POSIVA Report 2012-45



Current Status of Mechanical Erosion Studies of Bentonite Buffer


Sane, P. (ed.), Laurila, T., Olin, M. & Koskinen, K.



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ISBN 978-951-652-227-5


The performance of the bentonite buffer in KBS-3-type nuclear waste repository concept relies to a great extent on the buffer surrounding the canister having sufficient dry density. Loss of buffer material caused by erosion remains as the most significant process reducing the density of the buffer. The mechanical erosion, or pre-saturation erosion, is the process where flowing groundwater transports buffer material away from the deposition hole towards the deposition tunnel. This process reduces the overall buffer density and potentially creates localized regions of low density. In the worst case the process is assumed to last as long as the free volume between the pellets in the pellets filled regions is filled with groundwater.

With fixed environmental and material parameters a set of experiments was performed, testing the erosive properties of different buffer and backfill materials (MX-80 and Friedland Clay) in different groundwater conditions. The method used was a pinhole erosion test using two size-scales; 100mm and 400mm of cell length. The purpose of the pinhole tests was to test the scenario where piping channel is formed in the buffer and water flows through a single channel.

The erosion data was produced with two methods, firstly the time-related erosion rates measured in-situ during the measurement and secondly the overall mass loss in the sample cell measured after dismantling of the test. It was observed that erosion in piping channels decreases rapidly (~24 h) and irreversibly to a level that is an order of magnitude lower than the peak values.


Mechanical erosion, piping, MX-80, Friedland clay, erosion model, free swelling.


Current Status of Mechanical Erosion Studies of Bentonite Buffer (pdf) (4.2 MB)


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