High temperature speciation of the nickel/iron system for use with the MULTEQ model

High temperature solubility predictions for the nickel-iron system to calculate the precipitation of hematite, magnetite, nickel ferrites, nickel metal and nickel oxide are critical to the understanding of PWR system corrosion, corrosion product transport, and radiation field behavior. This complex system is difficult to model because of the low solubilities and possible non-stoichiometric nickel ferrite precipitates. This work reviewed the available data in the literature and derived a model for iron and nickel hydrolysis products, nickel and nickel oxide precipitates, iron oxide precipitates, and nickel ferrite precipitates. This model forms the basis of the latest revision to the EPRI MULTEQ Database, used in programs such as the EPRI MULTEQ High Temperature Chemistry Calculator. The results show an updated magnetite/nickel-ferrite/nickel system, the features of which include: The iron hydrolysis/magnetite/hematite system was re-derived from the best available data; Fe(OH)₃^- and Ni(OH)₃^- do not appear as significant species in high temperature conditions. Nickel ferrite is predicted to be the more stable product at PWR operating conditions, with iron concentrations determined by the solubility of this precipitate. Since corrosion of PWR surfaces leads to the release of excess nickel, nickel concentrations are determined by Ni/NiO solubility. (author)