Roller pressing or extrusion - seeking the best pellet production methods
Posiva has been testing manufacturing methods for bentonite pellets to be used in the disposal of spent nuclear fuel. The practical research work was conducted at the Espoo and Jyväskylä facilities of the VTT Technical Research Centre of Finland. The work aimed to reveal the optimal composition, length and manufacturing method for the pellets.
Development engineer Keijo Haapala from Posiva, why is Posiva conducting research on bentonite pellets?
"We wish to survey the properties of different bentonite pellets. We also want to make sure that bentonite pellets can be manufactured using the materials needed in the Finnish nuclear waste repository."
What is the role of the bentonite pellets in the final disposal of nuclear waste?
"They are needed to fill up the disposal holes and tunnels. In a disposal hole, bentonite blocks are placed around the canister, and bentonite pellets are used to fill the gap between the blocks and rock. Similarly, the gap between the filling blocks and the walls and ceiling of the deposition tunnels are filled using pellets. Briquette pellets have been tested in ONKALO at Olkiluoto as a part of a buffer test built to 40 percent of the full size. A full-scale test will begin in the ONKALO demonstration facilities in 2014. A full size copper canister will be used in the test together with bentonite blocks. Bentonite pellets will be used to fill the gap between the blocks and rock."
Have tests already revealed anything of the quality of the pellets?
"The preliminary test results have been positive. The pellets don't crumble when handled, and their length seems to be appropriate. The length of the pellets produced by extrusion varies. The amount of fine material from the production process is low compared to the amount of pellets.
The purpose of the test is to subject the pellets to compression to test their strength and handling properties. VTT will keep the first bentonite pellets produced by extrusion and test them further in a laboratory in Espoo."
Can different methods be used to produce bentonite pellets?
"They have been produced by extrusion and by roller pressing. Pellet research has been carried out in Canada as a joint project between Posiva, the Swedish SKB and the Canadian NWMO. The work was carried out by Atomic Energy of Canada Limited (AECL). They manufactured roller pressed briquettes from various materials. The properties of the briquettes were studied and the results of the development work were reported. Posiva has so far produced six tonnes of briquettes of MX-80 bentonite for testing purposes.
Previously, a 200 kg batch of bentonite pellets was produced by extrusion at VTT Jyväskylä. The aim was to find the proper mix of raw materials. There were differences between clay types and moisture levels. At the beginning of September, more extensive extrusion tests were conducted in Jyväskylä to test two different moisture levels and pellets of two diameter sizes. After testing the new pellet batches, we will be able to compare the results with those of roller pressed briquettes.An efficient extrusion method beats the roller press in production rates."
What are the most important criteria for bentonite pellets used in final disposal of nuclear waste?
"The pellet materials must have adequate swelling properties. The moisture level of the pellets must be right, and they must be able to fill the required space. For example, pellets produced by extrusion must not be too long to avoid bridging when filling the gaps."