Browsing by Author "Rushton, MJD"

Now showing 1 - 3 of 3
  • No Thumbnail Available
    Item
    From solid solution to cluster formation of Fe and Cr in α-Zr
    (Elsevier B.V., 2015-12-01) Burr, PA; Wenman, MR; Gault, B; Moody, MP; Ivermark, M; Rushton, MJD; Preuss, M; Edwards, L; Grimes, RW
    To understand the mechanisms by which the re-solution of Fe and Cr additions increase the corrosion rate of irradiated Zr alloys, the solubility and clustering of Fe and Cr in model binary Zr alloys was investigated using a combination of experimental and modelling techniques — atom probe tomography (APT), x-ray diffraction (XRD), thermoelectric power (TEP) and density functional theory (DFT). Cr occupies both interstitial and substitutional sites in the α-Zr lattice; Fe favours interstitial sites, and a low-symmetry site that was not previously modelled is found to be the most favourable for Fe. Lattice expansion as a function of Fe and Cr content in the α-Zr matrix deviates from Vegard's law and is strongly anisotropic for Fe additions, expanding the c-axis while contracting the a-axis. Matrix content of solutes cannot be reliably estimated from lattice parameter measurements, instead a combination of TEP and APT was employed. Defect clusters form at higher solution concentrations, which induce a smaller lattice strain compared to the dilute defects. In the presence of a Zr vacancy, all two-atom clusters are more soluble than individual point defects and as many as four Fe or three Cr atoms could be accommodated in a single Zr vacancy. The Zr vacancy is critical for the increased apparent solubility of defect clusters; the implications for irradiation induced microstructure changes in Zr alloys are discussed. © 2015 Elsevier B.V.
  • No Thumbnail Available
    Item
    Thermal conductivity and energetic recoils in UO2 using a many-body potential model
    (IOP Science, 2014-11-14) Qin, MJ; Cooper, MWD; Kuo, EY; Rushton, MJD; Grimes, RW; Lumpkin, GR; Middleburgh, SC
    Classical molecular dynamics simulations have been performed on uranium dioxide (UO2) employing a recently developed many-body potential model. Thermal conductivities are computed for a defect free UO2 lattice and a radiation-damaged, defect containing lattice at 300 K, 1000K and 1500 K. Defects significantly degrade the thermal conductivity of UO2 as does the presence of amorphous UO2, which has a largely temperature independent thermal conductivity of ∼1.4Wm−1 K−1. The model yields a pre-melting superionic transition temperature at 2600 K, very close to the experimental value and the mechanical melting temperature of 3600 K, slightly lower than those generated with other empirical potentials. The average threshold displacement energy was calculated to be 37 eV. Although the spatial extent of a 1 keV U cascade is very similar to those generated with other empirical potentials and the number of Frenkel pairs generated is close to that from the Basak potential, the vacancy and interstitial cluster distribution is different. © 2014, IOP Publishing Ltd.
  • No Thumbnail Available
    Item
    Thermal conductivity variation in uranium dioxide with gadolinia additions
    (Elsevier, 2020-11) Qin, MJ; Middleburgh, SC; Cooper, MWD; Rushton, MJD; Puide, M; Kuo, EY; Grimes, RW; Lumpkin, GR
    By combining experimental observations on Gd doped fuel with a theoretical understanding, the variation in thermal conductivity with Gd concentration and accommodation mechanism has been modelled. Four types of Gd accommodation mechanisms have been studied. In UO2−x, isolated substitutional Gd3+ ions are compensated by oxygen vacancies and {2Gd'u:V"o}x defect clusters. In UO2, isolated substitutional Gd3+ ions are compensated by U5+ ions and {Gd'u:U'u}x defect clusters. The results indicate that defect clusters can be considered as less effective phonon scatterers and therefore result in less thermal conductivity degradation. The thermal conductivity predicted for UO2 with {Gd/u:U'u}x defect clusters is in good agreement with experimental data for UO2 with 5 wt% Gd2O3. This supports the previous theoretical results that Gd is accommodated through defect clusters {Gd'u:U'u}x in UO2 in the presence of excess oxygen. © 2020 Elsevier B.V.