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Recent publications

Workreport 2018-25



Downscaled Tests on Buffer Pellet Behavior in Steel Cells


Halonen, M., Adesola, F.O.



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The erosion of Posiva pillow-shaped buffer pellets was studied in four medium-scale laboratory tests during April 2016 – January 2017. The pellets were poured inside confined rigid steel cylinders with a diameter of 350 mm and sample height ranging from 800 to 900 mm and were subjected to a constant inflow of 10 g/L saline solution with an inflow rate ranging from 0.002 to 0.1 L/min. Two tests were run in parallel and the target test duration was four months.

The objective of the tests reported in this document was to help make a distinction between erosion in bentonite buffer blocks and that in gap filling pellets, which in turn would provide information on which of the two buffer components would be more susceptible to erosion. Only pellets were used in these tests to better understand the clay homogenization under different watering circumstances and to provide further information for modelling studies.

Radial and axial pressure was measured from inside the cylinder, and the flow rate and pressure of the input solution was monitored for the duration of the tests. Erosion was measured with two different methods: the traditional periodic measurement where a sample of the outflow solution was taken daily and analyzed for eroded material, and a continuous, sedimentation-based method, which aimed to gather the clear majority of the eroded material in sedimentation bottles and tanks for later measurement.

After completion, the contents of the cylinders were sampled and measured for water ratio, bulk density and water potential (suction).

The tests subjected to low inflow rates proved troublesome for the test system as the pellet filling was able to block the flow, which in turn lead to development of high pressures inside the test system. This means that under certain conditions (inflow rate 0.01 l/min) the Posiva pillow pellet filling is capable of sealing flow channels.


Buffer pellets, bentonite, erosion


WR 2018-25_web (pdf) (7.6 MB)


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