A summary of the results of heater experiments conducted at the Stripa Mine in Sweden is given. These results for the induced temperature, displacement and stress fields, are compared with the original predictions for these parameters which were made using both analytical and finite-element calculations assuming that the material properties Of the rock remained temperature independent. Discrepancies between the measured and the predicted results are discussed. Additional calculations based on a limited amount of laboratory data for the temperature dependence of these material properties, are described. These new predictions are found to agree better with the measured field data.
Among the many alternatives that have been suggested for isolating nuclear wastes from the biosphere (Department of Energy, 1979) burial in deep underground caverns in a geologically stable formation appears to be the most practicable. The rock in a geological site used for a radioactive waste repository will be subjected to two major types of perturbations:
the perturbation due to the excavation of the repository and
the thermal loading due to the radioactive decay of the waste.
Civil and mining engineering experience can be relied upon to take into account the mechanical perturbation in the design and construction of the repository (Hoek, 1979). In contrast, there is little experience in the effects of thermo-mechanical loading of underground structures especially in hard rock.
Thermally induced stresses are important for repository design in a number of respects. First, high compressive stress may cause borehole decrepitation. thereby reducing the thermal conductivity of the rock and consequently causing increased temperatures in the waste canisters and also complicating waste retrieval should that be necessary. Second, if the waste is to remain retrievable for several decades, an objective that probably will become a policy in the United States (Batch and Heath. 1979), then the thermal stresses must be taken into consideration in the design of the excavations for the repository. Finally. thermally induced tensile stresses will lead to higher fracture permeability either by increasing the aperture or by extending the length of discontinuities. Conversely, compressive thermal stress is likely to reduce the permeability.
In addition to this thermo-mechanical loading of the rock the heating caused by the decay of the radioactive wastes is likely to cause convection of groundwater which is of concern since this could reduce the time for transport of radio-nucleides to the biosphere. Chemical reactions, such as corrosion of the canisters by the groundwater also may be accelerated by the heating. To gain a basic understanding of the behavior of a hard rock mass'' under the special conditions that will arise in an underground repository in the presence of heat-generating, radioactive wastes, a series of heating experiments have been conducted in a granite body at a depth of 338 m in an abandoned iron-ore mine at Stripa, Sweden, about 150 km west of Stockholm.