Phase identifications in crud from commercial boiling water reactors at the Idaho National Laboratory by transmission electron microscopy

This paper reports results of research in which transmission electron microscopy was used to characterize phases in four samples of "crud" (activated corrosion products) from three commercially operating boiling water reactors. Samples A (from Reactor A) and B (from Reactor B) were collected by scraping crud deposits formed on the outsides of fuel pins in a hot cell. Both pins had thick deposits of tenacious crud. Samples C and D (both from Reactor C) were collected by sucking pool water through filters after brushing or scraping fuel pins in the pool. Sample A contained nanocrystalline areas and single euhedral crystals of franklinite (ZnFe₂O₄ also known as zinc ferrite), single crystals of hematite (α-Fe₂O ₃), a nanocrystalline iron-oxide mixture that also probably includes goethite (α-FeOOH) and magnetite (Fe₃O₄) or maghemite (γ-Fe₂O₃) but does not include significant quantities of akaganéite (β-FeOOH)or lepidocrocite (γ-FeOOH), crystalline silica (probably quartz, α-SiO2), and an unidentified high-Ba, high-S phase. Sample B contained euhedral crystals of franklinite, a single crystal of willemite (Zn₂SiO₄, several areas of nanocrystalline akaganéite (β-FeOOH), amorphous silica, and an unidentified Fe-Cr phase with significant concentrations of Ni and Si. Samples A and B contained halite (NaCl), and Sample A contained small quantities of sylvite (KCl). Sample C contained hematite, probably magnetite, at least one kind of clay, and a high-Pb particle. Several particles from Sample C have thick, high-Zr central parts surrounded by thin areas consisting of Fe oxides (primarily hematite); these particles may represent fragments of the cladding and immediately adjacent crud. Phases identified in Sample D include corundum (α-Al₂O₃), hematite, an unidentified aluminosilicate that is probably a clay, and a high-Zr phase that was not analyzed in detail but may be part of the cladding. Because significant concentrations of chloride would not be tolerated in reactor cooling systems, the halite and sylvite in Samples A and B are assumed to represent contaminants. Despite its nominal composition, akaganéite is a chloride-bearing phase, and also indicates contamination of Sample B. Corundum and clays are also likely contaminants. The presence of contaminants such as these must be considered in interpreting bulk chemical analyses. Data from transmission electron microscopy provides detailed information about the microstructures, crystal structures, and compositions of a small number of tiny areas. Although this information cannot be obtained using other techniques, it should not be considered statistically representative and must be interpreted in the context of data representing larger volumes of material.