The purpose of this work was to investigate the swelling behaviour of artificially wetted highly compacted bentonite buffer blocks used in deposition holes for nuclear waste containment. The target was to make a preliminary study if homogeneous and rapid swelling of the bentonite is possible to achieve in the gap between the buffer and the rock wall of the deposition hole by artificial wetting. It is expected that the thermal, mechanical, chemical and hydraulic properties of the whole deposition system are better when the bentonite-rock gap is closed. In practice it is desirable that the bentonite material has a volume increase sufficient enough to create pressure against the rock surface. Uniform bentonite buffer swelling into the gap would prevent rock scaling while lowering the risks of bentonite piping and erosion due to potential water flow. In this work, two preliminary sets of tests were done on bentonite block swelling and water filtration through pellets. The majority of work was within the primary tests, where bentonite blocks were confined in a test chamber and swelling was induced by artificial wetting. Water was poured directly into the gap from the top, without the aid of pipes or pressure. After the initial watering, no additional water or moisture was made available for the system during the test duration. The resulting bentonite swelling pressure in both the radial and vertical directions was measured over time. 13 different test scenarios were investigated, including: varying block size of 10 cm height disks or 30 cm blocks, eccentric aligned blocks with gap sizes of 5 and 45 mm, gap sizes of 25 mm or 50 mm, using gap filling with pellets or granular in combined with water, free upward swelling or confinement, addition of water at varying rates, and longer term test duration. In some cases, video images were taken during the swelling, to get an indication of the time until the gap was sealed. After the wetting tests, material properties of the swollen buffer were measured, including measurements of moisture content, SHORE hardness, and density. The results showed that the bentonite buffer with artificial wetting is uniformly swelling to fill the gap within the first day. A radial pressure of about 200 kPa is achieved within the first few days, with maximum values up to 1000 kPa. The pressure reached a maximum after about 2 weeks and then gradually declined due to moisture distribution from the gap towards the centre of the buffer block. The swollen material was well confined in the gap when an axial confining pressure was used from the start of testing. The vertical or axial uplift pressure due to swelling can be as much as double the radial pressure. Gap filling with pellets resulted in a higher and faster rate of radial pressure development, which was also better maintained at the time of vertical unloading. It was possible to completely fill the gap with water combined with Cebogel large pellets, yet finer Ibeco Seal granular was difficult to saturate. A hardness measurement could be used to indicate the density distribution over the cross section of the sample after testing. Moisture equilibrium between the gap and buffer was not reached within the short test duration. The results of this work indicate that artificial wetting could be a potential alternative to the buffer design. Uncertainties still exist that need to be further clarified, including the effects of upscaling and the required level of confining pressure needed in the first days to prevent uplift. It was possible to wet a gap filled with large pellets installed by pouring, yet there needs to be sufficient voids to allow free filtration.
|Place of Publication||Olkiluoto, Eurajoki, Finland|
|Number of pages||120|
|Publication status||Published - 2011|
|MoE publication type||D4 Published development or research report or study|
|Series||Posiva Working Report|
- deposition hole