Studies of buffers behaviour in KBS-3H concept. Work during 2002-2004

In the KBS-3H concept, the deposition tunnels are replace by horizontal 300-m long circular deposition drifts which are excavated from a niche in the transport tunnel. About 40 disposal containers will be deposited in each drift. In order to make the deposition process easier the buffer material and one copper canister are assembled in a steel disposal container, which then is pushed into the deposition drift. The disposal container consists of a perforated steel cylinder in which the buffer material and one copper canister are assembled. Distance blocks of bentonite are placed between each disposal container. The purpose of the distance blocks is to seal off each canister position from the other and to prevent transport of water and bentonite along the drift. The distance blocks also separate one canister from the other in order to get the right temperature of the canister. The total thickness of the distance blocks between the disposal containers is mainly determined by the thermal conductivity of the rock and is expected to be in the range of 3-6 m. The main objective for the KBS-3H concept is that the method provides a more efficient way of depositing the canisters in the rock. The reason is that the deposition tunnels of the KBS-3V concept are not needed and the reduction of rock excavation is therefore about 50 percent. This leads to a lower environmental impact during the construction of the repository but also to a reduced disturbance on the hydro-geological situation in the rock mass. Furthermore, the reduction in rock excavation leads to a significant cost saving for the excavation phase and backfilling of the repository. KBS-3H and KBS-3V are very similar with respect to the behaviour of the bentonite buffer. However, there are some differences that require special attention. An early survey of the differences yielded that there are a number of processes and functions that needed to be investigated for evaluating the feasibility of the concept: 1. The function of the distance block. Scenario analyses of different concepts for design and installation of the distance blocks for finding critical issues. 2. Sealing ability of the distance plugs during water inflow. 3. Influence of rapid increase of water pressure inside the distance blocks. 4. Piping and erosion phenomena of the swelling bentonite during the installation phase and during the water saturation phase. 5. Mechanical interaction between the container and the buffer during the homogenisation of the bentonite and breakage of the container. 6. Near field thermal and hydraulic evolution. These processes have been studied by Clay Technology in a number of tests and analyses during 2002 to 2004. The studies can be structured in the following way: 1. Test scaled 1:10 of a simulated part of a deposition tunnel with two canisters. 2. Design and planning of a large-scale test of the interaction between the bentonite and the perforated deposition container and manufacturing of components (Big Bertha). 3. Investigation of sealing/piping/erosion phenomena during wetting of the buffer material. a) Basic laboratory tests. b) Study of processes and scenarios in the scale 1:10. c) Study of processes and scenarios in full scale. 4. Investigation of the effect of rapid water pressure increase inside the distance blocks by model tests in the scale 1:10 and in full scale. 4. Modelling a) Modelling of the interaction between the bentonite and the perforated deposition container. b) Modelling of temperature conditions for design and safety analysis purpose. c) Modelling of the water saturation phase and the influence of the hydraulic properties of the rock. d) Modelling of the wetting of the test scaled 1:10. e) Scenario analyses and conceptual modelling of the function of different distance block concepts. These studies and the results and conclusions reached will be presented in this report. The report should be considered a state of the art report at the end of 2004. The studies are planned to continue during the coming years