Interaction of Corroding Iron with Eight Bentonites in the Alternative Buffer Materials Field Experiment (ABM2)

Abstract

Bentonite, a common smectite-rich buffer material, is in direct contact with corroding steel in many high-level radioactive waste repository designs. The interaction of iron with the smectite-rich clay may affect its swelling and sealing properties by processes such as alteration, redox reactions and cementation. The chemical interactions were investigated by analysing the Fe/clay interfaces of eight bentonite blocks which had been exposed to temperatures up to 130 °C for five years in the ABM2 borehole at the Äspö Hard Rock Laboratory managed by the Swedish Nuclear Fuel and Waste Management Co (SKB). Eleven interface samples were characterised by high spatial resolution methods, including scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and μ-Raman spectroscopy as well as by “bulk” methods X-ray diffraction, X-ray fluorescence and 57Fe Mössbauer spectrometry. Corrosion induced an iron front of 5–20 mm into the bentonite, except for the high-Fe bentonite where no Fe increase was detected. This Fe front consisted mainly of ferric (oxyhydr)oxides in addition to the structural Fe in the smectite fraction which had been partially reduced by the interaction process. Fe(II) was also found to extend further into the clay, but its nature could not be identified. The consistent behaviour is explained by the redox evolution, which shifts from oxidising to reducing conditions during the experiment. No indication of smectite alteration was found.

Unravelling the corrosion processes at steel/bentonite interfaces in in situ tests

“…The cooling phase also lasted for about 1 year. The experiment is outlined in detail in Kaufhold et al [19] and Hadi et al [20] The analysis of the steel-bentonite interfaces is described in detail by Wersin et al [21] It included 11 interface samples from 8 blocks, with 7 different bentonite materials: MX-80 from Wyoming (as the "caged" granular material in two different blocks), MX-80/quartz mixture (70:30) (as the "caged" granular material), Ibecoseal from Georgia, Ikosorb from Morocco, Kunigel VI from Japan, Rokle from the Czech Republic and Deponit CAN from Milos, Greece.…”

“…ABM1, ABM2, FEBEX: The analysis of the Fe/clay interfaces was based on a multimethod approach developed over the last few years and described in detail in Wersin and colleagues. [18,20,21] Two types of interface samples were prepared. The first type was utilised for microscopic analysis at high spatial resolution using SEM/EDX and μ-Raman spectroscopy.…”

“…In particular, this method enables the evaluation of the overall reduction level (Fe(II)/Fe tot ) and distinguishing between different Fe(III) and Fe(II) species in clay materials. [21,25,31] An example of the Mössbauer spectra from the FEBEX sample, which is presented in Figure 4, shows the relative proportions of the main Fe species (structural Fe(III), goethite, haematite and paramagnetic Fe(II)) as a function of the distance from the Fe/clay interface. Thus, at the direct contact point, the average oxidation state of Fe increases because of the accumulation of Fe(III) oxides (mainly goethite), but in parallel paramagnetic Fe(II) (i.e., sorbed Fe(II), ferrous hydroxide or structural Fe(II) from the clay fraction) has also formed.…”

Mössbauer spectrometry insights into the redox reactivity of Fe-bearing phases in the environment

“…The other experimental difficulty may come from the sensitivity of the sample to be studied to different parameters (humidity, oxygen, atmospheric pressure, temperature). It is then necessary to prepare and seal the Mössbauer sample in a glove box (even for field studies where drilling must be performed under N 2 ), transport it under liquid N 2 or in hermetically sealed containers (e.g., dry shipper) before transferring it to the Mössbauer spectrometer: different tricks are necessary to ensure a thin, homogeneous, and stable layer by mixing the sample with a chemically neutral powder, in a resin, but the sample can no longer be the object of further measurements [24][25][26][27][28][29]. But the stability of the sample can be checked by comparing the hyperfine structures observed after aging of the sample.…”

“…Fe II reacts with the clay by a complex interaction process, which may affect the swelling and sealing capacity of the clay barrier [135]. Large-scale in situ tests in underground research laboratories, such as the ABM experiment at the Äspö Hard Rock laboratory in Sweden [29] or the FEBEX experiment at the Grimsel Test site in Switzerland [28] have yielded valuable information regarding the corrosion and Fe-clay interaction process. The bentonite used in both cases has some Goethite/Hematite, roughly 10% of initial Fe, with the remaining 90% initial Fe being structural Fe II isolated into illite mesocrystals.…”