Publications
89 results found
Gasparrini C, Xu A, Short K, et al., 2020, Micromechanical testing of unirradiated and helium ion irradiated SA508 reactor pressure vessel steels: Nanoindentation vs in-situ microtensile testing, Materials Science and Engineering: A, ISSN: 0921-5093
In this paper, microtensile testing is demonstrated to be a viable technique for measuring irradiation hardening and reduction of ductility of ion irradiated hot isostatic pressed SA508 ferritic/bainitic steel. Ion irradiation with He2+ was used as a surrogate for neutron irradiation to reach a damage level of 0.6 dpa (Kinchin-Pease). The mechanical properties of four unirradiated microtensile steel specimens were measured and compared to the bulk properties: when averaged the 0.2% proof stress was 501.6 ± 56.0 MPa, in good agreement with the macrotensile 0.2% proof stress of 456.2 ± 1.7 MPa. On the basis of the agreement between microtensile and standard tensile 0.2% proof stress in the unirradiated material, it was possible to directly measure irradiation induced hardening from ion irradiation performed with He2+ ions to a dose of 0.6 dpa. Microtensile testing of the ion irradiated steel revealed an increase in 0.2% proof stress of approximately 730 MPa. The irradiation hardening measured by nanoindentation was 3.22 ± 0.29 GPa. Irradiation hardening was higher than that previously observed in neutron irradiated low alloy steels exposed to similar doses at low temperatures (<100 °C). The reason for the higher hardening was related to the presence of fine helium bubbles implanted in the irradiated layer that, alone, was calculated to produce a 707 ± 99 MPa increase in yield stress.
Jones LD, Vandeperre LJ, Haynes TA, et al., 2020, Theory and application of Weibull distributions to 1D peridynamics for brittle solids, Computer Methods in Applied Mechanics and Engineering, Vol: 363, Pages: 1-11, ISSN: 0045-7825
Peridynamics is a continuum mechanics modelling method, which is emerging as a solution for – in particular – the modelling of brittle fracture. The inherent variability of brittle fracture is captured well by the Weibull distribution, which describes the probability of fracture of a given material at a given stress. Recreating a Weibull distribution in peridynamics involves adjusting for the fact that the body is made up of a large number of bonds, and the distribution of strengths associated with these bonds must be different to the distribution of strengths associated with the peridynamic body. In the local case, where the horizon ratio, m=1 is used, Weibull’s original simple size scaling gives exact results, but the overlapping nature of non-local bonds that occurs in higher m cases, typically used in the peridynamics literature (such as m=3), causes a significant distortion of Weibull distributions. The cause of these distortions is spurious toughening and partial component failures as a result of the reduced localisation associated with larger horizon ratios. In order to remove these distortions, appropriate size scaling is used for the bonds, and a methodology that is capable of reflecting the heterogeneity of the material in the model, is proposed. The methodology described means Weibull parameters measured at specimen or component level can be reproduced for higher values of m.
Smutna J, Fogarty RM, Wenman MR, et al., 2020, Systematic development of ab initio tight-binding models for hexagonal metals, Physical Review Materials, Vol: 4, Pages: 043801-1-043801-18, ISSN: 2475-9953
A systematic method for building an extensible tight-binding model from ab initio calculations has been developed and tested on two hexagonal metals: Zr and Mg. The errors introduced at each level of approximation are discussed and quantified. For bulk materials, using a limited basis set of spd orbitals is shown to be sufficient to reproduce with high accuracy bulk energy versus volume curves for fcc, bcc, and hcp lattice structures, as well as the electronic density of states. However, the two-center approximation introduces errors of several tenths of eV in the pair potential, crystal-field terms, and hopping integrals. Environmentally dependent corrections to the former two have been implemented, significantly improving the accuracy. Two-center hopping integrals were corrected by taking many-center hopping integrals for a set of structures of interest, rotating them into the bond reference frame, and then fitting a smooth function through these values. Finally, a pair potential was fitted to correct remaining errors. However, this procedure is not sufficient to ensure transferability of the model, especially when point defects are introduced. In particular, it is shown to be problematic when interstitial elements are added to the model, as demonstrated in the case of octahedral self-interstitial atoms.
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.
King DJM, Yang M, Whiting TM, et al., 2020, G-phase strengthened iron alloys by design, Acta Materialia, Vol: 183, Pages: 350-361, ISSN: 1359-6454
Density functional theory (DFT) calculations were used to model G-phase precipitates of formula X6M16Si7 where X is Cr, Hf, Mn, Mo, Nb, Ta, Ti, V, W and Zr and M is either Fe or Ni. It was found that the occupancy of the d-orbital is correlated to the formation enthalpies of each structure. Past thermal expansion coefficient data was used to predict the lattice misfit between each G-phase and body centred cubic (BCC) Fe. All except Hf and Zr containing G-phases were predicted to have zero misfit between 581−843 K. Of the Ni containing G-phases, Mn6Ni16Si7 was predicted to have the most similar elastic properties to BCC Fe. DFT calculations of the substitution energies of Al, Cr Cu, Fe, Ge, Hf, Mo, Nb, P, Ta, Ti, V, Zr, and vacancies onto the Mn6Ni16Si7 G-phase from BCC Fe were performed. It was predicted that Cu, P and vacancies favour G-phase substitution. Suppression of the G-phase is predicted when Si content is reduced by half, at which point the BCC phase is favoured. It is hypothesised that including Zr to form a (Mn,Zr)6Ni16Si7 precipitate will allow for higher ageing temperature and expediate nucleation in an Fe alloy. Thermocalc was used to predict that a mixture of FebalCr9Ni4Si2(Mn0.6Zr0.4)1.2 (at.%) will produce a G-phase strengthened Fe alloy with potential for a good balance of strength, ductility and oxidation/corrosion resistance at room temperature. This alloy composition was experimentally determined to precipitate the G-phase in ≤24 h with cube-on-cube orientation to the BCC Fe matrix.
Jones LD, Vandeperre LJ, Haynes TA, et al., 2020, Modelling of Weibull Distributions in Brittle Solids Using 2-Dimensional Peridynamics, 1st European-Structural-Integrity-Society (ESIS) Virtual European Conference on Fracture (ECF), Publisher: ELSEVIER, Pages: 1856-1874, ISSN: 2452-3216
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- Citations: 4
Reali L, El Chamaa S, Balint DS, et al., 2020, Deformation and fracture of zirconium hydrides during the plastic straining of Zr-4, MRS ADVANCES, Vol: 5, Pages: 559-567, ISSN: 2059-8521
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- Citations: 1
Scatigno GG, Dong P, Ryan MP, et al., 2019, The effect of salt loading on chloride-induced stress corrosion cracking of 304L austenitic stainless steel under atmospheric conditions, Materialia, Vol: 8, Pages: 1-11, ISSN: 2589-1529
The effect of salt loading on chloride-induced stress corrosion cracking in 304 L was studied at atmospheric pressure. Stress relieved samples were uniaxially pre-strained to 5% and were loaded with nine levels of MgCl2, investigating Cl− deposition levels from 1.7 × 10−4 to 3.1 x 10−2 g cm−2. Samples were subject to 60 MPa stress, 90 °C at 70% relative humidity, for 480 h. A direct correlation between chloride deposition and the density of cracking and corrosion was observed between 5.7 × 10−4 and 1.96 × 10−2 g cm−2. Crack propagation rates were constant between salt loadings of 5.7 × 10−4 and 2.4 × 10−2 g cm−2 at 1–2 µm h−1.
Whiting T, Burr PA, King D, et al., 2019, Understanding the importance of the energetics of Mn, Ni, Cu, Si and vacancy triplet clusters in bcc Fe, Journal of Applied Physics, Vol: 126, ISSN: 0021-8979
Numerous experimental studies have found the presence of (Cu)-Ni-Mn-Si clusters in neutron irradiated reactor pressure vessel steels, prompting concerns that these clusters could lead to larger than expected increases in hardening, especially at high fluences late in life. The mechanics governing clustering for the Fe-Mn-Ni-Si system are not well-known; state-of-the-art methods use kinetic Monte Carlo (KMC) parameterised by density functional theory (DFT) and thermodynamic data to model the time evolution of clusters. However, DFT based KMC studies have so far been limited to only pairwise interactions due to lack of DFT data. Here we explicitly calculate the binding energy of triplet clusters of Mn, Ni, Cu, Si and vacancies in bcc Fe using DFT to show that the presence of vacancies, Si, or Cu stabilises cluster formation, as clusters containing exclusively Mn and/or Ni are not energetically stable in the absence of interstitials. We further identify which clusters may be reasonably approximated as a sum of pairwise interactions, and which instead require an explicit treatment of the three-body interaction, showing that the three-body term can account for as much as 0.3 eV, especially for clusters containing vacancies.
Weekes H, Dye D, Proctor JE, et al., 2019, The effect of pressure on hydrogen solubility in Zircaloy-4, Journal of Nuclear Materials, Vol: 524, Pages: 256-262, ISSN: 0022-3115
The effect of pressure on the room temperature solubility of hydrogen inZircaloy-4 was examined using synchrotron X-ray diffraction on small groundflake samples in a diamond anvil cell at pressures up to 20.9 GPa. Differentcombinations of hydrogen level/state in the sample and of pressure transmittingmedium were examined; in all three cases examined, it could be concluded thatpressure resulted in the dissolution of d hydrides and that interstitialhydrogen retards the formation of w Zr. A pressure of around 9 GPa was requiredto halve the hydride fraction. These results imply that the effect of pressureis thermodynamically analogous to that of increasing temperature, but that theeffect is small. The results are consistent with the volume per Zr atom of thea, d and w phases, with the bulk moduli of a and d, and with previousmeasurements of the hydrogen site molar volumes in the a and d phases. Theresults are interpreted in terms of their implication for our understanding ofthe driving forces for hydride precipitation at crack tips, which are in aregion of hydrostatic tensile stress on the order of 1.5 GPa.
Gasparrini C, Podor R, Fiquet O, et al., 2019, Uranium carbide oxidation from 873 K to 1173 K, Corrosion Science, Vol: 151, Pages: 44-56, ISSN: 0010-938X
Oxidation of UC was studied from 873 to 1173 K in air and in 10 Pa oxygen using a High Temperature Environmental SEM (HT-ESEM). Conversion to U 3 O 8 improved when using 873 K as the oxide product was a fine powder. At higher temperatures (973 K to 1173 K) oxidation slowed due to a densification process with formation of coarse fragments. The oxide fragmentation at 973 K and 1073 K and oxide pulverisation at 873 K were observed in situ in a HT-ESEM. Cracking induced fragmentation and pulverisation was linked to stresses generated from the volumetric transformation from UC to U 3 O 8 .
King DJM, Wenman MR, 2019, Comment on "The two-step nucleation of G-phase in ferrite", the authors: Y. Matsukawa et al. Acta Mater 2016; 116:104-133, Scripta Materialia, Vol: 163, Pages: 163-165, ISSN: 1359-6462
Recently, Matsukawa et al. [1] published a paper investigating the nucleation and growth of Mn6Ni16Si7 G-phase precipitates in duplex stainless steel using experimental and theoretical techniques. The G-phase cubic unit cells simulated by the authors for the theoretical analysis are non-physical, with lattice parameters of ≤0.550 nm, leading to a partially erroneous conclusion regarding the structure of the observed precipitates. In this comment we use density functional theory results from our previous study of the G-phase [2] to offer an alternative explanation to the experimental observations made by Matsukawa et al., viz, the Mn-Ni-Si precipitate is in an intermediate P1 structure, resulting from an energy minimum, in the transformation from BCC packing to G-phase.
Haynes TA, Podgurschi V, Wenman MR, 2019, The impact of azimuthally asymmetric carbon deposition upon pellet-clad mechanical interaction in advanced gas reactor fuel, JOURNAL OF NUCLEAR MATERIALS, Vol: 513, Pages: 62-70, ISSN: 0022-3115
El Chamaat S, Patel M, Wenman MR, et al., 2019, MULTISCALE STRESS-DIFFUSION ANALYSIS OF NOTCH-TIP HYDROGEN PROFILES IN ZIRCALOY-4, ASME Pressure Vessels and Piping Conference (PVP 2018), Publisher: AMER SOC MECHANICAL ENGINEERS
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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
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.
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.
King DJM, Burr PA, Middleburgh SC, et al., 2018, The formation and structure of Fe-Mn-Ni-Si solute clusters and G-phase precipitates in steels, Journal of Nuclear Materials, Vol: 505, Pages: 1-6, ISSN: 0022-3115
Solute clustering and G-phase precipitation cause hardening phenomena observed in some low alloy and stainless steels, respectively. Density functional theory was used to investigate the energetic driving force for the formation of these precipitates, capturing temperature effects through analysis of the system's configurational and magnetic entropies. It is shown that enrichment of Mn, Ni and Si is thermodynamically favourable compared to the dilute ferrite matrix of a typical A508 low alloy steel. We predict the ordered G-phase to form preferentially rather than a structure with B2-type ordering when the Fe content of the system falls below 10–18 at. %. The B2 → G-phase transformation is predicted to occur spontaneously when vacancies are introduced into the B2 structure in the absence of Fe.
King DJM, Middleburgh SC, Burr PA, et al., 2018, Density functional theory study of the magnetic moment of solute Mn in bcc Fe, Physical Review B, Vol: 98, ISSN: 2469-9950
An unexplained discrepancy exists between the experimentally measured and theoretically calculated magnetic moments of Mn in α-Fe. In this study, we use density functional theory to suggest that this discrepancy is likely due to the local strain environment of a Mn atom in the Fe structure. The ferromagnetic coupling, found by experiment, was shown to be metastable and could be stabilized by a 2% hydrostatic compressive strain. The effects of Mn concentration, vacancies, and interstitial defects on the magnetic moment of Mn are also discussed. It was found that the ground-state, antiferromagnetic (AFM) coupling of Mn to Fe requires long-range tensile relaxations of the neighboring atoms along ⟨111⟩ which is hindered in the presence of other Mn atoms. Vacancies and Fe interstitial defects stabilize the AFM coupling but are not expected to have a large effect on the average measured magnetic moment.
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.
El Chamaa S, Patel M, Davies C, et al., 2018, The effect of grain boundaries and second-phase particles on hydride precipitation in zirconium alloys, MRS Advances, Vol: 3, Pages: 1749-1754, ISSN: 2059-8521
Understanding the precipitation of brittle hydride phases is crucial in establishing a failure criterion for various zirconium alloy nuclear fuel cladding. Accordingly, it is important to quantify the sensitivity of hydride precipitation to the component microstructure. This experimental investigation focuses on two microstructural characteristics and their role as hydride nucleation sites: The grain size and the alloy chemical composition. Samples of commercially pure zirconium (Zr-702) and Zircaloy-4, each with a wide range of grain sizes, were hydrided to 100 ppm and micrographs of the hydride distribution were optically analyzed for inter-granular and intra-granular precipitate sites. For most grain sizes, it was found that a significantly lower fraction of the precipitated hydrides nucleated at grain boundaries in Zircaloy-4 than in Zr-702, suggesting that a higher SPP content encourages the formation of intra-granular hydrides. Moreover, this effect became more prominent as the grain size increased; large-grain specimens contained a higher fraction of intra-granular hydrides than small-grain specimens of both Zr-702 and Zircaloy-4, highlighting the potency of grain boundaries as nucleation sites and how SPPs can influence the hydride distribution profile.
Pavlov TR, Staicu D, Vlahovic L, et al., 2017, A new method for the characterization of temperature dependent thermo-physical properties, International Journal of Thermal Sciences, Vol: 124, Pages: 98-109, ISSN: 1290-0729
The proposed method is based on the laser flash technique. Radially distributed thermograms are calculated via a finite element model and used in an inverse method by optimizing either specific heat or thermal conductivity of a material. These properties are evaluated as a function of radius and respective temperature. Two approximations are introduced inferring the dependence of each property as a function of radius – a polynomial (PNOM) approximation and an iterative gradient (IG) approximation. The method was tested using synthetic thermograms and both approximations were capable of yielding excellent results. The IG approximation was more universal and less sensitive to initial fitting parameters. The PNOM approximation was less computationally expensive but was prone to artefacts (such as un-physical minima or maxima) and more dependent on initial fitting parameters. Both approximations were successfully used on experimental data from UO2 and isostatically pressed graphite. Thermal conductivity was within 5% of the reference empirical correlation for UO2 and within 7% of the reference curve for graphite.
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.
Platt P, Mella R, DeMaio W, et al., 2017, Peridynamic simulations of the tetragonal to monoclinic phase transformation in zirconium dioxide, Computational Materials Science, Vol: 140, Pages: 322-333, ISSN: 0927-0256
Whether present as a manufactured stabilised ceramic, or as an oxide layer on zirconium alloys, mechanical degradation in zirconia is influenced by the tetragonal to monoclinic phase transformation. Peridynamic theory was implemented within the Abaqus finite element framework to understand how the tetragonal to monoclinic phase transformation can itself cause fracture in zirconia. In 2D these simulations represent a single grain, transforming via an isometric dilational expansion, surrounded by a homogenous monoclinic oxide. The effect of transformation time, applied bi-axial pressure, and the fracture strain were assessed using the change in strain energy and the amount of damage in the oxide surrounding the transformed grain. Reducing the applied compressive stress or applying a tensile stress reduces the transformation strain energy. The introduction of a fracture strain leads to damage in the surrounding oxide region largely in the form of cracks, and reduces the transformation strain energy further by reducing the constraint on the transforming grain. The extent of the fracture, and reduction in constraint on the transformed grain, is more significant with the application of a biaxial tensile pressure.
Patel M, Waheed S, Wenman MR, et al., 2017, Discrete dislocation plasticity modeling of hydrides in zirconium under thermal cycling, MRS Advances, Vol: 2, Pages: 3353-3358, ISSN: 2059-8521
Understanding the ratcheting effect of hydrogen and hydride accumulation in response to thermal cycling is important in establishing a failure criterion for zirconium alloy nuclear fuel cladding. We propose a simple discrete dislocation plasticity model to study the evolution of the dislocation content that arises as a micro-hydride repeatedly precipitates and dissolves over a series of thermal cycles. With each progressive thermal cycle, we find a steady growth in the residual dislocation density in the vicinity of the hydride nucleation site; this corresponds to a gradual increase in the hydrogen concentration and, consequently, the hydride population. The simulated ratcheting in the dislocation density is consistent with experimental observations concerning the hysteresis in the terminal solid solubility of hydrogen in zirconium, which can be correlated to the plastic relaxation of hydrides.
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.
Haynes TA, Ball JA, Wenman MR, 2017, Modelling the role of pellet crack motion in the ( r -θ) plane upon pellet-clad interaction in advanced gas reactor fuel, Nuclear Engineering and Design, Vol: 314, Pages: 271-284, ISSN: 0029-5493
A finite element model of pellet fragment relocation in the r-θ plane of advanced gas-cooled reactor (AGR) fuel is presented under conditions of both ‘hard’ and ‘soft’ pellet-clad interaction. The model was able to predict the additional radial displacement of fuel fragments towards the cladding as well as the stress concentration on the inner surface resulting from the azimuthal motion of pellet fragments. The model was subjected to a severe ramp in power from both full power and after a period of reduced power operation; in the former, the maximum hoop stress in the cladding was found to be increased by a factor of 1.6 as a result of modelling the pellet fragment motion. The pellet-clad interaction was found to be relatively insensitive to the number of radial pellet crack. However, it was very sensitive to both the coefficient of friction used between the clad and pellet fragments and power ramp duration.
Sutton AP, Nazarov R, Majevadia JS, et al., 2016, First-principles calculation of the elastic dipole tensor of a point defect: Application to hydrogen in α-zirconium, Physical Review B, Vol: 94, ISSN: 1550-235X
The elastic dipole tensor is a fundamental quantity relating the elastic field and atomic structure of a point defect. We review three methods in the literature to calculate the dipole tensor and apply them to hydrogen in α-zirconium using density functional theory (DFT). The results are compared with the dipole tensor deduced from earlier experimental measurements of the λ tensor for hydrogen in α-zirconium. There are significant errors with all three methods. We show that calculation of the λtensor, in combination with experimentally measured elastic constants and lattice parameters, yields dipole tensor components that differ from experimental values by only 10%–20%. There is evidence to suggest that current state-of-the-art DFT calculations underestimate bonding between hydrogen and α-zirconium.
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.
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