Imperial College London

ProfessorRobinGrimes

Faculty of EngineeringDepartment of Materials

BCH Steele Chair in Energy Materials
 
 
 
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Contact

 

+44 (0)20 7594 6730r.grimes

 
 
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Location

 

B303cBessemer BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

381 results found

Kuganathan N, Grimes R, Rushton M, Kilner J, Gkanas Eet al., 2021, Self-diffusion in garnet-type Li7La3Zr2O12 solid electrolytes, Scientific Reports, Vol: 11, Pages: 1-10, ISSN: 2045-2322

Tetragonal garnet-type Li7La3Zr2O12 is an important candidate solid electrolyte for all-solid-state lithium ion batteries because of its high ionic conductivity and large electrochemical potential window. Here we employ atomistic simulation methods to show that the most favourable disorder process in Li7La3Zr2O12 involves loss of Li2O resulting in lithium and oxygen vacancies, which promote vacancy mediated self-diffusion. The activation energy for lithium migration (0.45 eV) is much lower than that for oxygen (1.65 eV). Furthermore, the oxygen migration activation energy reveals that the oxygen diffusion in this material can be facilitated at higher temperatures once oxygen vacancies form.

Journal article

Galvin COT, Rushton MJD, Cooper MWD, Andersson DA, Burr PA, Grimes RWet al., 2021, The predicted shapes of voids and Xe bubbles in UO2, Journal of Nuclear Materials, Vol: 543, Pages: 1-11, ISSN: 0022-3115

Morphology is a fundamental attribute when investigating voids and bubbles in UO 2 . This study uses molecular dynamics and Monte Carlo simulations to predict the lowest energy shapes for voids and bub- bles in UO 2 . The energies of the { 100 } , { 110 } and { 111 } surfaces have been calculated and used to predict the equilibrium void shape from Wulff construction. This equilibrium shape is compared to low energy faceted voids exhibiting different relative proportions of each family of terminating surfaces. It is found that the equilibrium Wulff shape does not represent the lowest energy morphology for nm void sizes at temperatures between 30 0 K and 120 0 K. Furthermore, the lowest energy faceted voids are slightly more energetically favourable than spherical voids, and as Xe is added, and bubble pressure increases, the faceted morphology becomes even more favourable than the spherical shape.

Journal article

Galvin COT, Grimes RW, Burr PA, 2021, A molecular dynamics method to identify the liquidus and solidus in a binary phase diagram, Computational Materials Science, Vol: 186, Pages: 1-4, ISSN: 0927-0256

A method is presented adopting the phase coexistence technique within molecular dynamics simulations to identify the liquidus and solidus of binary systems. The Compositional Moving Interface method is applied to the case study of the Cu–Ni system and compared against a thermodynamic end-point model where the input parameters are determined using the same MD potential. This is a simple and powerful method to predict the solidus and liquidus boundary of a binary phase diagram for mixed systems calculated from the dynamics of a simulation.

Journal article

Jackson M, Burr P, Grimes R, 2021, Intrinsic defect migration in Be12Ti, Intermetallics, Vol: 128, Pages: 1-10, ISSN: 0966-9795

Be12Ti is a leading candidate neutron-multiplier material for fusion breeder blankets; yet the evolution of the crystal defects under irradiation is poorly understood. Here, the migration of intrinsic defects in tetragonal Be12Ti was predicted using atomic scale computer simulation. Transport of titanium and beryllium through the interstitial, interstitialcy and vacancy-mediated models was considered, along with the migration of divacancy clusters, previously identified as important to the defect chemistry of Be12Ti. It was found that titanium defects exhibit much higher migration energies than beryllium for most migration pathways, leading to a dramatic difference in migration isotropy. Both beryllium vacancy and interstitial diffusion is close to isotropic with vacancy transport exhibiting the highest self-diffusion coefficient. Migration of beryllium di-vacancies is also isotropic with activation energy equal to that of isolated vacancies. The titanium interstitial exhibits significantly lower migration energy than its vacancy (1.00 eV and 6.75 eV respectively), with both mechanisms strongly anisotropic: the activation energy for [001] migration is at least 5 eV lower than other directions. Even the more exotic mixed titanium beryllium vacancy migration exhibits a much higher migration energy than [001] titanium interstitial transport. The framework used for predicting defect transport kinetics, including vacancy-mediated, interstitial and interstitialcy mechanisms, can be applied to any complex-structured intermetallic compound.

Journal article

Than YR, Grimes RW, 2020, Predicting radiation damage in beryllium, PHILOSOPHICAL MAGAZINE, Vol: 101, Pages: 306-325, ISSN: 1478-6435

Journal article

Qin MJ, Middleburgh SC, Cooper MWD, Rushton MJD, Puide M, Kuo EY, Grimes RW, Lumpkin GRet al., 2020, Thermal conductivity variation in uranium dioxide with gadolinia additions, Journal of Nuclear Materials, Vol: 540, ISSN: 0022-3115

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 [equation] defect clusters. In UO2, isolated substitutional Gd3+ ions are compensated by U5+ ions and [equation] 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 [equation] 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 [equation] defect clusters in UO2 in the presence of excess oxygen

Journal article

Than YR, Wenman MR, Grimes RW, 2020, Cu and Sb in tetragonal ZrO2 on fuel cladding, Journal of Applied Physics, Vol: 128, Pages: 135101-135101, ISSN: 0021-8979

Atomic scale simulations were used to predict defect formation in tetragonal ZrO2 doped with Cu and Sb. Both dopants form strong associations with oxygen vacancies impeding oxygen progression through the oxide. Sb suppresses the free oxygen vacancy population though Cu increases the concentration. Thus, while the addition of Sb is predicted to be beneficial against corrosion, Cu will show a more complex behavior. Previous simulations showed that Ni0 promotes molecular hydrogen dissociation. Neither Cu nor Sb exhibit this behavior despite Cu+ having the same electronic configuration as Ni0. Both Cu and Sb show a favorable response to applied local space charges.

Journal article

Pavlov T, lestak, Wenman M, Vlahovic, Robba, Cambriani, Staicu, Dahms, Ernstberger, Brown, Bradford M, Konings R, Grimes Ret al., 2020, Examining the thermal properties of unirradiated nuclear grade graphite between 750 and 2500 K, Journal of Nuclear Materials, Vol: 538, Pages: 1-11, ISSN: 0022-3115

This study presents the first high temperature measurements (between 750 K and 2500 K) of thermal conductivity, thermal diffusivity, specific heat and spectral emissivity of virgin graphite samples (type IM1-24) from advanced gas-cooled reactor (AGR) fuel assembly bricks. Scanning electron microscope (SEM) and X-ray computed tomography (XRT) techniques were used to verify the presence of Gilsocarbon filler particles (a characteristic microstructural feature of IM1-24 graphite). All thermal properties were investigated in two orthogonal directions, which showed the effective macroscopic thermal conductivity to be the same (to within experimental error). This can be linked to the morphology of the filler particles that consist of concentrically aligned graphitic platelets. The resulting spherical symmetry allows for heat to flow in the same manner in both macroscopic directions. The current thermal conductivity results were compared to other isotropic grade graphite materials. The significant discrepancies between the thermal conductivities of the individual grades are likely the result of different manufacturing processes yielding variations in the microstructure of the final product. Differences were identified in the filler particle size and structure, and possibly the degree of graphitization compared to other reported nuclear graphites.

Journal article

Grimes R, Than YR, 2020, Predicting Radiation Damage in Beryllium, Philosophical Magazine, ISSN: 1478-6435

Displacement damage in beryllium was predicted as a function of temperature and energyusing molecular dynamics simulations. An key aim of this study was to determine if averageresults from large displacement cascades correspond to values predicted by the Kinchin-Pease(K-P) model. The number of residual defects remaining after 1 ps increased linearly withprimary knock-on atom (PKA) energy from 0.5 keV to 2.5 keV, while the extent of residualdamage was largely temperature independent from 300 K to 1100 K. The same simulationmodel was used to predict the directionally averaged probability of displacement as a functionof displacement energy, P(EPKA), and thereby the threshold displacement energy at whichthe probability for displacement is 100%, E1.0d = 105 eV. There is an excellent correspondencebetween the K-P prediction using Ed = E1.0dand the number of residual defects remainingafter the initial recovery phase. Also, by utilising P(EPKA), a modification to the K-P modelis proposed that gives rise to an average model prediction when EPKA < 2E1.0d.

Journal article

Kuganathan N, Chroneos A, Grimes RW, 2020, Encapsulation of volatile fission products in a two-dimensional dicalcium nitride electride, Journal of Applied Physics, Vol: 128, Pages: 045112-045112, ISSN: 0021-8979

The efficient capture of volatile fission products released during spent fuel reprocessing is a crucial concern for the nuclear community. Here, we apply the density functional theory to examine the efficacy of a two-dimensional dicalcium nitride electride (Ca2N:ē) to encapsulate volatile fission products. Encapsulation is endoergic for Kr, Xe, Rb, and Cs meaning that they are not encapsulated. Conversely, strong encapsulation is exhibited for Br, I, and Te with respect to their atoms and dimers as reference states. The preference for Br, I, and Te encapsulation is a consequence of charge transfer from Ca2N:ē to form encapsulated anions. This makes the electride a promising material for the selective trapping of volatile Br, I, and Te.

Journal article

Galvin COT, Burr PA, Cooper MWD, Fossati PCM, Grimes RWet al., 2020, Using molecular dynamics to predict the solidus and liquidus of mixed oxides (Th,U)O2, (Th,Pu)O2 and (Pu,U)O2, Journal of Nuclear Materials, Vol: 534, ISSN: 0022-3115

Molecular dynamics (MD) was used to establish a mechanistic basis for the experimentally observed reduction in liquidus and solidus temperatures below the melting point of the end-members for the mixed oxides (Th, U)O2, (Th, Pu)O2 and (Pu, U)O2. This dip is found at additions of the oxide with higher melting point to the oxide with the lower melting point. There are many causes suggested for the dip; here the distribution of the cation Frenkel energy for the mixed oxides caused by the local environment is proposed as a contributor. Furthermore, a variant of the moving interface method which yields information on the position of the solidus and liquidus boundaries, is used to predict the phase diagrams of these systems.

Journal article

Grimes R, 2020, Interests and changing needs from arms control, disarmament, and non-proliferation for international security, International Cooperation for Enhancing Nuclear Safety, Security, Safeguards and Non-proliferation, Publisher: Springer Verlag, ISSN: 0930-8989

Conference paper

Than YR, Grimes RW, Bell BDC, Wenman MRet al., 2020, Understanding the role of Fe, Cr and Ni in Zircaloy-2 with special focus on the role of Ni on hydrogen pickup, Journal of Nuclear Materials, Vol: 530, Pages: 1-7, ISSN: 0022-3115

Ni as an alloying addition in Zircaloy leads to an increase in hydrogen pick-up fraction. Atomic scale simulations of tetragonal ZrO2, based on density functional theory, are used to identify a possible mechanism for this observation. First, defect formation energies associated with Ni but also Fe and Cr are used to predict relative defect cluster and defect charge concentrations using Brouwer diagrams. At low oxygen partial pressures (), expected in the vicinity of the oxide metal interface, a cluster consisting of an oxygen vacancy adjacent to a charge neutral Ni0 atom is identified as the most populous cluster. Further simulations show that a hydrogen molecule will dissociate in the vicinity of this cluster. No other cluster is both sufficiently populous and acts in this way. This differentiates Ni from the other alloying elements.

Journal article

Middleburgh SC, Ipatova I, Evitts LJ, Rushton MJD, Assinder B, Grimes RW, Lee WEet al., 2020, Evidence of excess oxygen accommodation in yttria partially-stabilized zirconia, Scripta Materialia, Vol: 175, Pages: 7-10, ISSN: 1359-6462

Yttria partially stabilized zirconia (ZrO2)x(Y2O3)½-x has been investigated to understand accommodation of excess oxygen into its structure. ZrO2 powder with 8 wt% Y2O3 additions was treated in 30 vol% H2O2 solution to promote oxidation of the material. A new Raman peak was observed after treatment at 840 cm−1, consistent with previous reports of solid state peroxide ions (O22−). This was corroborated using atomic scale simulation based on density functional theory; these also highlighted the near-zero solution enthalpy for excess oxygen in the monoclinic structure via the formation of a peroxide ion defect.

Journal article

Kuganathan N, Chroneos A, Grimes R, 2019, The encapsulation selectivity for anionic fission products imparted by an electride, Scientific Reports, Vol: 9, ISSN: 2045-2322

The nanoporous oxide 12CaO•7Al2O3 (C12A7) can capture large concentrations of extra-framework species inside its nanopores, while maintaining its thermodynamical stability. Here we use atomistic simulation to predict the efficacy of C12A7 to encapsulate volatile fission products, in its stoichiometric and much more effective electride forms. In the stoichiometric form, while Xe, Kr and Cs are not captured, Br, I and Te exhibit strong encapsulation energies while Rb is only weakly encapsulated from atoms. The high electronegativities of Br, I and Te stabilize their encapsulation as anions. The electride form of C12A7 shows a significant enhancement in the encapsulation of Br, I and Te with all three stable as anions from their atom and dimer reference states. Successive encapsulation of multiple Br, I and Te as single anions in adjacent cages is also energetically favourable. Conversely, Xe, Kr, Rb and Cs are unbound. Encapsulation of homonuclear dimers (Br2, I2 and Te2) and heteronuclear dimers (CsBr and CsI) in a single cage is also unfavourable. Thus, C12A7 offers the desirable prospect of species selectivity.

Journal article

Ghosh PS, Arya A, Kuganathan N, Grimes RWet al., 2019, Thermal and diffusional properties of (Th,Np)O2 and (U,Np)O2 mixed oxides, Journal of Nuclear Materials, Vol: 521, Pages: 89-98, ISSN: 0022-3115

Molecular dynamics (MD) simulations were performed to determine thermal expansion, thermal conductivity and diffusional properties of Th1-xNpxO2 and U1-xNpxO2 mixed oxides (MOX). The linear thermal expansion coefficient (LTEC) of Th1-xNpxO2 MOX increases with NpO2 concentration, while that of U1-xNpxO2 MOX decreases. The degradation of thermal conductivity in U1-xNpxO2 is predicted to be far less significant compared to Th1-xNpxO2 because defect-phonon scattering is less pronounced in U1-xNpxO2. Addition of 6.25 atom% NpO2 in ThO2 degrades the thermal-conductivity of ThO2 by 24.0–12.5% in the 750–1000 K temperature range whereas up to 50 atom% NpO2 doping in UO2 degrades the thermal-conductivity only by 13–2.3%. Analytical expressions have been derived that describe the predicted lattice parameters and thermal conductivities over the full temperature and compositional ranges. Oxygen diffusivity is higher in UO2 and NpO2 compared to ThO2. With the addition of Th4+ or U4+ to NpO2, the diffusivity decreases due to the increase in the migration barriers caused by the larger ionic radius of Th4+ or U4+. The addition of Np4+ to ThO2 or UO2 decreases oxygen diffusion due to the preference for the oxygen vacancy to be adjacent to Np4+, even though the migration barriers decrease due to the smaller size of Np4+. Our MD calculated binding energies of the oxygen vacancy can be correlated with the isolated oxygen Frenkel pair defect energies (O-FPisolated) of individual actinide oxides calculated using same interatomic potential set. Moreover, MD calculated oxygen vacancy binding energy is consistent with that calculated using density functional theory.

Journal article

Kuganathan N, Grimes RW, Chroneos A, 2019, Encapsulation of heavy metals by a nanoporous complex oxide 12CaO center dot 7Al(2)O(3), Journal of Applied Physics, Vol: 125, ISSN: 0021-8979

The nanoporous oxide 12CaO ⋅ 7Al2O3 (C12A7) offers the possibility of capturing large concentrations of environmentally damaging extra-framework species in its nanopores. Using density functional theory with a dispersion correction, we predict the structures and energetics of some heavy metals (Cr, Ni, Cu, Zn, Cd, Hg, and Pb) trapped by the stoichiometric and electride form of C12A7. In the stoichiometric form, while Zn, Cd, Hg, and Pb are encapsulated weakly, Cr, Ni, and Cu exhibit strong encapsulation energies. The electride form of C12A7 shows a significant enhancement in the encapsulation of Cr, Ni, Cu, and Pb. Successive encapsulation of multiple Cr, Ni, Cu, and Pb as single species in adjacent cages of C12A7 is also energetically favorable.

Journal article

Kenich A, Wenman MR, Grimes RW, 2018, Iodine defect energies and equilibria in ZrO2, JOURNAL OF NUCLEAR MATERIALS, Vol: 511, Pages: 390-395, ISSN: 0022-3115

Journal article

Grimes RW, 2018, Strengthening the UK-Argentinian science relationship, Global Policy, Vol: 9, Pages: 42-44, ISSN: 1758-5880

In 2016 it was recognised that cooperation in science would be an important element of the developing relationship between Argentina and the UK. This was possible because of strong historic UK-Argentina science ties. Also, both countries recognised it was crucial to strive for excellence in science and that international collaboration led to better science outcomes, especially for their respective economies and development. In November 2016 the two science Ministers signed a ‘Statement of Intent’, which identified science focus collaboration areas. These have progressed with the support of industry and Academies, national laboratories and Universities. An important factor in the sustainability of science links is to offer sufficient opportunities for the exchange of people.

Journal article

Than YR, Wenman MR, Bell BDC, Ortner SR, Swan H, Grimes RWet al., 2018, Modelling and experimental analysis of the effect of solute iron in thermally grown Zircaloy-4 oxides, Journal of Nuclear Materials, Vol: 509, Pages: 114-123, ISSN: 0022-3115

Simulations based on density functional theory (DFT) were used to investigate the behaviour of substitutional iron in both tetragonal and monoclinic ZrO2. Brouwer diagrams of predicted defect concentrations, as a function of oxygen partial pressure, suggest that iron behaves as a p-type dopant in monoclinic ZrO2 while it binds strongly to oxygen vacancies in tetragonal ZrO2. Analysis of defect relaxation volumes suggest that these results should hold true in thermally grown oxides on zirconium, which is under compressive stresses. X-ray absorption near edge structure (XANES) measurements, performed to determine the oxidation state of iron in Zircaloy-4 oxide samples, revealed that 3 + is the favourable oxidation state but with between a third and half of the iron, still in the metallic Fe0 state. The DFT calculations on bulk zirconia agree with the preferred oxidation state of iron if it is a substitutional species but do not predict the presence of metallic iron in the oxide. The implications of these results with respect to the corrosion and hydrogen pick-up of zirconium cladding are discussed.

Journal article

Kuganathan N, Ghosh P, Arya A, Grimes Ret al., 2018, Helium trapping and clustering in ThO2, Journal of Nuclear Materials, Vol: 507, Pages: 288-296, ISSN: 0022-3115

Helium, generated in nuclear fuel, accomodates into bubbles and degrades mechanical stability. Atomic scale simulations were used to study the interaction of He atoms with point defects and defect clusters. The incorporation of a single He atom was unfavourable at an octahedral interstitial site by 0.92 eV compared to the gas phase reference state, by 0.68 eV at an oxygen vacancy and by 0.32 eV at a Th vacancy. The decreasing values reflect the greater space available for the inert He atom. Defect clusters consisting of multiple oxygen and Th vacancies provide more space. Thus, incorporation at a di-vacancy required 0.31 eV, at a neutral tri-vacancy (NTV) 0.25 eV and at a tetra-vacancy 0.01 eV. Clusters formed of two and three NTVs exhibited no energy penalty for the incorporation of multiple He atoms. Relative to incorporation at an interstitial site, clusters offer space for multiple He and may therefore be effective traps to form proto-bubbles. A relationship was generated that describes the incorporation energy of the xth He atom, Ex (n, m), into a cluster consisting of n thorium vacancies and m oxygen vacancies. Solution energies for He, where equilibrium with the solution site is taken into account, were also determined.

Journal article

Bell BDC, Murphy ST, Grimes RW, Wenman MRet 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.

Journal article

Ghosh PS, Kuganathan N, Arya A, Grimes RWet 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

Journal article

Pavlov TR, Wangle T, Wenman MR, Tyrpekl V, Vlahovic L, Robba D, Van Uffelen P, Konings RJM, Grimes RWet 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.

Journal article

Kuganathan N, Arya A, Rushton M, Grimes Ret 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.

Journal article

Middleburgh SC, Claisse A, Andersson DA, Grimes RW, Olsson P, Mašková Set 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.

Journal article

Galvin C, Cooper MWD, Rushton MJD, Grimes RWet 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.

Journal article

Pavlov TR, Wenman MR, Vlahovic L, Robba D, Konings RJM, Van Uffelen P, Grimes RWet 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.

Journal article

Kuganathan N, Ghosh PS, Arya AK, Dey GK, Grimes RWet 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.

Journal article

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.

Journal article

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