Thermally Induced Bentonite Alterations in the SKB ABM5 Hot Bentonite Experiment

Abstract

Pilot sites are currently used to test the performance of bentonite barriers for sealing high-level radioactive waste repositories, but the degree of mineral stability under enhanced thermal conditions remains a topic of debate. This study focuses on the SKB ABM5 experiment, which ran for 5 years (2012 to 2017) and locally reached a maximum temperature of 250 °C. Five bentonites were investigated using XRD with Rietveld refinement, SEM-EDX and by measuring pH, CEC and EC. Samples extracted from bentonite blocks at 0.1, 1, 4 and 7 cm away from the heating pipe showed various stages of alteration related to the horizontal thermal gradient. Bentonites close to the contact with lower CEC values showed smectite alterations in the form of tetrahedral substitution of Si4+ by Al3+ and some octahedral metal substitutions, probably related to ferric/ferrous iron derived from corrosion of the heater during oxidative boiling, with pyrite dissolution and acidity occurring in some bentonite layers. This alteration was furthermore associated with higher amounts of hematite and minor calcite dissolution. However, as none of the bentonites showed any smectite loss and only displayed stronger alterations at the heater–bentonite contact, the sealants are considered to have remained largely intact.

Characterization of Bentonites from the In Situ ABM5 Heater Experiment at Äspö Hard Rock Laboratory, Sweden

“…However, in this work, no Mg-or Fe-rich trioctahedral smectites were detected despite the decrease in Mg at exchange sites and the increase in iron due to the corrosion of the C-steel heater. This is in agreement with other ABM5 studies [8,9]. Fully reducing conditions are important constraints for the alteration of diocthedral smectites to trioctahedral ones.…”

“…Some blocks looked rather intact, while others were highly fractured and very fragile due to the effect of water loss caused by the high temperature. Different bentonite samples were retrieved and analysed for performing different investigations (e.g., [8,9,17]). Most of the bentonite samples were preserved immediately inside vacuum-sealed aluminium-foil bags for avoiding water loss and oxidation due to their exposure to the air-atmosphere.…”

“…Estimated maximum temperatures at the heater contact of 240-250 • C were reached between blocks 22 and 8, with decreasing temperatures both at bottom and top part of the bentonite column (<188-156 • C, blocks 3 and 27), and as a function of the granite contact (Figure 2). In the hottest blocks, temperatures of more than 150 • C were reached at a distance of 6 cm from the heater, with the thickness of the bentonite block being 10 cm (see also [8,9]).…”

“…After the experiments, the buffer packages ABM1, ABM2 and ABM5 were retrieved and analysed, with the studies being focused on hydromechanical properties [5], geochemical and mineralogical alteration [5][6][7][8][9], cation exchange rearrangements [10,11], and redox chemistry evolution [12][13][14]. However, few studies have been performed on the pore water chemistry in these systems [15].…”

EURADWASTE’22 Paper – Host rocks and THMC processes in DGR

“…For the clay host rocks and bentonite, previous knowledge indicates that an increase in temperature due to the presence of heat-emitting wastes will induce strong and anisotropic THM coupled responses within the clayey materials [18]. Although the effect of temperatures higher than 100 • C has been studied concerning, for instance, mineralogical transformations of bentonites [16,28,29], less is known with respect to HM properties of clayey materials for this range of temperatures, mainly because of the testing experimental issues.…”