Buffer component manufacturing by uniaxial compression method: Small scale

The purpose of this work was to gain better understanding about the method of uniaxial compression for the production of bentonite buffer components. Manufacturing by uniaxial was done in laboratory conditions to produce small cylindrical samples. The goal was to produce samples with sufficient bulk density according to the design needs, as well as to have homogenous density distribution and minimal defects in the sample. The testing was done using MX-80 bentonite as the reference material according to the production line plan. The water content was typically 17 %, though a few additional tests were done in ranges from 13 % to 22 %. Compression was done at 100 MPa for a target bulk density of approximately 2100 kg/m3. The final compressed sample size typically was 100 mm in diameter by 100 mm in height. Four different mould arrangements were used, including moulds with straight or inclined (0.5°) walls, moulds with insert tube liners, 3-part opening mould, and dual direction pressing mould. Approximately 10 different types of lubricants were used to reduce friction, including single-use spray, powder or grease lubricants, permanent mould liner coatings and single use foil liners. Post compression assessment included measurement of bulk and dry density over the height of the samples and visual observation of defects and/or cracking. In some cases tensile and compressive strength were tested. Microscopic investigations of the surfaces provided information about lubricant penetration to the bentonite. The results showed that it was possible to obtain uniform, defect free bentonite blocks by uniaxial compression. The best samples were obtained with the inclined mould walls and using a high-quality lubricant so that the bulk density variation was under 0.5 % across the sample height. Copper spray was the best lubricant, as well as the Teflon coating. Yet permanent coatings are a better alternative since they do not get entrapped within the bentonite during mould filling and compaction. Microcracks in some samples were attributed to sample extraction by pressing out of the mould. The 3-part opening mould resulted in samples without cracks, even when pressed without lubricant. The dual direction pressing resulted in production of homogeneous samples double in height compared to the diameter.