  A serious concern in the safety and economy of a pressurized water nuclear
reactor is related to the accumulation of boron inside the metal oxide (mostly
NiFe${}_{2}$O${}_{4}$ spinel) deposits on the upper regions of the fuel rods.
Boron, being a potent neutron absorber, can alter the neutron flux causing
anomalous shifts and fluctuations in the power output of the reactor core. This
phenomenon reduces the operational flexibility of the plant and may force the
down-rating of the reactor. In this work an innovative approach is used to
combine first-principles calculations with thermodynamic data to evaluate the
possibility of B incorporation into the crystal structure of
NiFe${}_{2}$O${}_{4}$, under conditions typical to operating nuclear
pressurized water nuclear reactors. Analyses of temperature and pH dependence
of the defect formation energies indicate that B can accumulate in
NiFe${}_{2}$O${}_{4}$ as an interstitial impurity and may therefore be a major
contributor to the anomalous axial power shift observed in nuclear reactors.
This computational approach is quite general and applicable to a large variety
of solids in equilibrium with aqueous solutions.
