Alex Scrimshire, is a postdoctoral research associate at Sheffield Hallam University under the supervision of Prof Paul Bingham. The principle expertise of the presenting author is in Mössbauer Spectroscopy, as employed in the present work. The PhD topic of the presenting author pertains to Mössbauer spectroscopic studies of several catalytic and energy storage materials and connecting the findings of different characterisation techniques to the catalytic or electrochemical performance of these materials.
Structural role of iron in nepheline-based aluminosilicates for nuclear waste applications
Mostafa Ahmadzadeh1, Alex Scrimshire*2, Paul A. Bingham2, Ashutosh Goel3 and John McCloy1
1Materials Science & Engineering Program, Washington State University, Pullman, WA, USA
2Materials & Engineering Research Institute, Sheffield Hallam University, Sheffield, UK
3Department of Materials Science & Engineering, Rutgers, The State University of New York, Jersey,
Piscataway, NJ, USA
Vitrified high level nuclear wastes (HLW) are susceptible to nepheline (nominally NaAlSiO4) crystallization, which reduces the chemical durability of the final waste form and causes volume expansion which leads to cracking and hence increased surface area for leaching. Crystallization of such glasses is a function of the composition of starting glass-forming melt. Iron plays an important role in nuclear waste glasses, since it is present in a variety of concentrations which can variously affect processing as well as long-term corrosion behavior of such glasses. However, only a few studies have been conducted on HLW glasses with higher Fe concentrations. Moreover, the structural role of Fe in glasses is still not well understood because Fe can occur as both Fe2+ and Fe3+; can act as network modifier and/or network former; and can cluster when present at elevated concentrations. Therefore, a systematic study on the role of Fe in nuclear waste glasses for nepheline crystallization is necessary.
We have studied simplified HLW glass compositions along the NaAlSiO4–NaFeSiO4 join to assess the structural behavior of iron in nepheline-based aluminosilicates as a function of Fe-Al substitution, as it is known that Fe substitutes for Al sites in the nepheline structure. Glasses were prepared by a standard melt-quench method and then isothermally heat-treated to promote crystallization. Crystallization and thermal behavior of the prepared samples were investigated using X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. Back-scattered electron (BSE) images were obtained by scanning electron microscopy (SEM) to analyze the microstructure. Furthermore, coordination and redox state of iron for the glasses and crystals were determined using both Mössbauer spectroscopy and wet chemistry techniques.
The results show that Fe promotes the crystallization of nepheline over its high temperature polymorph (carnegieite) when substituted for Al in low additions. The glass transition temperature, Tg, and crystallization temperature, Tc, decrease with substituting Fe for Al, while staying within the range of 500°C<Tg<800°C and 700°C<Tc<850°C, respectively. BSE micrographs demonstrate that Fe oxides (i.e. hematite and magnetite) are present in cases even when their fractions are below the XRD detection limit. Fe redox determination measurements reveal that Fe2+/ΣFe is higher for the as-quenched glasses with lower Fe content at the expense of Al (Fe2+/ΣFe ~ 0.23±0.02 for Na(Al0.9Fe0.1)SiO4 glass). On the other hand, for high-Fe glasses, where there is not adequate Al in the structure to take the glass-forming role, iron tends to oxidize to Fe3+ and behave as a glass former, leading to lower Fe2+/ΣFe. Mössbauer spectra also indicate the presence of Fe magnetic oxides (i.e. magnetite, hematite, and maghemite) in heat-treated crystalline samples, which is consistent with the microstructural images.
In addition to the isothermal heat-treatment (IHT), a canister centerline cooling (CCC) treatment was applied to some of the compositions along the join. Obtaining the XRD results and Rietveld analyses of the CCC samples and comparing them to the IHT samples, we observe a difference in the fraction and type of the phases within the same compositions. It is found that, in lower-Fe compositions, a higher fraction of the phases (e.g., nepheline and carnegieite) crystallize during CCC compared to the equivalent IHT samples.
At the Hanford site, iron concentration varies from ~5-6 wt% Fe2O3 in high-aluminum high-sodium waste, to as high as >30 wt% Fe2O3 in high-Fe2O3 waste which forms ~12% of the total waste mass. Spinel formation, Nepheline formation, and viscosity have been shown to be the main constraints on waste loading for high-Fe2O3 wastes. The results of the current study, such as the dependency of crystallization temperature and redox state on the Fe content, provide fundamental insight into predicting the behavior of HLW glasses with various amount of Fe in a simplified composition, with an emphasis on nepheline crystallization.