Simulation of HTM processes in buffer-rock barriers based on the French HLW disposal concept

Document available in extended abstract form only. The main objectives of this paper are to gain experience with modelling and analysis of HTM processes in clay rock and bentonite buffer surrounding heat-generating radioactive waste. The French concept for HLW disposal in drifts with backfilled bentonite buffer considered in numerical calculations which are carried out by using the computer code CODE-BRIGHT developed by the Technical University of Catalonia in Barcelona. The French repository designed by ANDRA is located in the middle of the Callovo-Oxfordian argillaceous formation (COX) of 250 m thickness at a depth of 500 to 630 m below the surface. The French concept has been simplified at this simulation work. A drift is considered to be excavated at a depth of 500 m below the surface. It has a diameter of 2.2 m and a length of 20 m. A large volume of the rock mass around the drift is taken into account by an axisymmetric model of 100 m radius and 100 m length. In fact, this model represents a cylindrical rock-buffer-system with the central axis of the containers, as shown in Figure 1. Some points are selected in the buffer and the rock along the radial line (dash yellow line) in the middle of the drift for recording HTM parameters with time. The display and analysis of the results at this paper are chiefly along this line. The simulation work has been divided to two time steps. At the first step, the drift excavation and ventilation is simulated by reducing the stress normal to the drift wall down to zero and circulating gas along the drift wall with relative humidity of 85 %. Following the drift excavation and ventilation, the HLW containers and the bentonite are emplaced in the drift as the second step of the simulation. This is simulated by simultaneously applying the initial conditions of the buffer and the decayed heat emitting from the waste containers as thermal boundary conditions. Two materials (Clay rock and bentonite buffer) are taken into account at this simulation work, so that a lot of material HTM properties and parameters have to be considered. The material parameters are taken from the literatures, laboratory- und URL-experiments. Inspired by the French concept for disposal of HLW waste and the in-situ prevailing conditions data in the Bure-URL, coupled HM-coupled performances at the operation phase of the disposal and the HTM-coupled phenomena at the after-closure phase have been simulated and analyzed. The results include the display and interpretation of the temperature field development, the saturation and desaturation processes, mechanical stress and displacement, material damage processes, hydraulic swelling and thermal expansion at the buffer and clay rock media, which are jointly shown at this paper. The main conclusions from the modelling works are summarized as follows: (1) The drift excavation induces a redistribution of the rock stress with a minimum radial component, a maximum tangential component, and a middle component in the length direction. The deviatory stress results in deformation of the rock towards the open drift. In the area near the drift wall, the rock is damaged. The damaged zone extends into the rock mass to a distance of about 1.5 m. (2) The drift ventilation leads to a reduction of the pore-water pressure and even to a desaturation in the surrounding rock. The de-saturated zone reaches to a distance of about 4 m over 5 years. (3) The backfill of the drift with the unsaturated buffer enhances the desaturation of the rock. The bentonite-buffer takes up water from the rock, increasing the swelling pressure against the deformation and the damage of the surrounding rock. (4) The heat from the HLW containers transfers gradually into the buffer and the rock. The maximum temperature of 157 C is reached at the surface of the containers after about 2.5 years. The temperatures in the rock are limited below the conceptual criterion of 90 C, except for that of 93 C at the rock / buffer interface. (5) Heating causes evaporation of the pore water in the buffer near the containers and thus desaturation. In the opposite side, the water saturation in the buffer near the rock increases. (6) The increase of temperature in the saturated rock generates a significant rise of the pore water pressure up to 9 MPa due to the very low porosity and the large difference of the water expansion and the solid expansion. Simultaneously, the total rock stress increases, too. Because the effective stresses during the heating phase are still compressive, no fracturing can take place during the heating period. (7) The preliminary modelling results are comparable with those obtained from the similar calculations of HTM processes in the buffer-rock-barriers surrounding HLW canisters in horizontal and vertical boreholes in clay and in granite. (authors)