Coupled thermo-mechanical behavior of an underground repository has been numerically simulated using FLAC3D. It Is assumed that the repository unit consists of double tunnels disposed in parallel. The numerical model is symmetrical with respect to the center of the tunnels. Typical values for mechanical and thermal parameters were assigned to rock mass. Heat decay function was also used to consider a decrease of heat flux from canisters. The simulation has been conducted up to 100 years after the deposition of canisters. The temperature and stresses around the repository rapidly increased in the early time and reached the peak about 10 years after deposition. Thereafter they slowly decreased with time, Numerical results show mutual affection between tunnels in the respect of temperature increase and stress concentration. Tunnel is subjected to upward displacement in the opposite direction to the neighboring tunnel with a little amount of lateral shrinkage.
In order to understand thermo-mechanical response of rock mass around underground repositories to nuclear wastes, numerical analyses in 3D have been conducted using FLAC3D. Layout of the repository system is determined according to the KBS-3 concept (Jang et al, 1997). In this study, a symmetrical model is used assuming that two repository tunnels are disposed in parallel. This model allows analyzing the effect of heat load of one repository on mechanical and thermal behaviors of the other one. Mechanical and thermal data for rock mass domestically produced are used as input values to meet the design condition in Korea. The repository tunnels undergo high stresses since they are constructed at great depth. Main concern of this research is to investigate mechanical stability of the repository tunnels suffered by thermal load under such condition. The variation of temperature of rock mass around the repository with time is also dealt with.
2.1 Geometry In this study, two repository tunnels spaced in parallel are considered. The tunnels are assumed placed at the depth of 500m under the surface with a distance of 30m between them. Because of the symmetry of the tunnel arrangement only a half of the whole structure is built in the model as shown in Fig. 1. The tunnels will be assumed to infinitely extend. Therefore, FLAC model consists of one unit section with a repository tunnel and a deposition hole. The tunnel is 5m high and 4m wide. The deposition holes are 2m in diameter and 8m in height with a spacing of 5m. 2.2 Procedure Three sides except symmetrical plane of the model are truncated within a certain distance from the tunnel to reduce computing time. The whole boundaries but the top arc constraint from lateral or vertical movement. Equally distributed forces corresponding to the overload of layers are applied to the upper boundary. Preliminary calculation is executed to obtain equilibrium state of stresses of the model without excavation, assigning initial stresses of the whole zones. Stress with depth is calculated from the following equation obtained from field measurements in other region
