  We used a thermodynamics-informed first principles scheme, to calculate the
temperature-dependent surface energies of nickel oxide (NiO) and nickel ferrite
(NiFe2O4), two compounds that are known to deposit on the fuel rods in nuclear
pressurized water reactors (PWR). These calculations predict a negative surface
free energy for nickel ferrite when formed from ions in solution under PWR
conditions of temperature, pressure and species concentration. Under these
conditions the thermodynamics of bulk nickel ferrite yields a positive change
in free energy for formation of the solid from dissolved ions. These results
and their analysis in terms of classical nucleation theory have suggested a new
fundamental relation - the reflected nucleation curve - that reveals a
previously unrecognized aspect of the theory of phase stability. These
calculations also suggest the presence of stable octahedral nickel ferrite
clusters in PWR coolant that would not be observed in the coolant outside of
service conditions, and that are not removed by the combination of filtration
and capture of solvated ions currently used to purify the PWR coolant.
