Browsing by Author "Kuo, EY"

Now showing 1 - 7 of 7
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    Density and structural effects in the radiation tolerance of TiO2 polymorphs
    (IOP Publishing, 2013-08-05) Qin, MJ; Kuo, EY; Whittle, KR; Middleburgh, SC; Robinson, M; Marks, NA; Lumpkin, GR
    The radiation response of TiO2 has been studied using molecular dynamics. The simulations are motivated by experimental observations that the three low-pressure polymorphs, rutile, brookite and anatase, exhibit vastly different tolerances to amorphization under ion-beam irradiation. To understand the role of structure we perform large numbers of simulations using the small thermal spike method. We quantify to high statistical accuracy the number of defects created as a function of temperature and structure type, and reproduce all the main trends observed experimentally. To evaluate a hypothesis that volumetric strain relative to the amorphous phase is an important driving force for defect recovery, we perform spike simulations in which the crystalline density is varied over a wide range. Remarkably, the large differences between the polymorphs disappear once the density difference is taken into account. This finding demonstrates that density is an important factor which controls radiation tolerance in TiO2. © 2013, IOP Publishing Ltd.
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    Molecular dynamics simulations of thermal conductivity of UO2, PuCrO3 and PuAlO3
    (Australian Institute of Physics, 2014-02-04) Qin, MJ; Kuo, EY; Robinson, M; Marks, NA; Lumpkin, GR; Middleburgh, SC
    The thermal conductivities of the PuCrO3 and PuAlO3 precipitates in UO2 fuel have been calculated using non-equilibrium molecular dynamics simulations. The PuCrO3 phase showed a markedly lower thermal conductivity than UO2, which will impact the microstructure, fission product distribution and gas release properties of UO2-based fuels. The PuAlO3, in both its orthorhombic and rhombohedral structures, showed greater thermal conductivity in comparison to PuCrO3, lower than UO2 at low temperatures but higher at elevated temperatures. Additions of Al with Cr to doped fuels is therefore likely to have a beneficial impact on the thermal conductivity of the fuel as opposed to solely doping with Cr.
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    Technetium and ruthenium incorporation into rutile TiO2
    (Elsevier, 2013-10-01) Kuo, EY; Qin, MJ; Thorogood, GJ; Whittle, KR; Lumpkin, GR; Middleburgh, SC
    The incorporation of TcO2 into the rutile phase of TiO2 has been theoretically examined to understand the potential of the system for use as a wasteform. The effects of defect clustering and the transmutation of Tc into Ru were considered. Results suggest that as a single defective species, Tc has a temperature-dependent, moderate solubility into rutile TiO2, which increases when clustering is considered. The {2TcTi} cluster that preferentially forms was found to have a Tc–Tc bond length similar to the cation distance in TcO2. The transmutation of Tc to Ru has two effects: first, the preferred binary cluster morphology changes from that of first nearest neighbour for {2TcTi} to that of second nearest neighbour for {2RuTi}. Second, the defect solubility is lower for single RuTi defects and Ru-containing defect clusters. This will likely result in the formation of a secondary phase as transmutation proceeds, if Tc is added to rutile TiO2 to its solution limit. The varying solubility of RuO2 in TiO2 with temperature was compared with previously published experimental data with encouraging results. The use of two different DFT codes (one plane-wave and one local-orbital) was used to find the correct magnetic ordering in the defective lattices, which increased the confidence in the methods and therefore the results. © 2013, Elsevier B.V.
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    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.
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    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.
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    Tilting and distortion in rutile-related mixed metal ternary uranium oxides: a structural, spectroscopic, and theoretical investigation
    (American Chemical Society, 2021-01-29) Murphy, GL; Zhang, Z; Tesch, R; Kowalski, PM; Avdeev, M; Kuo, EY; Gregg, DJ; Kegler, P; Alekseev, EV; Kennedy, BJ
    A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO4 oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of β-CdUO4, MnUO4, CoUO4, and MgUO4 are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of β-CdUO4 is best described by space group Cmmm whereas MnUO4, CoUO4, and MgUO4 are described by the lower symmetry Ibmm space group and are isostructural with the previously reported β-NiUO4 [Murphy et al. Inorg. Chem.2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A2+ and U6+ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO6 polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO4 oxides results in reduction of the tilt angle. This is demonstrated for MnUO4 where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO4 is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO4 family of oxides. © 2021 American Chemical Society
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    Transmutation of ABO4 compounds incorporating technetium-99 and caesium-137
    (IOP Publishing, 2017-01-13) Kuo, EY; Qin, MJ; Thorogood, GJ; Huai, P; Ren, CL; Lumpkin, GR; Middleburgh, SC
    The stability of a series of ABO4ABO4 minerals incorporating radioactive 99Tc99Tc during the latter’s β-decay to 99Ru99Ru was investigated theoretically using density functional theory (DFT) computations. The compounds investigated were KTcO4KTcO4, RbTcO4RbTcO4 and CsTcO4CsTcO4. The stability of the latter, CsTcO4CsTcO4, during transmutation, when the caesium consists of the radioactive isotope 137Cs137Cs, was also investigated. For each of the compounds, two similar possible crystal structure types—scheelite and pseudoscheelite—were considered. As the 99Tc99Tc decays, or the 137Cs137Cs decays to 137Ba137Ba, reaction enthalpies were calculated for possible decompositions or precipitations of the transmuting compounds. All the possible decompositions or precipitations investigated had positive reaction enthalpies, suggesting that the transmuting compounds are all chemically stable. Volume and lattice parameter changes, however, suggest that KTcO4KTcO4 would also be structurally stable during transmutation to KRuO4KRuO4, but that CsTcO4CsTcO4 would not be structurally stable during its transmutation to BaRuO4BaRuO4. © 2017 IOP Publishing Ltd.