Abstract:

The disposal of used nuclear fuel using the KBS-3 concept requires the excavation of thousands of 1.8-m diameter 8 m deep boreholes. The heat generating spent fuel placed in these boreholes will increase the stresses on the boundary of these boreholes. Three in-situ experiments have been carried out to assess the performance of the borehole wall as the excavation-induced and thermally-induced stresses are applied. The three experiments utilized similar configurations and concepts. The APSE and CAPS experiments were carried out in heterogeneous igneous crystalline rock mass. The heterogeneity is caused by alteration, sealed fractures, my-lonite shear zone and open flowing fractures. The APSE experiment utilized external heaters while CAPS used a central heater. The diameter of the APSE experiment was more than 3 times greater than the diameter of the CAPS experiments. Despite the different loading path and differences in scale the rock mass spalling strength deter-mined from the CAPS and APSE results was essentially the same, approximately 50% of the intact laboratory uniaxial compressive strength. Both experiments also showed that a small confining pressure applied to the wall of the borehole was sufficient to control fracture propagation associated with the spalling process. The POSE Experiment was the first attempt to establish the spalling strength for a heterogeneous mixture of strongly foliated mica gneiss with coarse grained pegmatite granite veins (Migmatite). The rock mass is essentially massive, i.e., no open frac-tures. The results from the POSE experiment are still being reviewed as the experi-ment is still in progress. The results obtained to date are not as easy to interpret as those obtained in APSE and CAPS, in part because there is still some uncertainty as to the orientation of the in-situ stress in the vicinity of the experiment.

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