A COMPARISON OF DEFECT ENERGIES IN MGO USING MOTT-LITTLETON AND QUANTUM-MECHANICAL PROCEDURES

The authors compare the predictions of Mott-Littleton calculations, based on empirical interatomic potentials, with predictions based on self-consistent solutions of the Schrodinger equation for embedded clusters. Simple vacancy and substitutional defects in MgO are modelled using both the classical Mott-Littleton and quantum mechanical methods. Particular attention is paid to the size of the quantum mechanical cluster, the different ways that polarisation is taken into account and the choice of basis set. Results are presented for closed-shell systems only, namely V"Mg and Vo vacancies and for Li'Mg, Na'Mg, AlMg, Fo and Clo substitutional impurities. They find a respectable level of agreement between the quite distinct approaches. This both validates the classical calculations and indicates useful generalisations combining the two approache

Thermophysical properties and oxygen transport in (Thx,Pu1-x)O2

Using Molecular Dynamics, this paper investigates the thermophysical properties and oxygen transport of (Thx,Pu1−x)O2 (0 ≤ x ≤ 1) between 300-3500 K. In particular, the superionic transition is investigated and viewed via the thermal dependence of lattice parameter, linear thermal expansion coefficient, enthalpy and specific heat at constant pressure. Oxygen diffusivity and activation enthalpy are also investigated. Below the superionic temperature an increase of oxygen diffusivity for certain compositions of (Thx,Pu1−x)O2 compared to the pure end members is predicted. Oxygen defect formation enthalpies are also examined, as they underpin the superionic transition temperature and the increase in oxygen diffusivity. The increase in oxygen diffusivity for (Thx,Pu1−x)O2 is explained in terms of lower oxygen defect formation enthalpies for (Thx,Pu1−x)O2 than PuO2 and ThO2, while links are drawn between the superionic transition temperature and oxygen Frenkel disorder

Pipe and grain boundary diffusion of He in UO₂

Molecular dynamics simulations have been conducted to study the effects of dislocations and grain boundaries on He diffusion in UO2. Calculations were carried out for the {100}, {110} and {111} h110i edge dislocations, the screw h110i dislocation and Σ5, Σ13, Σ19 and Σ25 tilt grain boundaries. He diffusivity as a function of distance from the dislocation core and grain boundaries was investigated for the temperature range 2300 - 3000 K. An enhancement in diffusivity was predicted within 20 Å of the dislocations or grain boundaries. Further investigation showed that He diffusion in the edge dislocations follows anisotropic behaviour along the dislocation core, suggesting that pipe diffusion occurs. An Arrhenius plot of He diffusivity against the inverse of temperature was also presented and the activation energy calculated for each structure, as a function of distance from the dislocation or grain boundar

Xe diffusion and bubble nucleation around edge dislocations in UO2

AbstractRecently it has been suggested that dislocations, generated by radiation damage, may increase the rate of fission gas diffusion from the fuel grains, an affect which is at present not incorporated into fuel performance codes. Therefore, we perform molecular dynamics simulations employing empirical potentials to investigate the diffusion of Xe atoms around edge dislocations in UO2 to establish the importance of this pathway for fission gas release. The results suggest that for isolated atoms near the dislocation the activation energy for Xe diffusion is dramatically reduced relative to the bulk. However, Xe atoms diffusing along the dislocation cluster together to form small bubbles, these bubbles incorporate all of the isolated mobile Xe atoms thereby inhibiting fast diffusion of Xe along the dislocation core

Simulations of threshold displacement in beryllium

Atomic scale molecular dynamics simulations of radiation damage have been performed on beryllium. Direct threshold displacement simulations along a geodesic projection of directions were used to investigate the directional dependence with a high spatial resolution. It was found that the directionally averaged probability of displacement increases from 0 at 35 eV, with the energy at which there is a 50% chance of a displacement occurring is 70 eV and asymptotically approaching 1 for higher energies. This is, however, strongly directionally dependent with a 50% probability of displacement varying from 35 to 120 eV, with low energy directions corresponding to the nearest neighbour directions. A new kinetic energy dependent expression for the average maximum displacement of an atom as a function of energy is derived which closely matches the simulated data

Solubility and partitioning of impurities in Be alloys

The most energetically favourable accommodation processes for common impurities and alloying elements in Be metal and Be-Fe-Al intermetallics were investigated using atomic scale simulations. Fe additions, combined with suitable heat treatments, may scavange Al and Si through their incorporation into the FeBe₅ intermetallic. In the absence of Fe, Al and Si will not be associated with Be metal. Li and Mg are also not soluble, but may react with other impurities if present (such as Al or H). Mg may also form the MgBe₁₃ intermetallic phase under certain conditions. He and H exhibit negligible solubility in all phases investigated and whilst He will tend to form bubbles, H can precipitate as BeH₂. Similarly, C additions will form the stable compound Be₂C. Finally, oxygen exhibits a strong affinity to Be, exhibiting both some degree of solubility in all phases considered here (though especially metallic Be) and a highly favourable energy of formation for BeO

Effects of Gallium Doping in Garnet-Type Li7La3Zr2O12 Solid Electrolytes

Garnet-type Li7La3Zr2O12 (LLZrO) is a candidate solid electrolyte material that is now being intensively optimized for application in commercially competitive solid state Li+ ion batteries. In this study we investigate, by force-field-based simulations, the effects of Ga3+ doping in LLZrO. We confirm the stabilizing effect of Ga3+ on the cubic phase. We also determine that Ga3+ addition does not lead to any appreciable structural distortion. Li site connectivity is not significantly deteriorated by the Ga3+ addition (>90% connectivity retained up to x = 0.30 in Li7–3xGaxLa3Zr2O12). Interestingly, two compositional regions are predicted for bulk Li+ ion conductivity in the cubic phase: (i) a decreasing trend for 0 ≤ x ≤ 0.10 and (ii) a relatively flat trend for 0.10 < x ≤ 0.30. This conductivity behavior is explained by combining analyses using percolation theory, van Hove space time correlation, the radial distribution function, and trajectory density

Elastic and thermal properties of hexagonal perovskites

We systematically investigate the mechanical and thermal properties of the P6₃cm hexagonal perovskites with composition A³+B³+O₃ for potential use in thermal barrier coatings. In spite of the structural anisotropy, the elastic constants are essentially isotropic. The thermal expansion is, however, strongly anisotropic, while the thermal conductivity is relatively isotropic. The thermal conductivities of the hexagonal perovskites are much larger than those of the orthorhombic perovskites

Melting behavior of (Th,U)O2 and (Th,Pu)O2 mixed oxides

© 2016 Elsevier B.V.The melting behaviors of pure ThO2, UO2 and PuO2 as well as (Th,U)O2 and (Th,Pu)O2 mixed oxides (MOX) have been studied using molecular dynamics (MD) simulations. The MD calculated melting temperatures (MT) of ThO2, UO2 and PuO2 using two-phase simulations, lie between 3650-3675 K, 3050–3075 K and 2800–2825 K, respectively, which match well with experiments. Variation of enthalpy increments and density with temperature, for solid and liquid phases of ThO2, PuO2 as well as the ThO2 rich part of (Th,U)O2 and (Th,Pu)O2 MOX are also reported. The MD calculated MT of (Th,U)O2 and (Th,Pu)O2 MOX show good agreement with the ideal solidus line in the high thoria section of the phase diagram, and evidence for a minima is identified around 5 atom% of ThO2 in the phase diagram of (Th,Pu)O2 MOX

High temperature measurements and condensed matter analysis of the thermo-physical properties of ThO2

Values are presented for thermal conductivity, specific heat, spectral and total hemispherical emissivity of ThO2 (a potential nuclear fuel material) in a temperature range representative of a nuclear accident - 2000 K to 3050 K. For the first time direct measurements of thermal conductivity have been carried out on ThO2 at such high temperatures, clearly showing the property does not decrease above 2000 K. This could be understood in terms of an electronic contribution (arising from defect induced donor/acceptor states) compensating the degradation of lattice thermal conductivity. The increase in total hemispherical emissivity and visible/near-infrared spectral emissivity is consistent with the formation of donor/acceptor states in the band gap of ThO2. The electronic population of these defect states increases with temperature and hence more incoming photons (in the visible and near-infrared wavelength range) can be absorbed. A solid state physics model is used to interpret the experimental results. Specific heat and thermal expansion coefficient increase at high temperatures due to the formation of defects, in particular oxygen Frenkel pairs. Prior to melting a gradual increase to a maximum value is predicted in both properties. These maxima mark the onset of saturation of oxygen interstitial sites