Publications
390 results found
Bell BDC, Murphy ST, Grimes RW, et al., 2018, The effect of Sn–VO defect clustering on Zr alloy corrosion, Corrosion Science, Vol: 141, Pages: 14-17, ISSN: 0010-938X
Density functional theory simulations were used to study Sn defect clusters in the oxide layer of Zr-alloys. Clustering was shown to play a key role in the accommodation of Sn in ZrO2, with the {SnZr:VO}×bound defect cluster dominant at all oxygen partial pressures below 10−20atm, above which SnZr×is preferred. {SnZr:VO}×is predicted to increase the tetragonal phase fraction in the oxide layer, due to the elevated oxygen vacancy concentration. As corrosion progresses, the transition to SnZr×, and resultant destabilisation of the tetragonal phase, is proposed as a possible explanation for the early first transition observed in Sn-containing Zr–Nb alloys.
Ghosh PS, Kuganathan N, Arya A, et al., 2018, Phase stability, electronic structures and elastic properties of (U,Np)O-2 and (Th,Np)O-2 mixed oxides, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 20, Pages: 18707-18717, ISSN: 1463-9076
Pavlov TR, Wangle T, Wenman MR, et al., 2018, High temperature measurements and condensed matter analysis of the thermo-physical properties of ThO2, SCIENTIFIC REPORTS, Vol: 8, ISSN: 2045-2322
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.
Kuganathan N, Arya A, Rushton M, et al., 2018, Trapping of volatile fission products by C60, Carbon, Vol: 132, Pages: 477-485, ISSN: 0008-6223
Carbon based filters provide important safety barriers that remove volatile fission products from gas streams. The capacity and efficiency of a filter to trap fission products depends upon the strength of the interaction between the fission products and the filter material. In this study, we apply density functional theory together with a dispersion correction (DFT + D) to predict structures and energies of volatile fission product atoms and molecules trapped by buckminsterfullerene (C60). Endohedral encapsulation energies and exohedral association energies show that Rb and Cs are strongly trapped as ions, each transferring approximately one electron to C60. Kr and Xe are weakly trapped atoms with Xe showing a preference for exohedral association and Kr for endohedral encapsulation. Br, I and Te, while strongly trapped from atoms (and assuming charge from C60) are thermodynamically more stable as neutral covalently bonded Br2, I2 and Te2 molecules weakly trapped through van der Waals forces, exohedrally. Heteronuclear CsBr and CsI were also considered. Both molecules were non-bonded to C60 with similar association energies to those exhibited by Br2, I2 and Te2.
Middleburgh SC, Claisse A, Andersson DA, et al., 2018, Solution of hydrogen in accident tolerant fuel candidate material: U₃Si₂, Journal of Nuclear Materials, Vol: 501, Pages: 234-237, ISSN: 0022-3115
Hydrogen uptake and accommodation into U 3 Si 2 , a candidate accident-tolerant fuel system, has been modelled on the atomic scale using the density functional theory. The solution energy of multiple H atoms is computed, reaching a stoichiometry of U 3 Si 2 H 2 which has been experimentally observed in previous work (reported as U 3 Si 2 H 1.8 ). The absorption of hydrogen is found to be favourable up to U 3 Si 2 H 2 and the associated volume change is computed, closely matching experimental data. Entropic effects are considered to assess the dissociation temperature of H 2 , estimated to be at ∼800 K – again in good agreement with the experimentally observed transition temperature.
Galvin C, Cooper MWD, Rushton MJD, et al., 2017, Oxygen Diffusion in Gd-doped Mixed Oxides, Journal of Nuclear Materials, Vol: 498, Pages: 300-306, ISSN: 0022-3115
Molecular dynamics simulations have been performed to investigate oxygen transport in (UxPux−1)0.95Gd0.05O1.975, (UxThx−1)0.95Gd0.05O1.975 and (PuxThx−1)0.95Gd0.05O1.975 between 1000 and 3200 K. Oxygen diffusivity and corresponding activation energies are examined and compared to values for the undoped (UxPux−1)O2, (UxThx−1)O2 and (PuxThx−1)O2 systems where compositions between end members display enhanced diffusivity. Below the superionic transition oxygen diffusivity for the Gd doped systems is orders of magnitude greater compared to their undoped counterparts. However, enhanced diffusivity for doped mixed actinide cation compositions is not observed compared to doped end members. Changes in activation energy suggest changes in diffusion regime, which correspond to the creation of thermally activated oxygen defects.
Pavlov TR, Wenman MR, Vlahovic L, et al., 2017, Measurement and interpretation of the thermo-physical properties of UO2 at high temperatures: the viral effect of oxygen defects, Acta Materialia, Vol: 139, Pages: 138-154, ISSN: 1359-6454
Values are reported of specific heat, thermal conductivity and thermal diffusivity of UO2 from 1500 K to 2900 K based on laser flash measurements. Experiment is complemented by the development of solid state physics models that aid in the interpretation of the results. Specific heat is shown to exhibit a smooth maximum at 2715 K ± 100 K, consistent with a competition between two processes - oxygen defect interactions (net attraction) and saturation of oxygen interstitial sites. The specific heat model and measurements show, for the first time that a gradual pre-melting transition is consistent with high temperature literature values – enthalpy increment measurements and independently measured high temperature oxygen defect concentrations. Thermal conductivity exhibits a minimum consistent with: 1) an increase in electronic thermal conductivity via polaron production and mobilization and 2) degradation in lattice thermal conductivity due to phonon - phonon scattering and phonon - defect scattering. It is predicted that the high concentration of oxygen defects should contribute significantly to electrical conductivity and thermal expansion at high temperatures.
Kuganathan N, Ghosh PS, Arya AK, et al., 2017, Energetics of halogen impurities in thorium dioxide, Journal of Nuclear Materials, Vol: 495, Pages: 192-201, ISSN: 0022-3115
Defect energies for halogen impurity atoms (Cl, Br and I) in thoria are calculated using the generalized gradient approximation and projector augmented plane wave potentials under the framework of density functional theory. The energy to place a halogen atom at a pre-existing lattice site is the incorporation energy. Seven sites are considered: octahedral interstitial, O vacancy, Th vacancy, Th-O di-vacancy cluster (DV) and the three O-Th-O tri-vacancy cluster (NTV) configurations. For point defects and vacancy clusters, neutral and all possible defect charge states up to full formal charge are considered. The most favourable incorporation site for Cl is the singly charged positive oxygen vacancy while for Br and I it is the NTV1 cluster. By considering the energy to form the defect sites, solution energies are generated. These show that in both ThO 2-x and ThO 2 the most favourable solution equilibrium site for halides is the single positively charged oxygen vacancy (although in ThO 2 , I demonstrates the same solubility in the NTV1 and DV clusters). Solution energies are much lower in ThO 2-x than in ThO 2 indicating that stoichiometry is a significant factor in determining solubility. In ThO 2 , all three halogens are highly insoluble and in ThO 2-x Br and I remain insoluble. Although ½Cl 2 is soluble in ThO 2-x alternative phases such as ZrCl 4 exist which are of lower energy.
Jackson M, Burr PA, Grimes RW, 2017, Defect processes in Be12X (X=Ti, Mo, V, W), Nuclear Fusion, Vol: 57, Pages: 1-10, ISSN: 0029-5515
The stability of intrinsic point defects in Be12X intermetallics (where X = Ti, V, Mo or W) are predicted using density functional theory simulations and discussed with respect to fusion energy applications. Schottky disorder is found to be the lowest energy complete disorder process, closely matched by Be Frenkel disorder in the cases of Be12V and Be12Ti. Antitisite and X Frenkel disorder are of significantly higher energy. Small clusters of point defects including Be divacancies, Be di-interstitials and accommodation of the X species on two Be sites were considered. Some di-interstitial, divacancy and X2Be combinations exhibit negative binding enthalpy (i.e. clustering is favourable), although this is orientationally dependent. None of the Be12X intermetallics are predicted to exhibit significant non-stoichiometry, ruling out non-stoichiometry as a mechanism for accommodating Be depletion due to neutron transmutation.
Bell BDC, Murphy ST, Grimes RW, et al., 2017, The effect of Nb on the corrosion and hydrogen pick-up of Zr alloys, Acta Materialia, Vol: 132, Pages: 425-431, ISSN: 1359-6454
Zr-Nb alloys are known to perform better in corrosion and hydrogen pick-up than other Zr alloys but the mechanism by which this happens is not well understood. Atomistic simulations using density functional theory of both tetragonal and monoclinic ZrO2 were performed, with intrinsic defects and Nb dopants. The overall defect populations with respect to oxygen partial pressure were calculated and presented in the form of Brouwer diagrams. Nb is found to favour 5 + in monoclinic ZrO2 at all partial pressures, but can exist in oxidation states ranging from 5 + to 3 + in the tetragonal phase. Nb5+ is charge balanced by Zr vacancies in both phases, suggesting that contrary to previous assumptions, Nb does not act as an n-type dopant in the oxide layer. Clusters containing oxygen vacancies were considered, Nb2+ was shown to exist in the tetragonal phase with a binding energy of 2.4 eV. This supports the proposed mechanism whereby low oxidation state Nb ions (2 + or 3+) charge balance the build-up of positive space-charge in the oxide layer, increasing oxygen vacancy and electron mobility, leading to near-parabolic corrosion kinetics and a reduced hydrogen pick-up. Previous experimental work has shown that tetragonal ZrO2 transforms to the monoclinic phase during transition, and that during transition a sharp drop in the instantaneous hydrogen pick-up fraction occurs. The oxidation of lower charge state Nb defects to Nb5+ during this phase change, and the consequent temporary n-doping of the oxide layer, is proposed as an explanation for the drop in hydrogen pick-up during transition.
Pavlov T, Vlahovic L, Staicu D, et al., 2017, A new numerical method and modified apparatus for the simultaneous evaluation of thermo-physical properties above 1500 K: A case study on isostatically pressed graphite, Thermochimica Acta, Vol: 652, Pages: 39-52, ISSN: 0040-6031
This paper presents a new numerical inverse method coupled with an improved apparatus based on the laser flash (LF) technique for the measurement of thermo-physical properties of materials at high temperatures. Using this thermal conductivity, specific heat capacity, thermal diffusivity and spectral emissivity have been measured at temperatures above 1500 K. The method improves the characterization of input parameters such as laser power profile, which was shown to impact thermal conductivity and specific heat capacity by 15–20%. Convective heat losses are characterized semi-empirically and are not fitted. The apparatus has been enhanced via the adoption of a spectropyrometer for the simultaneous measurement of spectral emissivity within an uncertainty of 5% (equivalent to 0.3–0.7% error in temperature in the range 1500–3000 K)The results obtained on isotropic, isostatically pressed, graphite are in good agreement with literature values (around 1500 K) and extend the available data up to 2800 K. Additionally, a model has been developed based on the theory of Debye and Klemens for predicting the temperature dependence of thermal conductivity, specific heat capacity and thermal diffusivity of isotropic graphite. The model is in good agreement with the new experimental data and previous lower temperature data and therefore provides confidence in the new experimental approach.
Fossati PC, Grimes RW, 2017, Cation ordering and oxygen transport behaviour in Sr1−3x/2LaxTiO3 perovskites, Journal of Materials Chemistry A, Vol: 5, Pages: 5321-5331, ISSN: 2050-7496
Molecular dynamics simulations and genetic algorithms are used to identify the mechanisms by which oxygen is transported through the Sr1−3x/2LaxTiO3 family of perovskites as a function of x. Across this compositional range the relative stability of ordered structures and random arrangements of cations and vacancies on the A sublattice is established. O and Ti Frenkel pair formation is then predicted for the considered compositions. These results show that Ti defects are more favourable for all La-containing compositions, but have a larger defect volume than O defects for low x. Oxygen diffusion at high temperature is determined using Molecular dynamics simulations. Two types of oxygen transport mechanisms are identified, that each contribute to oxygen diffusion in ordered structures. These mechanism involve transient O and Ti defects respectively, and provide means for O ions to swap positions. Depending on cation ordering, they can be correlated in space, which causes qualitatively different diffusion patterns.
Pilania G, Whittle KR, Jiang C, et al., 2017, Using Machine Learning To Identify Factors That Govern Amorphization of Irradiated Pyrochlores, CHEMISTRY OF MATERIALS, Vol: 29, Pages: 2574-2583, ISSN: 0897-4756
Structure–property relationships are a key materials science concept that enables the design of new materials. In the case of materials for application in radiation environments, correlating radiation tolerance with fundamental structural features of a material enables materials discovery. Here, we use a machine learning model to examine the factors that govern amorphization resistance in the complex oxide pyrochlore (A2B2O7) in a regime in which amorphization occurs as a consequence of defect accumulation. We examine the fidelity of predictions based on cation radii and electronegativities, the oxygen positional parameter, and the energetics of disordering and amorphizing the material. No one factor alone adequately predicts amorphization resistance. We find that when multiple families of pyrochlores (with different B cations) are considered, radii and electronegativities provide the best prediction, but when the machine learning model is restricted to only the B = Ti pyrochlores, the energetics of disordering and amorphization are critical factors. We discuss how these static quantities provide insight into an inherently kinetic property such as amorphization resistance at finite temperature. This work provides new insight into the factors that govern the amorphization susceptibility and highlights the ability of machine learning approaches to generate that insight.
Hodgson APJ, Jarvis KE, Grimes RW, et al., 2017, Advances in the development of a dissolution method for the attribution of iridium source materials, Journal of Radioanalytical and Nuclear Chemistry, Vol: 311, Pages: 1193-1199, ISSN: 0236-5731
To assist in nuclear forensic investigations, new techniques are required to evaluate radioactive materials that may be discovered outside of regulatory control. Using a recently developed pressure digestion method for iridium powder, assessments have been made of this techniques suitability for undertaking iridium target material evaluations. In addition to determining the reaction conditions necessary for total dissolution, these investigations have provided an insight into the elemental impurities that are present within unirradiated iridium targets that are used in QSA Global radiography sources, and established the speciation of the iridium solutions that are formed during this process.
Kuganathan N, Ghosh PS, Galvin COT, et al., 2016, Fission gas in thoria, Journal of Nuclear Materials, Vol: 485, Pages: 47-55, ISSN: 0022-3115
The fission gases Xe and Kr, formed during normal reactor operation, are known to degrade fuel performance, particularly at high burn-up. Using first-principles density functional theory together with a dispersion correction (DFT + D), in ThO₂ we calculate the energetics of neutral and charged point defects, the di-vacancy (DV), different neutral tri-vacancies (NTV), the charged tetravacancy (CTV) defect cluster geometries and their interaction with Xe and Kr. The most favourable incorporation point defect site for Xe or Kr in defective ThO₂ is the fully charged thorium vacancy. The lowest energy NTV in larger supercells of ThO₂ is NTV3, however, a single Xe atom is most stable when accommodated within a NTV1. The di-vacancy (DV) is a significantly less favoured incorporation site than the NTV1 but the CTV offers about the same incorporation energy. Incorporation of a second gas atom in a NTV is a high energy process and more unfavourable than accommodation within an existing Th vacancy. The bi-NTV (BNTV) cluster geometry studied will accommodate one or two gas atoms with low incorporation energies but the addition of a third gas atom incurs a high energy penalty. The tri-NTV cluster (TNTV) forms a larger space which accommodates three gas atoms but again there is a penalty to accommodate a fourth gas atom. By considering the energy to form the defect sites, solution energies were generated showing that in ThO₂−x the most favourable solution equilibrium site is the NTV1 while in ThO₂ it is the DV.
Ghosh PS, Arya A, Dey GK, et al., 2016, A computational study on the superionic behaviour of ThO₂, Physical Chemistry Chemical Physics, Vol: 18, Pages: 31494-31504, ISSN: 1463-9076
This study reports the density functional theory (DFT) and classical molecular dynamics (MD) study of the lattice dynamical, mechanical and anionic transport behaviours of ThO2 in the superionic state. DFT calculations of phonon frequencies were performed at different levels of approximation as a function of isotropic dilation (ε) in the lattice parameter. With the expansion of the lattice parameter, there is a softening of B1u and Eu phonon modes at the X symmetry point of the Brillouin zone. As a result of the nonlinear decrease at the X point, the B1u and Eu phonon modes cross each other at ε = 0.03, which is associated with a sharp increase in the narrow peak of the phonon density of states, signifying a higher occupation and hence a higher coupling of these modes at high temperatures. The mode crossing also indicates anionic conductivity in the 〈001〉 direction leading to occupation of interstitial sites. Moreover, MD and nudged elastic band calculated diffusion barriers indicate that 〈001〉 is the easy direction for anion migration in the normal and superionic states. With a further increase in the lattice parameter, the B1u mode continues to soften and becomes imaginary at a strain (ε) of 0.036 corresponding to a temperature of 3430 K. The calculated temperature variation of single crystal elastic constants shows that the fluorite phase of ThO2 remains elastically stable up to the superionic regime, though the B1u phonon mode is imaginary in that state. This leads to anionic disorder at elevated temperatures. Tracking of anion positions in the superionic state as a function of time in MD simulations suggests a hopping model in which the oxygen ions migrate from one tetrahedral site to another via octahedral interstitial sites.
Pavlov T, Vlahovic L, Staicu D, et al., 2016, Experimental evaluation of the high temperature thermo- physical properties of UO2, Top Fuel 2016, Publisher: American Nuclear Society, ISSN: 0003-018X
High temperature properties of UO2 are reported, in particular thermal conductivity, specific heat capacity, thermal diffusivity and melting point. All are measured with a single laser flash apparatus coupled with a numerical inverse method. The thermal conductivity, spectral emissivity, specific heat capacity, thermal diffusivity and melting point are in very good agreement with established literature values indicating the validity of the methodology and its potential for measuring these properties up to melting. The melting point was identified to be 3118 K ± 28 K. The thermal conductivity exhibits a minimum between 1800 K and 2100 K due to the competition between phonon scattering and an increase in the concentration of free charge carriers. The substantial increase in specific heat can be predominantly attributed to the formation of oxygen Frenkel pairs.
Valu SO, Benes O, Manara D, et al., 2016, The high-temperature heat capacity of the (Th,U)O-2 and (U,Pu)O-2 solid solutions, JOURNAL OF NUCLEAR MATERIALS, Vol: 484, Pages: 1-6, ISSN: 0022-3115
Galvin C, Cooper MWD, Rushton MJD, et al., 2016, Thermophysical properties and oxygen transport in (Thx,Pu1-x)O2, Scientific Reports, Vol: 6, ISSN: 2045-2322
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 dependenceof lattice parameter, linear thermal expansion coefficient, enthalpy and specific heat at constant pressure. Oxygen diffusivityand activation enthalpy are also investigated. Below the superionic temperature an increase of oxygen diffusivity for certaincompositions of (Thx,Pu1−x)O2 compared to the pure end members is predicted. Oxygen defect formation enthalpies are alsoexamined, as they underpin the superionic transition temperature and the increase in oxygen diffusivity. The increase in oxygendiffusivity for (Thx,Pu1−x)O2 is explained in terms of lower oxygen defect formation enthalpies for (Thx,Pu1−x)O2 than PuO2and ThO2, while links are drawn between the superionic transition temperature and oxygen Frenkel disorder.
Liu X-Y, Cooper MWD, McClellan KJ, et al., 2016, Molecular dynamics simulation of thermal transport in UO₂ containing uranium, oxygen, and fission-product defects, Physical Review Applied, Vol: 6, ISSN: 2331-7019
Uranium dioxide (UO₂) is the most commonly used fuel in light-water nuclear reactors and thermal conductivity controls the removal of heat produced by fission, thereby governing fuel temperature during normal and accident conditions. The use of fuel performance codes by the industry to predict operational behavior is widespread. A primary source of uncertainty in these codes is thermal conductivity, and optimized fuel utilization may be possible if existing empirical models are replaced with models that incorporate explicit thermal-conductivity-degradation mechanisms during fuel burn up. This approach is able to represent the degradation of thermal conductivity due to each individual defect type, rather than the overall burn-up measure typically used, which is not an accurate representation of the chemical or microstructure state of the fuel that actually governs thermal conductivity and other properties. To generate a mechanistic thermal conductivity model, molecular dynamics (MD) simulations of UO₂ thermal conductivity including representative uranium and oxygen defects and fission products are carried out. These calculations employ a standard Buckingham-type interatomic potential and a potential that combines the many-body embedded-atom-method potential with Morse-Buckingham pair potentials. Potential parameters for UO₂+z and ZrO₂ are developed for the latter potential. Physical insights from the resonant phonon-spin-scattering mechanism due to spins on the magnetic uranium ions are introduced into the treatment of the MD results, with the corresponding relaxation time derived from existing experimental data. High defect scattering is predicted for Xe atoms compared to that of La and Zr ions. Uranium defects reduce the thermal conductivity more than oxygen defects. For each defect and fission product, scattering parameters are derived for application in both a Callaway model and the corresponding high-temperature model typically used in fuel-performance codes. Th
Middleburgh SC, Grimes RW, Lahoda EJ, et al., 2016, Non-stoichiometry in U3Si2, Journal of Nuclear Materials, Vol: 482, Pages: 300-305, ISSN: 0022-3115
Uranium silicides, in particular U3Si2, are being explored as an advanced nuclear fuel with increased accident tolerance as well as competitive economics compared to the baseline UO2 fuel. Here we use density functional theory calculations and thermochemical analysis to assess the stability of U3Si2 with respect to non-stoichiometry reactions in both the hypo- and hyper-stoichiometric regimes. We find that the degree of non-stoichiometry in U3Si2 is much smaller than in UO2 and at most reaches a few percent at high temperature. Non-stoichiometry impacts fuel performance by determining whether the loss of uranium due to fission leads to a non-stoichiometric U3Si2±x phase or precipitation of a second U-Si phase. We also investigate the U5Si4 phase as a candidate for the equilibrium phase diagram.
Cooper MWD, Rushton MJD, Grimes RW, et al., 2016, Development of Xe and Kr empirical potential for CeO2, ThO2, UO2 and PuO2, combining DFT with high temperature MD, Journal of Condensed Matter Physics, Vol: 28, ISSN: 2160-6919
The development of embedded atom method (EAM) many-bodypotentials for actinide oxides and associated mixed oxide (MOX) systems hasmotivated the development of a complementary parameter set for gas-actinide andgas-oxygen interactions. A comprehensive set of density functional theory (DFT)calculations were used to study Xe and Kr incorporation at a number of sites in CeO2,ThO2, UO2 and PuO2. These structures were used to fit a potential, which was used togenerate molecular dynamics (MD) configurations incorporating Xe and Kr at 300 K,1500 K, 3000 K and 5000 K. Subsequent matching to the forces predicted by DFT forthese MD configurations was used to refine the potential set. This fitting approachensured weighted fitting to configurations that are thermodynamically significantover a broad temperature range, while avoiding computationally expensive DFTMDcalculations. The resultant gas potentials were validated against DFT trappingenergies and are suitable for simulating combinations of Xe and Kr in solid solutionsof CeO2, ThO2, UO2 and PuO2, providing a powerful tool for the atomistic simulationof conventional nuclear reactor fuel UO2 as well as advanced MOX fuels.
Galvin COT, Cooper MWD, Fossati PCM, et al., 2016, Pipe and grain boundary diffusion of He in UO₂, Journal of Physics: Condensed Matter, Vol: 28, ISSN: 0953-8984
Molecular dynamics simulations have been conducted to study the effects of dislocations andgrain 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 grainboundaries. He diffusivity as a function of distance from the dislocation core and grain boundarieswas investigated for the temperature range 2300 - 3000 K. An enhancement in diffusivitywas predicted within 20 Å of the dislocations or grain boundaries. Further investigation showedthat He diffusion in the edge dislocations follows anisotropic behaviour along the dislocationcore, suggesting that pipe diffusion occurs. An Arrhenius plot of He diffusivity against the inverseof temperature was also presented and the activation energy calculated for each structure,as a function of distance from the dislocation or grain boundary
Jackson ML, Fossati PCM, Grimes RW, 2016, Simulations of threshold displacement in beryllium, Journal of Applied Physics, Vol: 120, ISSN: 0021-8979
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.
Burr PA, Middleburgh SC, Grimes RW, 2016, Solubility and partitioning of impurities in Be alloys, Journal of Alloys and Compounds, Vol: 688, Pages: 382-385, ISSN: 0925-8388
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.
Ghosh PS, Kuganathan N, Galvin COT, et al., 2016, Melting behavior of (Th,U)O2 and (Th,Pu)O2 mixed oxides, Journal of Nuclear Materials, Vol: 479, Pages: 112-122, ISSN: 1873-4820
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.
Chernatynskiy A, Auguste A, Steele B, et al., 2016, Elastic and thermal properties of hexagonal perovskites, Computational Materials Science, Vol: 122, Pages: 139-145, ISSN: 0927-0256
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.
Jackson ML, Burr PA, Grimes RW, 2016, Resolving the structure of TiBe12, Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials, Vol: 72, Pages: 277-280, ISSN: 2052-5206
There has been considerable controversy regarding the structure of TiBe12, which is variously reported as hexagonal and tetragonal. Lattice dynamics simulations based on density functional theory (DFT) show the tetragonal phase space group I4/mmm to be more stable over all temperatures, while the hexagonal phase exhibits an imaginary phonon mode, which, if followed, would lead to the cell adopting the tetragonal structure. We then report the predicted ground state elastic constants and temperature dependence of the bulk modulus and thermal expansion for the tetragonal phase.
Hodgson APJ, Jarvis KE, Grimes RW, et al., 2016, Development of an iridium dissolution method for the evaluation of potential radiological device materials, JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY, Vol: 307, Pages: 2181-2186, ISSN: 0236-5731
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Bell BDC, Murphy ST, Burr PA, et al., 2016, The influence of alloying elements on the corrosion of Zr alloys, Corrosion Science, Vol: 105, Pages: 36-43, ISSN: 0010-938X
Density functional theory (DFT) and autoclave corrosion tests in 360 °C water were used to investigate the influence of Sb, Sc, Nb and Sn on the corrosion and hydrogen pick-up (HPU) of Zr-alloys. Sc was shown to have a strongly detrimental effect on alloy corrosion resistance. The Nb–Sb–Zr ternary alloy exhibited significantly improved corrosion resistance over Zr–Nb and ZIRLO, and had little measurable HPU after 195 days. The ratio of View the MathML source was shown to transition smoothly with applied space charge, implying Sb can act as a buffer to charge imbalance in the oxide layer.
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