First-principles investigation of boron incorporation into CRUD under Pressurized Water Reactor conditions
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First-principles investigation of boron incorporation into CRUD under Pressurized Water Reactor conditions Zs. Rák, C. J. O’Brien, and D. W. Brenner Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA ABSTRACT The accumulation of boron within the porous nickel ferrite (NiFe2O4, NFO) deposits on nuclear fuel rods is a major technological problem with important safety and economical implications. In this work, first-principles results are combined with experimental thermochemical data to analyze the energetics of vacancy formation in NFO and the possibility of B incorporation into the structure of NFO. Under solid-solid equilibrium conditions, the calculations suggest that vacancy formation and B incorporation into the NFO structure is energetically unfavorable, the main limiting factors being the narrow stability domain of NFO and the precipitation of B2O3, Fe3BO5, and Ni3B2O6 as secondary phases. Assuming solid-liquid equilibrium between NFO and the surrounding aqueous solution saturated with respect to NFO, the calculations predict that in operating PWR environment, Ni vacancies are likely to form. Under these conditions the possibility of B incorporation at the Ni vacancy sites cannot be excluded. INTRODUCTION CRUD (Chalk River Unidentified Deposit) is predominately a nickel-ferrite spinel (NiFe2O4) corrosion deposit that accumulates on hot surfaces of nuclear fuel rods during reactor operation. The presence of CRUD decreases the heat transfer between the fuel rods and coolant and induces local corrosion on the fuel clad surface. Besides these unwanted effects boron, a strong neutron absorber, can accumulate within the CRUD, triggering shifts in the neutron flux and fluctuations in the reactor power level. This can lead to down-rating of the power plant with significant economic losses. Therefore, it is crucial to understand and predict the mechanisms by which B is trapped into the CRUD. As a first step, the vacancy formation in NFO and B incorporation as a point defect into the crystal structure of NFO are investigated within the framework of the Density Functional Theory (DFT). To obtain the defect formation energies, theoretical results are combined with experimental thermochemical data. The atomic/ionic chemical potentials needed for the defect calculations are obtained assuming two types of chemical equilibrium conditions: (i) solid-solid (or solid-gas) equilibrium between NFO and atomic reservoirs of elemental solids (or gas in the case of oxygen) and (ii) solid-liquid equilibrium between NFO and the surrounding aqueous solution that contain solvated ions of Ni2+ and Fe2+/3+. The later approach allows the extrapolation of the results to conditions of temperature and pressure characteristic to operating PWRs. DEFECT FORMATION UNDER SOLID-SOLID EQUILIBRIUM Under solid-solid equilibrium conditions the energy required to create a defect D in charge state q in a crystalline solid can be written as [1, 2]:
H f Dq E Dq E0 ni i Ei q E
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