CRUD (acronym for Chalk River Unidentified Deposit) is predominately a nickel-ferrite (NiFe2O4) deposit that accumulates on hot surfaces of nuclear fuel rods during reactor operation. The presence of CRUD modifies the heat transfer between the fuel rods and coolant and can induce localized corrosion of surface the fuel clad. Besides these unwanted effects, boron, which is a strong neutron absorber, can accumulate within the CRUD, triggering shifts in the core neutron flux and fluctuations in the reactor power level. Therefore it is crucial to understand and predict the mechanisms by which B is trapped into the CRUD. As a first step, the incorporation of B as a point defect into the crystal structure of NiFe2O4 has been investigated using the Density Functional Theory (DFT) framework. To obtain the formation energies of various interstitial and substitutional B-defects, theoretical results have been combined with experimental thermo-chemical data. Assuming solid-solid equilibrium conditions, the main factors that limit the incorporation of B are:
The relatively narrow stability domain in which the host NiFe2O4 is stable
The formation of ternary Fe-B-O and Ni-B-O compounds.
The present study also investigates the incorporation of B assuming solid-liquid equilibrium between NiFe2O4 and the surrounding aqueous solution under conditions of pressure, temperature and pH characteristic to PWRs.