Solubility of nickel ferrite in high-temperature pure or oxygenated water

In the primary circuit of a light water reactor, corrosion products of component materials are activated by neutron irradiation in the reactor core and adhere to the out-core circuit, which causes the increase in radiation dose rates of plants. Elucidation of the transport behavior of the corrosion products in the primary circuit is one of the main targets of water chemistry and has been actively investigated worldwide. Here, the solubility of nickel ferrite is measured at 423, 473, and 523 K in a pure or oxygenated water system, which is similar to boiling water reactor conditions, using a specially designed batch autoclave system. Thermodynamic analysis is performed by a procedure minimizing Gibbs free energy of the system at the final state. On the basis of both the analysis and the experimental results, it is shown that the dissolution mechanism of NiFe2O4 under the condition where no redox reaction takes place consists of both NiFe2O4 dissolution and Fe2O3 precipitation equilibria. The calculated value of the solubility at 423 K using literature values of the thermodynamic data agree with the experimental value, but at 473 and 523 K they deviate somewhat from the experimental ones. By fitting to the experimental results at these temperatures, the thermodynamic data of NiFe2O4 for 473 and 523 K are reanalyzed, and new values are proposed