. Currently granite is the most popular rock for underground nuclear waste repository in many countries including Canada, Sweden. Finland, and Japan due to its occurrence in large and relatively uniform masses, high mechanical strength, chemical stability, and small economic value. With consideration of the characteristics of Korean geology as well as the spent fuel, vertical emplacement in boreholes in crystalline rock was recommended as the most reliable method (Choi et al., 1999). For the further reference to the design development of the repository, it is necessary to investigate the influence of parameters on the stability of repository. With the determination of the most important parameters, it would be possible to adjust repository design more efficiently. In this study, a three-dimensional finite difference code, FLAC3D, was used to assess the influence of the parameters. From the modelling for different cases, important parameters could be chosen and the interaction effect between those parameters was investigated using fractional factorial design.