Elemental mapping and iron oxidation state measurement of synthetic low-activity waste feeds

During the conversion of nuclear waste feed into glass, iron bearing precursors are incorporated into the melt and affect various properties of the melt such as viscosity and density. Laboratory-scale measurements to determine iron oxidation state could be used in cases where samples are subjected to ex-situ heat treatment and kinetic relationships developed. Typically, measurements of iron oxidation state have been limited to synchrotron X-ray absorption techniques, Mossbauer spectroscopy, or destructive wet chemical techniques. In this work, we present our method development which employs electron probe microanalysis (EPMA) to measure the oxidation state of iron. This method could also serve broader applications, not only in nuclear waste vitrification but also in commercial glass making, geology, and archaeology. Using EPMA, we measured the Fe L-edge X-ray emission spectra of nuclear waste feed samples prepared with a reduced iron precursor and heat treated to various temperatures in air. EPMA based measurements provide a cost-effective and rapid alternative to measure iron oxidation state. Furthermore, the method described in this paper allows for spatially resolved measurements with a minimum step size of 100 µm. We demonstrate that iron oxidation state can be calculated using EPMA, and we compare the calculated iron oxidation state to that measured by synchrotron Fe K-edge X-ray absorption measurements. This work demonstrates that, for nuclear waste feeds prepared with reduced iron, the oxidation state of the iron is influenced by the thermal history. Fast-drying the feed slurry produced more reduced iron in feed than previously observed in feeds prepared by slowly drying and crushing into a powder.