During the last few years, a reference concept for direct disposal of spent fuel from nuclear reactors in salt formations has been under investigation. The stability of the underground excavations and the extent of the disturbed zones due to the excavation will have a primary influence on the short term safety assessment of a such waste repository. The purpose of this paper is to present the results of a series of thermomechanical analyses of the main access drift in the vicinity of an emplacement area for spent fuels. The finite element calculations were performed with the program MAUS taking into account the nonlinear, time and temperature dependent behaviour of the rock salt. A failure criterion was also considered. The results of the numerical analyses show that the thermal load induces a clearly higher closure rate of the drift but only a minor additional load on the pillar. The development of the brittle zones around the drifts as a consequence of the tensile stresses are determinated.
The reference concept for direct disposal of spent fuel consists of the emplacement of self-shielded containers (POLLUX-containers) on the floor of the storage drifts in rock salt at a level of 850 m below the surface (Closs et al. (1984)). Immediately after the emplacement the drifts will be backfilled with crushed salt. The disposal areas are limited by two parallel main access drifts. At the emplacement time, the heat generation power of the POLLUX Container will be up to 7.5 kW, depending on the intermediate storage time. The temperature increase in the disposal drifts will affect the thermo-mechanical behaviour of the surrounding rock salt. As a consequence, new problems in the stability of the pillar between the emplacement area and the main drift may arise. In this paper, the temperature fields, the thermally induced drift closure, the resulting stress-strain fields in the pillar and the development of the disturbed zones in rock salt around the drifts are numerically estimated. The calculations were performed with the finite element code MAUS (Mechanical Analysis of Underground Storage) specifically developed for the investigation of nuclear waste repository structures (Albers (1985)), taking into account the complex material behaviour of the rock salt by elevated temperature. The Temperature development in the disposal field was calculated with the program FAST-BEST (Korthaus (1983)).
The thermo-mechanical behaviour of rock salt is a highly nonlinear function of temperature, stress and time (Langer (1982)). With regard on the radioactive waste repository the long term creep deformation ought to be considered the most important part of the constitutive behaviour of rock salt. The complex thermo-mechanical behaviour of rock salt was described in the finite element analysis by a thermo-elastic material model with temperature dependent steady state creep. However, this has not been considered in the calculation because there is not sufficient data available. A strength criterion to determine whether or not the rock salt has failed, was considered in the numerical modelling.
