The new insight on iron-based oxide formation in lead-bismuth eutectic

Liquid lead-bismuth eutectic (LBE) is a heavy metal alloy considered as a promising coolant and spallation target material for Generation IV nuclear reactor concepts. For public acceptance of LBE as a reliable technology, better understanding of LBE coolant chemistry is crucial. Chemical interactions between steel corrosion products and dissolved oxygen in LBE should be precisely determined. The research presented here shows thermal cycling experiments in lab-scale stagnant LBE autoclaves in which the LBE is repeatedly cooled down and heated up and the dissolved oxygen concentration is measured by means of a potentiometric oxygen sensor. Magnetite (Fe3O4) is confirmed to be an important oxide to describe oxygen-iron interaction in LBE. The thermal cycling experiments indicated, however, the presence of another oxide, besides Fe3O4, participating in LBE bulk chemistry in conditions relevant for LBE-cooled reactors (200 °C – 400 °C, 1e-8 wt. % – 1e-5 wt. % dissolved oxygen concentration). Theoretically grounded variations of iron impurity content, nickel impurity content and cooling rate in the experiments indicated that the oxide could be hematite (Fe2O3). Thermodynamic equilibrium calculations suggested that Fe2O3 formation through the reaction of Fe3O4 with dissolved oxygen could explain the observed oxygen trends. Because Fe2O3 could form upon LBE cooling, the oxygen control strategy should account for additional oxygen consumption in the reactor cold leg.