Eighteen years of steel–bentonite interaction in the FEBEX in situ test at the Grimsel Test Site in Switzerland

Abstract

Corrosion of steel canisters containing buried high-level radioactive waste is a relevant issue for the long-term integrity of repositories. The purpose of the present study was to evaluate this issue by examining two differently corroded blocks originating from a full-scale in situ test of the FEBEX bentonite site in Switzerland. The FEBEX experiment was designed initially as a feasibility test of an engineered clay barrier system and was recently dismantled after 18 years of activity. Samples were studied by Fspatially resolved`and Fbulk`experimental methods, including Scanning Electron Microscopy, Elemental Energy Dispersive Spectroscopy (SEM-EDX), μ-Raman spectroscopy, X-ray Fluorescence (XRF), X-ray Diffraction (XRD), and 57 Fe Mössbauer spectrometry, with a focus on Fe-bearing phases. In one of the blocks, corrosion of the steel liner led to diffusion of Fe into the bentonite, resulting in the formation of large (width > 140 mm) red, orange, and blue colored halos. Goethite was identified as the main corrosion product in the red and orange zones while no excess Fe 2+ (compared to the unaffected bentonite) was observed there. Excess Fe 2+ was found to have diffused further into the clay (in the blue zones) but its speciation could not be unambiguously clarified. The results indicate the occurrence of newly formed octahedral Fe 2+ either as Fe 2+ sorbed on the clay or as structural Fe 2+ inside the clay (following electron transfer from sorbed Fe 2+). No other indications of clay transformation or newly formed clay phases were found. The overall pattern indicates that diffusion of Fe was initiated when oxidizing conditions were still prevailing inside the bentonite block, resulting in the accumulation of Fe 3+ close to the interface (up to three times the original Fe content), and continued when reducing conditions were reached, allowing deeper diffusion of Fe 2+ into the clay (inducing an increase of 10-12% of the Fe content).

Reinforced concrete structures: A review of corrosion mechanisms and advances in electrical methods for corrosion monitoring

“…We must note that even if a thick corrosion layer develops at the vicinity of the rebar, a "virtual" diffusion of Fe 2+ can still occur across this layer through electron transfer in the Fe(III) layer, i.e. sorption of one Fe 2+ on one side and release of another Fe 2+ on the other side, as proposed for Fe diffusion at the steel-bentonite interface [235].…”

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

“…Given the parameters of analysis (resolution, dwell time), the acquisition time per block was ~12 h. Output data from the operating software (Smartsem ® by ZEISS for the SEM part and Genesis ® by AMETEK for the EDX) were collected and treated with a MATLAB in-house algorithm in order to establish chemical profiles, large scale elemental mappings, and backscatter images. The main data obtained from the SEM-EDX survey are presented as "Al-normalized" chemical profiles of the major elements, representing the atomic ratio of a given element over Al as a function of the distance to the interface [15]. The so-called "Al-normalized" values were computed directly from the quantification results [31,32].…”

“…XRF analyses of major elements were performed on a PW 2400 Philips spectrometer and corrected with the internal Philips software ×40 on the basis of a set of international rock standards. Loss on ignition (LOI) was determined by mass The main data obtained from the SEM-EDX survey are presented as "Al-normalized" chemical profiles of the major elements, representing the atomic ratio of a given element over Al as a function of the distance to the interface [15]. The so-called "Al-normalized" values were computed directly from the quantification results [31,32].…”

“…Less work has focussed on parameters that are more representative of repository conditions, such as low liquid/solid and low Fe/bentonite ratios, variable redox conditions, higher temperatures (up to 140 • C), realistic dimensions and longer timescales. Examples of such studies are the FEBEX mock-up test [11], the FEBEX in situ test at the Grimsel Test Site [12][13][14][15] or the ABM in situ tests at the Äspö Hard Rock Laboratory [16][17][18][19][20][21][22][23]. In general, similar patterns of neo-formed iron minerals at the Fe/bentonite interface have been observed, notably magnetite, ferric (oxyhydr)oxides (e.g., hematite, goethite, lepidocrocite) and siderite.…”

Swelling characteristics of bentonite after long-term dissolution in alkaline solution

“…The safe disposal of high-level radioactive waste (HLRW) presents a significant challenge for many countries. Deep geological disposal, in many approaches, includes HLRW in sealed canisters, buffer/backfill materials and host rocks, and this is internationally recognized as an effective method for the disposal of HLRW (Komine, 2008; Chen et al , 2011; Hadi et al , 2019). Bentonite is generally recognized as an ideal buffer/backfill material between the canister containing HLRW and the host rock in deep geological repositories due to its low hydraulic conductivity, high swelling capacity and good self-sealing capacity (Dixon & Gray, 1985; Komine & Ogata, 2004; Ye et al , 2010; Xiang et al , 2019a).…”