Mineralogical investigations of the first package of the alternative buffer material test – II. Exchangeable cation population rearrangement

Abstract

AB ST R ACT : Bentonites are candidate materials for the encapsulation of radioactive waste. In the 'Alternative Buffer Material test' (ABM), compacted ring-shaped blocks of eleven different buffer materials (mainly bentonites) were packed vertically on top of each other with an iron tube as heater in the centre. These buffer materials started with various exchangeable cation populations (EC population ). The first 'ABM package' was terminated 28 months after installation and the bentonites had been exposed to the maximum temperature (130ºC) for about one year. The aim of the present study is first: to describe modification of the cation exchange population, and second to understand the influence of the groundwater on cation exchange at different scales. No significant horizontal variation of any exchangeable cation (EC) was detected between 1 and 7 cm distance from contact with the iron tube. Large total differences of the EC populations , however, were observed for the individual blocks after the field experiment (n = 21 blocks) with respect to the composition of the reference materials. The average cation exchange capacity (CEC) values of the analysed bentonites (n = 9 blocks) decreased by 5.5 meq/100 (1.1À8.8 meq/100 g) after the experiment. Exchangeable Na + and Mg 2+ decreased on average, whereas Ca 2+ increased. This trend was pronounced in the top region of the parcel (upper seven blocks). Although most changes occurred on the large scale of the whole test parcel, small but important changes were also recorded in the vertical direction on the centimetre scale. The observed differences cannot be explained assuming simply that a bentonite reacts only with neighbouring blocks, which would mean that the system was more or less closed. The differences are much larger and the only conclusion from this observation is that the whole package seems to be influenced by the groundwater which was added from a water tank at the experiment site, enabling at least partial equilibration between the different blocks.

Long-term effects of an iron heater and Äspö groundwater on smectite clays: Chemical and hydromechanical results from the in situ alternative buffer material (ABM) test package 2

“…In general, the changes in exchangeable cation populations in ABM2 samples were clearly more pronounced than observed previously Dohrmann et al, 2013). Dohrmann et al (2013) found that the exchangeable cation compositions in ABM parcel 1 varied considerably between the blocks, and that blocks located in the upper part of the parcel were richer in Ca than blocks located in the bottom part of the parcel. Those authors also noted that the influence of neighbouring blocks on cation compositions was much less important than the position within the test parcel.…”

Distinguishing between more and less suitable bentonites for storage of high-level radioactive waste

“…In addition, Ca-rich bentonites do not tend to release colloidal particles as Na-bentonites do. Furthermore, the larger selectivity of Ca(Mg) towards the interlayer and the fact that most host-rock waters do contain some Ca will cause fast cation exchange if Na-bentonites are contacted with such solutions (Kaufhold et al, 2013;Dohrmann et al, 2013b). A general recommendation of the type of exchangeable cation initially present may be redundant because of the fast equilibration of the bentonite with the surrounding water.…”

“…Depending on the barrier concept and particularly on the amount of rock water in contact with the bentonite barrier, the selection of the type of exchangeable cation is only relevant for the early phase of the buffer. In the Äspö rock laboratory the cation exchange (equilibration with rock fluids) was fast (Dohrmann et al, 2013b) so that the cation initially present was less important than the composition of the host-rock water. The pH-buffering effect discussed above, however, is also more important in the early stages of the barrier because of the particularly high pH of the cement fluids at the beginning.…”

“…Therefore, no general recommendation can be given. Recent up-scale tests showed quick equilibration of the interlayer cation population with the surrounding water (Dohrmann et al, 2013b) and that the type of interlayer cation initially present is less important (van Geet & Dorhmann, 2016, this issue).…”

“…The interlayer composition (type and amount of exchangeable cations) will soon re-equilibrate if bentonite is in contact with saline waters. Such cation exchange processes are surprisingly fast, even in highly compacted bentonite blocks (Dohrmann et al, 2013b). Cation exchange may affect physico-chemical and geotechnical properties such as swelling or hydraulic conductivity.…”