Identification of the Fe3O4-Fe2O3 reaction in liquid lead-bismuth eutectic

Liquid lead–bismuth eutectic (LBE) is a heavy metal alloy with great potential as a coolant and spallation target material for Generation IV nuclear reactors. For LBE technology to be widely accepted as reliable, a comprehensive understanding of LBE coolant chemistry is crucial. Specifically, precise determination of chemical interactions between corrosion products and dissolved oxygen in LBE is essential. For that purpose, the thermal cycling method is used in the present research, in which a well-controlled batch of LBE is repeatedly cooled while a potentiometric oxygen sensor monitors the dissolved oxygen concentration. According to the literature, magnetite (Fe ₃O ₄) is confirmed to be an important oxide to describe oxygen–iron interaction in LBE. However, thermal cycling experiments indicate the presence of another oxide, besides Fe ₃O ₄, participating in LBE bulk chemistry under conditions relevant for LBE-cooled reactors (200 °C–400 °C, 10 ⁻⁸ wt.%–10 ⁻⁵ wt.% dissolved oxygen concentration). By varying iron impurity content, nickel impurity content and cooling rate in the experiments, the oxide has been identified as hematite (Fe ₂O ₃). Thermodynamic equilibrium calculations show that Fe ₂O ₃ formation through the reaction of Fe ₃O ₄ with dissolved oxygen can explain the observed oxygen trends. As Fe ₂O ₃ could form during LBE cooling, an oxygen control strategy should take into account additional oxygen consumption in the reactor cold leg.