| The recent emphasis in the United States on developing abundant domestic sources of energy, together with an increasing awareness of the environmental hazards of fossil fuels, has led to a fresh look at the challenges of nuclear energy within the science and engineering community. One of these challenges is controlling the precipitation of porous oxide deposits onto the nuclear fuel rod cladding from the primary coolant during operation of pressurized light-water reactors (PWRs). These deposits, called CRUD (an acronym for Chalk River Unidentified Deposits), are a major concern to reactor operation because they reduce fuel lifetime and efficiency by reducing heat transfer to the coolant, promote corrosion, and depress neutron flux. This dissertation provides fundamental insights into the process by which CRUD is formed in PWRs by providing a framework linking the results of first-principles calculations to experimental data. The technique developed to facilitate the investigation is referred to as Density Functional Theory (DFT) referenced semi-empirical thermodynamics; It links 0K first-principles calculations with high temperature thermodynamics by redefining the reference chemical potentials of the constituent elements. The technique permits aqueous chemistry to be incorporated into thermodynamic calculations and allows for the prediction of temperature and pressure dependent free energies of materials that are experimentally inaccessible or have not yet been measured. The ability to extend accurate first-principles calculations to high temperatures and aqueous environments allows the stability of crystal surfaces, calculated with DFT techniques, to be predicted at conditions representative of an operating PWR. Accurate values of surface energies are used in fulfilling the principal goal of this dissertation, which is to investigate the aqueous thermodynamics of formation of nickel oxide (NiO) and nickel ferrite (NiFe 2O4) crystallites as representative CRUD components. Specifically, this dissertation investigates the thermodynamics of the homogeneous nucleation of crystallites and proposes a mechanism for their agglomeration. This work also contributes to a more complete understanding of CRUD by generating improved thermodynamic data to enable modeling of the incorporation of boron into CRUD through the formation of Bonaccordite (Ni2FeBO5) which has formed in PWRs operating at high-power conditions. |
