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Summary:
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Methods for backfilling and sealing of disposal tunnels in an underground repository for spent nuclear fuel are studied in cooperation between Finland (Posiva Oy) and Sweden (Svensk Kärnbränslehantering AB, SKB) in “Backfilling and CLOsure of the deep repository” (Baclo) programme. Baclo phase III included modelling task force SP1: Finite element modelling of deformation of the backfill due to swelling of the buffer. The objective of the finite element modelling of the backfill was to study the interaction between the buffer and backfilling. The calculations aimed to find out how large deformations can happen in the buffer-backfill interface causing loosening of the buffer bentonite above the canister. The criterion used was that the dry density of the buffer right above the canister should be higher than 1.95 tn/m3. This report presents the results of the VTT’s modelling calculations and comparison between different calculation approaches. The analytical and finite element calculations of ClayTechnology AB are reported separately.
The modelling calculations were conducted with SKB’s and Posiva’s deposition tunnel geometry. Posiva’s tunnel is clearly smaller than SKB’s and also the backfilling degrees may differ according to present design proposals. Based on current plans, 60…80% of the total volume of deposition tunnels will be backfilled with pre-compacted blocks and the remaining space will be filled with bentonite pellets. The basic assumption in these modelling was that the buffer is totally saturated generating swelling pressure up to 7 MPa and backfill is in unsaturated state. This was evaluated to present a “worst case scenario” with the highest risk to lead in decrease in dry density of the buffer. Most of the modellings were done using material properties determined for Friedland clay blocks, but also Asha case was tested. Besides these the boundary conditions of the problem were varied, like friction between tunnel wall and pellets and missing pellets or free spaces due to erosion. It was also planned to include some preliminary 3D studies into this work, but the Plaxis 3D proved to be unsuitable for that. Therefore most of the results gained were based on axisymmetric 2D calculations done with Plaxis 2D. The chosen material model was linear elastic continuum material. Some tests were done with elasto-plastic Mohr-Coulomb material model, but they failed due to the unrealistic stress concentrations in tunnel corner.
When the input assumptions and different calculation methods (2D, 3D and analytical calculations) are compared to each other it can be concluded that the vertical deformations in the interface between buffer and backfill vary between 80…120 mm for the SKB tunnel geometry. Based on 2D calculations for the Posiva geometry the vertical deformations in the interface are about 80 mm. The dry density requirement of 1.95 tn/m3 will be exceeded if the loosening in the buffer is supposed to happen evenly. However, in reality the loosening is non-linear being highest near the interface. The 3D modelling of ABAQUS refer to the possibility that this requirement can be fulfilled meaning that the loosening of buffer right above canister will remain on acceptable level.
The modelling process should be an essential part of the tunnel backfill design. These modellings are preliminary and they include many assumptions and simplifications (like the deformations of block-block interfaces). In the future the modelling should include saturated backfill, 3D models, sophisticated material models and tunnel floor granules.
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