Preliminary results from water content and density measurements of the backfill and buffer in the prototype repository at Aespoe HRL

Document available in extended abstract form only. Since 2001 the Prototype Repository at Aespoe Hard Rock Laboratory has been carried out as a large-scale experimental installation of the KBS-3 Swedish/Finnish concept for final disposal of spent nuclear fuel. The Prototype Repository consists of a total of six full-scale deposition holes with a centre distance of 6 m, located in a TBM tunnel at a depth of 450 m. Each deposition hole is fitted with a full-scale bentonite buffer, consisting of altogether 14 blocks and a full-scale canister, Figure 1. The canisters are equipped with heaters to simulate the heat from spent nuclear fuel. There are two sections of the installation; The inner section (I) consisting of four deposition holes (no. 1-4) with buffer and canister, and the outer section (II) consisting of two deposition holes (no. 5-6). The deposition tunnel is filled with a mixture of crushed rock and bentonite (30% of bentonite). A massive concrete plug, designed to withstand full water and swelling pressures, separates the test area from the open tunnel system and a second plug separates the two sections. This layout provides two more or less independent test sections. The outer section was opened and retrieved during 2011. The backfill was excavated with a back-hoe loader in layers of two metres. Samples were taken in these layers with the object of determining density and water content. Important items of the backfill to examine were the contact between backfill and the tunnel wall and the contact between the buffer and backfill in the deposition holes. The water content of the backfill was determined by drying samples in an oven at a temperature of 105 C for 24 h and the density was determined by weighting the sample both in air and merged into paraffin oil with known density. Altogether more than 900 tons of backfill material was excavated from the tunnel and more than 1100 samples, distributed over 11 sections, were taken for determining the water content and density. Preliminary results from the excavation are showing no evidence of piping and erosion in the backfill and the contact between the backfill and the tunnel surface was intact, i.e. no gaps between the tunnel wall and the filling were observed. Furthermore, the measurements of the water content and the density are indicating that the backfill was fully saturated, although the filling close to the roof had a higher water content compared to the central part of the tunnel resulting in a lower density. Samples were taken with the object of determining density and water content when removing the buffer (MX-80) in the two deposition holes. Samples were also taken for analysis of possible biological, chemical and mechanical changes in the buffer. The buffer was not expected to be water saturated in all parts. Furthermore, the water intake has occurred via the rock which means that density and water content was not expected to be axisymmetrical. In order to get a picture of the variation in density and water content of the buffer in the two deposition holes, an extensive sampling of the buffer was made. The examination was made in 8 radial directions in each of the 14 blocks. Most of the samples were taken by core drilling from the top of each of the bentonite blocks. Altogether more than 3500 determinations of water content and density were made on the buffer in each of the deposition holes. The preliminary analyses of the buffer are indicating that the buffer was not fully saturated and that the water uptake was not axisymmetric. An axial swelling of the buffer, especially the upper part of the buffer, causing decrease in density, was also observed. (authors)