Publications
390 results found
Kuganathan N, Barron NJ, Grimes RW, 2023, Initial stage of helium clustering in UN and PuN, JOURNAL OF NUCLEAR MATERIALS, Vol: 583, ISSN: 0022-3115
Kuganathan N, Chroneos A, Grimes RW, 2023, Heavy metals in yttrium silicide electride (Y<sub>5</sub>Si<sub>3</sub>:e<SUP>-</SUP>), JOURNAL OF APPLIED PHYSICS, Vol: 133, ISSN: 0021-8979
Kuganathan N, Chroneos A, Grimes RW, 2023, Vacancy defects in nitrogen doped diamond, PHYSICA B-CONDENSED MATTER, Vol: 655, ISSN: 0921-4526
Kuganathan N, Barron NJ, Grimes RW, 2023, Incorporation of volatile fission products in UN and PuN and comparison to oxides, JOURNAL OF NUCLEAR MATERIALS, Vol: 576, ISSN: 0022-3115
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Wenman M, Grimes R, Tate J, et al., 2022, Parliament Committees. Delivering nuclear power: Written evidence from Imperial College London (NCL0026), Publisher: UK Parliment
Kuganathan N, Chroneos A, Grimes RWW, 2022, Confinement of volatile fission products in the crystalline organic electride Cs<SUP>+</SUP>(15C5)<sub>2</sub>.e<SUP>-</SUP>, JOURNAL OF APPLIED PHYSICS, Vol: 132, ISSN: 0021-8979
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Kuganathan N, Grimes R, Chroneos A, 2022, Nitrogen-vacancy defects in germanium, AIP Advances, Vol: 12, ISSN: 2158-3226
While nitrogen doping has been investigated extensively in silicon, there is only limited information on its interaction with vacancies in germanium, despite most point defect processes in germanium being vacancy controlled. Thus, spin polarized density functional theory calculations are used to examine the association of nitrogen with lattice vacancies in germanium and for comparison in silicon. The results demonstrate significant charge transfer to nitrogen from nearest neighbour Ge and strong N-Ge bond formation. The presence of vacancies results in a change in nitrogen coordination (from tetrahedral to trigonal planar) though the total charge transfer to N is maintained. A variety of different nitrogen vacancy clusters are considered all of which demonstrated strong binding energies. Substitutional nitrogen remains an effective trap for vacancies even if it has already trapped one vacancy.
Stephens GF, Than YR, Neilson W, et al., 2021, The accommodation of lithium in bulk ZrO<sub>2</sub>, SOLID STATE IONICS, Vol: 373, ISSN: 0167-2738
Kuganathan N, Chroneos A, Grimes RW, 2021, One-dimensional yttrium silicide electride (Y5Si3:e−) for encapsulation of volatile fission products, Journal of Applied Physics, Vol: 129, Pages: 1-7, ISSN: 0021-8979
Better ways are needed to capture radioactive volatile fission products (Kr, Xe, Br, I, Te, Rb, and Cs) discharged during the reprocessing of spent nuclear fuel in order to reduce the volumes of produced waste and minimize environmental impact. Using density functional theory, we examine the efficacy of a one-dimensional yttrium silicide electride (Y5Si3:e−) as a host matrix to encapsulate these species. Endoergic encapsulation energies calculated for Kr, Xe, Rb, and Cs imply they are not captured by Y5Si3:e−. Encapsulation is exoergic for Br, I, and Te with respect to their atoms and dimers as reference states, meaning that they can be captured effectively due to their high electronegativities. This is further supported by the formation of anions due to charge transfer between Y5Si3:e− and Br (I and Te). The selectivity of this material for these volatile species makes it promising for use in nuclear filters.
Kuganathan N, Grimes R, Rushton M, et 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.
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.
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.
Galvin COT, Rushton MJD, Cooper MWD, et 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.
Grimes R, Than YR, 2020, Predicting radiation damage in beryllium, Philosophical Magazine, Vol: 101, Pages: 306-325, 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.
Qin MJ, Middleburgh SC, Cooper MWD, et 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
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.
Pavlov T, lestak, Wenman M, et 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.
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.
Burr PA, Rushton MJD, Chroneos A, et al., 2020, Fundamental Point Defect Properties in Ceramics, Comprehensive Nuclear Materials: Second Edition, Pages: 50-73, ISBN: 9780081028667
This article describes the general principles of point defects in ceramics and how they affect material properties. First, an overview of intrinsic atomic point defects is provided, followed by a description of the reaction processes that lead to atomic disorder and the accommodation of non-stoichiometry. The thermodynamics of defect clustering are reviewed next. Extrinsic point defects are then considered to show how doping affects defect concentrations. Following this, electronic defects are considered. For both the dilute and concentrated defects, particular attention is given to the structural response of the lattice and the impact this has on material properties. The reactions governing the concentration of electronic and atomic point defects are combined to form a Brouwer diagram. Finally, the migration processes of point defects in ceramics are described, including the interstitial, interstitialcy and vacancy-mediated mechanisms, as well as atypical collective and diffusion processes.
Galvin COT, Burr PA, Cooper MWD, et 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.
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
Than YR, Grimes RW, Bell BDC, et 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.
Middleburgh SC, Ipatova I, Evitts LJ, et 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.
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.
Ghosh PS, Arya A, Kuganathan N, et 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.
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.
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
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.
Than YR, Wenman MR, Bell BDC, et 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.
Kuganathan N, Ghosh P, Arya A, et 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.
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