Description of basic design for buffer

This report describes the basic design of the buffer for a KBS-3V repository. The basic design is also known as “the reference design”. This report is mainly based on the design basis of buffer "Design basis for buffer components" (Juvankoski and Marcos 2009) and on the Facility Description 2006 (Tanskanen 2007). This report on hands has been used in turn as initial data in updating the Facility Description 2009 (in Finnish “Loppusijoituslaitoksen luonnossuunnitelma 2009”, Saanio et al. 2009). In the KBS-3V repository concept single canisters containing spent nuclear fuel are emplaced in individual vertical deposition holes drilled in the floor of deposition tunnels in the bedrock at about 420 m depth. The diameter of the deposition holes are 1.75 m and the depths from 6.60 m to 8.25 m. In the basic design disk type bentonite blocks are installed at the bottom of the hole and on top of the spent fuel canister. Ring type bentonite blocks surround the fuel canister. In the basic design MX-80 bentonite is the reference bentonite material. The gap between the deposition hole rock surface and the bentonite blocks is 25 mm and the gap between the bentonite blocks and fuel canister is 10 mm. In the basic design the gaps will not be filled at emplacement time. Bentonite blocks have a diameter of 1.70 m and a height of 0.40 m. At installation the water content of blocks is about 16% and the bulk density of blocks is about 2100 kg/m³. At target state the saturated density in the buffer is about 2000 kg/m3 . In the basic design the swelling of bentonite is assumed to be achieved due to inflowing water leaking from the rock over time. If the amount of inflowing water from the rock is small, the swelling of bentonite can be delayed. The increase of temperature of the rock could result in the spalling of the upper parts of the deposition hole, if the buffer does not exert swelling pressure on it. On the other hand, if water inflow to the hole is high, it can cause problems with the installation and may cause erosion of bentonite. It is important to be prepared for both dry and wet holes. This in turn affects the installation schedule. In order to ensure the successful installation and behaviour of the buffer the details of the deposition hole are important, e.g. wall surface flatness, evenness of the base and the chamfer required on the top of the hole for the disposal of spent fuel canisters of the Olkiluoto nuclear power plants. All of these must be designed so that the requirements set for the buffer are met. This report will be followed by a detailed design report "Description of detailed design of buffer", in which some issues already dealt in this report, will be treated in more detail. It will also contain alternative gap handling methods (open gap, wetting, filling).