Characterization of a redox-modified clay mineral with respect to its suitability as a barrier in radioactive waste confinement

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Abstract

Engineered barriers for high-level nuclear waste (HLW) consist of excavated repositories in sub-surface rock formations where canisters holding the radionuclide are stored. Clay minerals, particularly the swelling 2:1 types, are used as backfill material, both in the canisters and in the bore hole, in order to prevent radionuclide transport to surrounding groundwater. One of the most important risks that can occur is the corrosion of the canister, which could be coupled with reduction of iron (Fe) in the clay structure. Such changes could greatly decrease the long-term stability of the clay and, consequently, of the barriers themselves. In order to test the potential effects of such redox interactions, an Fe-bearing clay mineral from a commercial source located in the Kutch region, India, was selected for study. This particular mineral is one of the candidate clay minerals to serve as such a barrier material, and is the one with the largest structural Fe content. Results from it should, therefore, provide maximum insight into the potential effects of redox interactions between the barrier and its surroundings. The unaltered clay was characterized by X-ray powder diffractometry (XRD), thermal gravimetric analysis (TGA/DTGA), Fourier-transform infrared (FTIR) spectroscopy, and variable-temperature Mössbauer spectroscopy. The chemically reduced and reoxidized forms of the clay were characterized by variable-temperature Mössbauer spectroscopy and chemical analysis. In the unaltered state the clay is comprised of smectite, maghemite, superparamagnetic goethite, and hematite, with a possible trace of kaolinite. In the reduced state the Fe (oxyhydr)oxides were dissolved. Upon reoxidation no six-line pattern was observed, indicating that the Fe remained only in the structure of the silicates. The final structure of the reduced–reoxidized clay contained more defects than the original clay, as revealed by greater quadrupole splitting values for structural Fe(III) in the clay. These findings indicate that upon exposure to natural redox cycles the Kutch clay could undergo permanent changes in its mineralogical composition and clay mineral structure, but further study is required to ascertain the effects that such changes would have on its long-term stability as a barrier material.

Introduction

High-level radioactive nuclear waste (HLW) consists of either spent nuclear fuel or products generated from its reprocessing and requires careful planning for long-term disposal and storage. One strategy being developed is to place the HLW in metal canisters which, in turn, are to be placed within excavated chambers in solid rock repositories well beneath the earth's surface. Typically bentonite (or bentonite–sand) is planned to be used as backfill emplaced within and around the metal canisters containing the HLW in order to serve as an isolating material to prevent migration of the radionuclide and water infiltration. Bentonite is a smectite-rich rock where montmorillonite is the major component. This kind of barrier is preferred for repositories because of its properties of low hydraulic conductivity and high cation exchange and adsorption capacities (Pusch, 1992). In order for bentonite barriers to perform their required functions, however, the smectite component must be stable over prolonged periods of time under potentially variable redox conditions. Alteration of montmorillonite to non-swelling 1:1 or 2:1 phyllosilicates could result in loss of swellability and of sorptive and cation exchange capacities.

Such alterations have an intrinsic relationship with iron (Fe) in the crystal structure of the mineral and the redox conditions surrounding it, both of which play a prominent role in determining clay structure and properties (see for example Stucki, 2006, Stucki et al., 2002, Lee et al., 2006). The swell ability of the clay decreases with increased Fe reduction level (Stucki et al., 1984a, Lear and Stucki, 1989) and superimposed layers tend to collapse one upon the other (Wu et al., 1989, Shen and Stucki, 1994). Cation exchange capacity increases markedly upon reduction (Khaled and Stucki, 1991, Chen et al., 1987, Lear and Stucki, 1989). These studies show that the reduction of Fe(III) to Fe(II) may result in an increase in layer charge and ordering in the hk lattice planes (Stucki and Tessier, 1991). Furthermore, aqueous suspensions of ferruginous smectite have an increased viscosity subsequent to Fe reduction due to increased interparticle attraction. Other associated processes also can occur in the barrier such as Si, Al, and Mg release in high pH due to solution leaking from the canisters. These cations can form new minerals such as zeolites and feldspathoids (Mashal et al., 2005).

In order to evaluate the potential influence of redox reactions in situ with products from canister corrosion, the effects of redox reactions on the candidate barrier clays must be evaluated. The objective of the present study was, therefore, to characterize the effects of redox reactions in a representative Fe-bearing barrier clay material in its oxidized, reduced, and reduced–reoxidized states in order to understand its suitability under the types of conditions expected in nuclear repository barriers.

Section snippets

Materials and methods

Bentonite from the Kutch Region, India, was investigated in its as-received state from the Swedish Nuclear Fuel and Waste Management Company (sample #8939). This clay was selected because it has the largest Fe content among about one dozen candidate clay materials to form HLW barriers, and thus was believed to serve as a representative clay to magnify potential effects of redox alterations. Chemical reduction was performed by the method of Stucki et al. (1984b). About 30 mg of clay was

Unaltered Kutch clay

Wet chemical analysis found the Fe(II) and total Fe contents to be 0.18 mmol g 1 and 1.36 mmol g 1, respectively. The Fe was distributed in a ratio of about 3:1 between the structure of the silicate and other Fe-bearing phase(s), respectively (see Table 4 below).

XRD analysis of the unaltered Kutch clay (Fig. 1) revealed a broad d001 peak at 1.2 nm, characteristic of 2:1 clay minerals, and a small reflection at 0.72 nm, characteristic of interstratification of the smectite. The 2:1 clay mineral

Conclusions

Dioctahedral smectite is the principal silicate component of Kutch clay, with perhaps a trace of kaolinite. The macroscopic magnetic behavior of the sample (attraction by a hand magnet) is due to the presence of maghemite, but hematite and superparamagnetic goethite are also present. The presence of these Fe-bearing minerals accounts for about one-fourth of the total Fe content of the sample, and the smectite Fe content is unusually large (∼ 5.5% of sample mass).

Upon reduction, partial

Acknowledgments

The authors thank the Swedish Nuclear Fuel and Waste Management Company and the International Collaborative Agreement between the University of Illinois (UIUC) and the French National Research Agency (CNRS) for financial support of this study.

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