Uniaxial compaction of backfill blocks

Document available in extended abstract form only. Finnish nuclear waste management organization Posiva is investigating the KBS-3-method for spent nuclear fuel repositories and the industrialization of the component manufacturing. The objective of this project was to sort out the different factors affecting the manufacturing of the backfill block production. The main parameters affecting compaction were varied in the tests: the water content of the materials, compaction pressure, material temperature and the compaction speed. Materials tested were Friedland clay in two granule sizes, and the mixture of crushed rock and AC200 bentonite (60/40). In the first step of the testing, the laboratory scale, compaction pressure was altered between 10 and 50 MPa. The effects of pressure on the block density were clear, and 25 MPa pressure was selected for further testing. Pressure of 25 MPa produced high enough dry density, when the water content was correctly selected. The water content was the other tested variable. Based on the previous tests, the desired water contents were selected to be 7% for Friedland-clay and 9% for the mixture. The water content in the both materials was varied between 6% and 13%. The water content clearly effected the achieved dry density. With higher water content the material started to stick into the compacting piston surfaces. When varying the material temperature it was noticed that while the temperature has no effect on the achieved density, it has a huge effect on the material tendency to stick to the piston surfaces. At lower temperatures, below 10 C, the tendency to stick into the surfaces was greatly reduced. As a conclusion from the laboratory scale tests, the following parameters were selected for the industrial scale tests: - Compaction pressure should be around 25 MPa; - Water content for Friedland-clay should be around 8±0.5 % and for the mixture of crushed rock and bentonite (60/40) 12±0.5%; - Temperature alteration was considered to be too difficult to be studied in the compaction factory. It was decided to compact the Friedland-clay in the state of delivery. The first batch of Friedland clay was only two tonnes, and the compaction was easy. Correct compaction parameters were easy to find for the machine and the end result was excellent. The second batch of 25 ton was delivered few moths later and the water content varied between 5.7 % and 9.3%. The big bags with the lowest water content were extremely difficult to compact. The maximum compaction pressure which was used was only 6 MPa, resulting into too low dry densities. Water content adjustment was done to a part of the delivery batch on site. After the water content had been adjusted to around 8%, the compaction could be done. However the compaction result was inferior to the first batch, but still in acceptable limits. Based on this test it was concluded that the Friedland-clay requires some time to cure after the water adjustment. After this experience all of the remaining Friedland-clay batches were mixed beforehand as well as the mixture of crushed rock and bentonite. The mixing was mainly done with BSH-mixer, which was twin shaft 44 rpm mixer. The water adding was done with a pressure nozzle, which proved to provide a suitable drop size. Some tonnes were also mixed with Eirich intensive mixer. The major difference between the mixers was the mixing time, although there were small indications that using the Eirich mixer could result to smaller variance in the block density. The compacted batch was too small to be certain of this. The smaller granule size (0-1 mm) Friedland-clay causes much dust in the factory environment, hence, it was decided to try bigger granules (1-6 mm) as solution to the problem. The dusting was not significantly better with the larger granules, however, the blocks were much better. The compaction speed was higher since the air was able to get out of the block faster, and the mechanical strength was also greater. The compaction of the mixture of crushed rock and bentonite (60/40) was very challenging due to material sticking onto the pistons. In order to produce blocks a lubricant was applied to piston surfaces. Lubricant helped to the sticking but is not acceptable from long-term safety point of view. After finding the right compaction factors the process was able to manufacture blocks with great accuracy and constancy. - 80-90 % of the Friedland-clay blocks were within the ±0.1 mm height tolerance and average dry density variation was around ±20 kg/m3; - 80-98% of the blocks made out of the mixture of crushed rock and bentonite (60/40) were within the ±0.1 mm height tolerance and the average dry density variation was around ±20 kg/m3