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It has long been recognized from low-temperature defect production experiments and molecular dynamics (MD) simulations of displacement events in metals that the surviving defect fraction relative to the calculated Norgett-Robinson Torrens (NRT) displacements per atom (dpa) ranges from ∼1 for near-threshold displacements (representative of electron irradiation) to ∼0.3 for energetic displacement cascades (relevant for fast neutrons or energetic heavy ion irradiations). This correlated point defect recombination that occurs within a few picoseconds following the creation of a primary knock-on atom (PKA) has recently been formalized in the athermal recombination corrected dpa (ARC-DPA) parameter. However, at elevated temperatures additional thermally-stimulated correlated defect recombination processes need to be considered. For example, at irradiation temperatures above defect recovery Stage I (typically ∼40–100 K for most metals), mobile self-interstitial atoms induce additional thermally-activated correlated defect recombination that can amount to 70–80% reduction in the surviving defect fraction for low PKA energies and ∼20 to 30% reduction for high PKA energies. Since most nuclear material applications involve operation above room temperature, it is generally important to include this additional correlated defect recombination in radiation effects models. Published experimental defect recovery and defect production studies on 26 metals are analyzed to extract the surviving defect fraction following thermally-activated correlated recombination as a function of PKA energy. These experimental estimates of the athermal- plus thermal-activated correlated recombination-corrected DPA (CRC-DPA) are compared with new long-term MD simulation results on Cu and Fe. The analysis indicates the CRC-DPA fraction of surviving defects in most elements for irradiation at temperatures where interstitials are mobile is ∼25% of the NRT DPA over a wide range of PKA energies relevant for electron, ion and fast neutron irradiation conditions. Limited experimental and kinetic Monte Carlo studies suggest a further reduction to ∼10% of the NRT DPA over a wide range of PKA energies when correlated recombination due to thermally activated vacancy migration is considered. This temperature-dependent CRC-DPA should be used as the defect production parameter (rather than NRT DPA or ARC-DPA) for materials irradiated at all temperatures where defects are mobile. [Display omitted] [ABSTRACT FROM AUTHOR]

High helium generation rates in irradiated materials leads to the formation of small He–vacancy clusters that can evolve into larger bubbles and voids. An equation of state that accurately reproduces their pressure–volume relationship is necessary to understand and predict the behaviour of these He–vacancy defects. Previous research has employed equations of state of varying complexity, including the ideal gas, van der Waals, and hard sphere models. We recently used ab initio calculations to determine the energetics of helium-vacancy clusters and applied the results to develop a new three-body interatomic potential that describes the behaviour of helium in iron. This potential was employed in molecular dynamics simulations to determine the conditions for mechanical equilibrium between small helium-stabilized bubbles and an iron matrix, and to systematically map the pressure–volume relationship for the bubbles at a range of temperatures. These atomistic results are compared to an existing equation of state and a modification is proposed for bubbles with high helium densities. [ABSTRACT FROM AUTHOR]

Abstract: The SRIM (formerly TRIM) Monte Carlo simulation code is widely used to compute a number of parameters relevant to ion beam implantation and ion beam processing of materials. It also has the capability to compute a common radiation damage exposure unit known as atomic displacements per atom (dpa). Since dpa is a standard measure of primary radiation damage production, most researchers who employ ion beams as a tool for inducing radiation damage in materials use SRIM to determine the dpa associated with their irradiations. The use of SRIM for this purpose has been evaluated and comparisons have been made with an internationally-recognized standard definition of dpa, as well as more detailed atomistic simulations of atomic displacement cascades. Differences between the standard and SRIM-based dpa are discussed and recommendations for future usage of SRIM in radiation damage studies are made. In particular, it is recommended that when direct comparisons between ion and neutron data are intended, the Kinchin–Pease option of SRIM should be selected. [Copyright &y& Elsevier]

Abstract: Under the DOE Deep Burn program TRISO fuel is being investigated as a fuel form for consuming plutonium and minor actinides, and for greater efficiency in uranium utilization. The result will thus be to drive TRISO particulate fuel to very high burn-ups. In the current effort the various phenomena in the TRISO particle are being modeled using a variety of techniques. The chemical behavior is being treated utilizing thermochemical analysis to identify phase formation/transformation and chemical activities in the particle, including kernel migration. Density functional theory is being used to understand fission product diffusion within the plutonia oxide kernel, the fission product’s attack on the SiC coating layer, as well as fission product diffusion through an alternative coating layer, ZrC. Finally, a multiscale approach is being used to understand thermal transport, including the effect of radiation damage induced defects, in a model SiC material. [Copyright &y& Elsevier]

Abstract: Multiscale modeling schemes encompass models from the atomistic to the continuum scale. Phenomena at the mesoscale have typically been simulated using models based on reaction rate theory, such as mean field rate theory (MFRT) or Monte Carlo. These mesoscale models are appropriate for application to problems that involve intermediate-length scales, and timescales from those characteristic of diffusion to long-term microstructural evolution (∼μs to years). Although the MFRT and Monte Carlo models can be used simulate the same phenomena, some of the details are handled quite differently in the two approaches. Models employing the rate theory have been extensively used to describe radiation-induced phenomena such as void swelling and irradiation creep. The primary approximations in such models are time and spatial averaging of the radiation damage source term, and spatial averaging of the microstructure into an effective medium. Kinetic Monte Carlo models can account for these spatial and temporal correlations; their primary limitation is the computational burden, which is related to the size of the simulation cell. Even with modern computers, the maximum simulation cell size and the maximum dose (typically much less than 1dpa) that can be simulated are limited. In contrast, even very detailed MFRT models can simulate microstructural evolution for doses up 100dpa or greater in clock times that are relatively short. Within the context of the effective medium, essentially any defect density can be simulated. A direct comparison of MFRT and object kinetic MC simulations has been made in the domain of point defect cluster dynamics modeling, which is relevant to the evolution (both nucleation and growth) of radiation-induced defect structures. Overall, the agreement between the two methods is best for irradiation conditions that produce a high density of defects (lower temperature and higher displacement rate) and for materials that have a relatively high density of fixed sinks such as dislocations. [Copyright &y& Elsevier]

Abstract: The nature of residual damage from atomic displacement cascades in iron has been investigated by diffuse X-ray scattering. A single crystal iron sample was irradiated at ∼60°C in the High Flux Isotope Reactor at the Oak Ridge National Laboratory to a fluence of 1×1023 n/m2 (E >0.1MeV), or ∼0.01 dpa. Diffuse X-ray scattering measurements were carried out at the Advanced Photon Source at the Argonne National Laboratory on three specimens: unirradiated, as-irradiated, and irradiated and annealed for one hour at 450°C (IA). The specimens were cooled to ∼40K to minimize thermal diffuse scattering. Subsequent measurements of the specimen lattice parameter were completed at ORNL. The diffuse scattering in the as-irradiated specimen was dominated by interstitial defect clusters exhibiting a tetragonal distortion consistent with a 〈100〉 type defect. Substantial recovery of this defect component was observed following the anneal at 450°C. A lattice parameter increase was observed following irradiation, which is also consistent with a substantial population of interstitial type defects. A net lattice parameter decrease was observed in the IA specimen, indicating loss of the interstitial defects with a residual population of vacancy-type defects. [Copyright &y& Elsevier]

Abstract: To understand the effects of He on irradiated metals requires modeling of helium-vacancy cluster evolution. A new method of solving the two-dimensional master equation (ME) describing He-vacancy cluster evolution has been applied to calculate helium bubble evolution in a stainless steel irradiated with alpha particles near room temperature and annealed in the temperature range of 600–900°C. For the first time, the evolution of the helium bubble size distribution function was precisely calculated in 2-D phase space and good agreement with experimental results was obtained. The results indicate that Brownian motion of bubbles via surface vacancy diffusion provides a reasonable explanation for bubble evolution during annealing, most bubbles are found to be near the equilibrium state during the evolution at temperatures of 700°C and higher, lack of vacancies at temperatures lower than 700°C prevents bubble growth, and use of a non-ideal He equation of state (EOS) increases the bubble density and size relative to the case when the ideal EOS is used. [Copyright &y& Elsevier]

Apostolopoulos and Kotsina pointed out our unstated assumption in the discussion of the energy partitioning in SRIM that leads to Eqs. (4) When we checked the energy conservation in SRIM by comparing the ion beam energy with the sum of all four energy loss mechanisms, i.e. the ion beam energy lost to ionization and phonons and the target atom energy lost to ionization and phonons, we always found a small excess energy that we assumed was due to finite accuracy in the process of binning. Apostolopoulos and Kotsina discovered that the ion beam energy lost to phonons (EiP in our paper) also is included in the target energy lost to phonons (ETP); therefore EiP was double counted. [Extracted from the article]

Abstract: Stacking fault tetrahedra (SFTs) are formed during irradiation of f.c.c. metals and alloys with low stacking fault energy. The high number density of SFTs observed suggests that they should contribute to radiation-induced hardening and, therefore, be taken into account when estimating mechanical property changes of irradiated materials. The key issue is to describe the interaction between a moving dislocation and an individual SFT, which is characterized by a small physical scale of about 100nm. In this paper we present results of an atomistic simulation of edge and screw dislocations interacting with small SFTs at different temperatures and strain rates and present mechanisms which can explain the formation of defect-free channels observed experimentally. [Copyright &y& Elsevier]

The high number density of stacking fault tetrahedra (SFTs) observed in irradiated fcc metals suggests that they should contribute to radiation-induced hardening and, therefore, taken into account when estimating mechanical properties changes of irradiated materials. The central issue is describing the individual interaction between a moving dislocation and an SFT, which is characterized by a very fine size scale, ∼100 nm. This scale is amenable to both in situ TEM experiments and large-scale atomic modelling. In this paper we present results of an atomistic simulation of dislocation–SFT interactions using molecular dynamics (MD). The results are compared with observations from in situ deformation experiments. It is demonstrated that in some cases the simulations and experimental observations are quite similar, suggesting a reasonable interpretation of experimental observations. [Copyright &y& Elsevier]

Surface modification of structural and functional materials under bombardment by energetic ions is observed under different conditions and can be either an unavoidable effect of the irradiation or an intentional modification to enhance materials properties. Understanding the basic mechanisms is necessary for predicting property changes. The mechanisms activated during ion irradiation are of atomic scale and atomic scale modeling is the most suitable tool to study these processes. In this paper, we present results of an extensive simulation program aimed at developing an understanding of primary surface damage in iron induced by energetic particles. We simulated 25 keV self-ion bombardment of Fe thin films with (1 0 0) and (1 1 0) surfaces at room temperature. A large number of simulations, ∼400, were carried out allow a statistically significant treatment of the results. The particular mechanism of surface damage depends on how the destructive supersonic shock wave generated by the displacement cascade interacts with the free surface. Three scenarios were primarily observed, with the limiting cases being damage created far below the surface with little or no impact on the surface itself, and extensive direct surface damage on the timescale of a few picoseconds. In some cases, formation of large 〈1 0 0〉 vacancy loops beneath the free surface was observed, which may explain some earlier experimental observations. [ABSTRACT FROM AUTHOR]

We used molecular dynamics modeling of atomic displacement cascades to characterize the nature of primary radiation damage in 3C–SiC. We demonstrated that the most commonly used interatomic potentials are inconsistent with ab initio calculations of defect energetics. Both the Tersoff potential used in this work and a modified embedded-atom method potential reveal a barrier to recombination of the carbon interstitial and carbon vacancy which is much higher than the density functional theory (DFT) results. The barrier obtained with a newer potential by Gao and Weber is closer to the DFT result. This difference results in significant differences in the cascade production of point defects. We have completed both 10 keV and 50 keV cascade simulations in 3C–SiC at a range of temperatures. In contrast to the Tersoff potential, the Gao-Weber potential produces almost twice as many C vacancies and interstitials at the time of maximum disorder (∼0.2 ps) but only about 25% more stable defects at the end of the simulation. Only about 20% of the carbon defects produced with the Tersoff potential recombine during the in–cascade annealing phase, while about 60% recombine with the Gao-Weber potential. The Gao-Weber potential appears to give a more realistic description of cascade dynamics in SiC, but still has some shortcomings when the defect migration barriers are compared to the ab initio results. [ABSTRACT FROM AUTHOR]

The radiation growth of zirconium is studied using a reaction–diffusion model which takes into account intra-cascade clustering of self-interstitial atoms and one-dimensional diffusion of interstitial clusters. The observed dose dependence of strain rates is accounted for by accumulation of sessile dislocation loops during irradiation. The computational model developed and fitted to available experimental data is applied to study deformation of Zr single crystals under irradiation up to hundred dpa. The effect of cold work and the reasons for negative prismatic strains and co-existence of vacancy and interstitial loops are elucidated. [ABSTRACT FROM AUTHOR]

The results of a computational study of helium–vacancy clusters in tungsten are reported. A recently developed atomistic kinetic Monte Carlo method employing empirical interatomic potentials was used to investigate the behavior of clusters composed of three interstitial-helium atoms near {1 1 1}, {1 1 0} and {1 0 0} free surfaces. Multiple configurations were examined and the local energy landscape was characterized to determine cluster mobility and the potential for interactions with the surface. The clusters were found to be highly mobile if far from the surface, but were attracted and bound to the surface when within a distance of a few lattice parameters. When near the surface, the clusters were transformed into an immobile configuration due to the creation of a Frenkel pair; the vacancy was incorporated into what became a He 3 –vacancy complex. The corresponding interstitial migrated to and became an adatom on the free surface. This process can contribute to He retention, and may be responsible for the observed deterioration of the plasma-exposed tungsten surfaces. [ABSTRACT FROM AUTHOR]

The anisotropic mobility of hexagonal dislocation networks (HDNs) in a series of (1 0) twist boundaries under applied shear stress has been studied at the atomic scale in α-iron. A strong angular effect on the HDN mobility is found to be correlated with the dislocation core structure. The vector form of the Orowan equation and differential displacement maps of dislocation cores are used to account for the HDN behavior under loading. [Copyright &y& Elsevier]

Abstract: In this paper, kinetic Monte Carlo method was applied to investigate the long time evolution of cascade damage prepared by molecular dynamics simulations in α-Fe up to recoil energy of more than 200keV. We conducted thorough investigation on how the surviving defects vary with cascade damage energy and annealing temperature. The results can be used for input parameters of rate equations to simulate microstructural evolution under irradiation. The study also suggested that neighboring sub-cascades evolves almost independently during annealing, and that the temperature dependence of the annealing results can be explained by the temperature dependence of vacancy-migration and vacancy-dissociation probabilities. [Copyright &y& Elsevier]

Abstract: A new empirical Fe–He potential has been developed by fitting results obtained from first-principles calculations. Both the formation and relaxation energies of single He defects and small He clusters were accounted for in the fitting process. The new potential consists of a repulsive pair-potential term and a three-body interaction term, and was applied in combination with three commonly used iron interatomic potentials, and a potential describing the behavior of helium in vacuum. As an application of the new potential, the stability of He–vacancy clusters at zero temperature was evaluated. The calculated results were similar for all three Fe potentials, and the new potentials provide results that are more consistent with ab initio calculations than those obtained from previous Fe–He potentials. [Copyright &y& Elsevier]

Abstract: The oxide dispersion strengthened (ODS) steels are being developed and investigated for fission and fusion structural applications in Japan, Europe, and the United States. In this paper, the fracture toughness and tensile properties of an ODS steel with nominal composition Fe–12Cr–2.5W–0.4Ti–0.25Y2O3 (designated 12YWT) were investigated and compared to commercial reduced-activation ferritic/martensitic (RAFM) steels. Small, 1.6-mm thick and 3.2-mm wide, 3-point bend specimens were used for fracture toughness characterization of this steel. Specimens were fatigue pre-cracked to initial crack length (a) to width (W) ratio of 0.45 and tested quasi-statically in the temperature range from −50°C to 550°C. Specimens tested up to 50°C exhibited elastic–plastic cleavage fracture that was typical for the transition region in ferritic steels. The master curve transition temperature, T 0, for the 12YWT alloy was determined to be 102°C. Specimens tested at 100°C and higher exhibited ductile stable crack growth. In these cases, the J-integral at the onset of stable crack growth (J Ic) was determined from the J–R curves. Their equivalent values in terms of stress intensity factor, , were about 93MPa√m at 100°C and decreased to 53MPa√m at 550°C. This study showed that oxide dispersion strengthening resulted in significant decreases in the toughness properties compared to commercial RAFM steels, although appreciable level of toughness was still retained. Tensile tests were performed at temperatures between room temperature and 800°C. As expected, this material exhibited very high yield strength, ∼1300MPa, at room temperature. For comparison, the yield strength of commercial RAFM steels is about 550MPa. Yield strength of 12YWT decreases as test temperature increases and at 800°C it is about 323MPa. [Copyright &y& Elsevier]

Abstract: Oxide dispersion strengthened ferritic steels are being considered for a number of advanced nuclear reactor applications because of their high strength and potential for high temperature application. Since these properties are attributed to the presence of a high density of very small (nanometer-sized) oxide clusters, there is interest in examining the radiation stability of such clusters. A novel experiment has been carried out to examine oxide nanocluster stability in a mechanically alloyed, oxide dispersion strengthened ferritic steel designated 12YWT. Pre-polished specimens were ion irradiated and the resulting microstructure was examined by atom probe tomography. After ion irradiation to ∼0.7dpa with 150keV Fe ions at 300°C, a high number density of ∼4nm-diameter nanoclusters was observed in the ferritic matrix. The nanoclusters are enriched in yttrium, titanium and oxygen, depleted in tungsten and chromium, and have a stoichiometry close to (Ti+Y):O. A similar cluster population was observed in the unirradiated materials, indicating that the ultrafine oxide nanoclusters are resistant to coarsening and dissolution under displacement cascade damage for the ion irradiation conditions used. [Copyright &y& Elsevier]

Abstract: The destruction process of large non-truncated stacking fault tetrahedra (SFTs) induced by gliding dislocations was examined by in situ transmission electron microscope straining experiments. Three different destruction processes were observed: a triangular Frank loop remained after the collapse (Type 1), the whole SFT was incorporated into a gliding dislocation as multiple super jog segments (Type 2), and an apex portion of the original SFT remained as a smaller SFT while the base portion was annihilated (Type 3). The remnants of Type 1 and 2 destruction processes were similar to those of previous models proposed by Kimura, indicating that these processes are based on dislocation reactions as assumed in Kimura models. On contrary, the Type 3 process, which was entirely different from Kimura models, is occasionally accompanied by vacancy migration. [Copyright &y& Elsevier]

Abstract: Electronic structure calculations were used to study the relaxation, formation and binding energies of small helium clusters in iron. We considered three He defect configurations: two He atoms in an interstitial position and two and three He atoms located in one vacancy. To study He–vacancy clusters containing more He atoms, we used a multi-scale approach and constructed an empirical potential fitted to both formation and relaxation energies of a single He defect and small He clusters obtained from the first principles calculations. The potential consists of a repulsive pair-interaction part and a many-body attractive term describing the cohesion. The potential was used to study stability of He–vacancy clusters at zero temperature. The binding energy of a He atom to a He-cluster varies from 1.3eV to 1.9eV depending on the cluster size. When more than six He atoms are placed into a vacancy an Fe self-interstitial atom (SIA) is produced. The SIA binding energy to a He–di-vacancy cluster decreases from 5.0eV to 0.7eV as the number of He atoms increases. The results obtained are consistent with experimental observations of helium desorption reported in the literature. [Copyright &y& Elsevier]

Abstract: The collapse of stacking-fault tetrahedra (SFT) by gliding dislocations was observed in in situ straining experiments in a transmission electron microscope (TEM). A stacking-fault tetrahedron was collapsed by intersection with a gliding perfect dislocation: only the base portion divided by the gliding plane of the dislocation annihilated, while the apex portion remained intact. As a result of analysis on evolution of atom configuration induced by intersection with perfect dislocation in SFT, it was found that an unusual atom configuration inevitably appeared in one of the ledges formed on stacking-fault planes, which is traditionally called I-ledge: the atoms on adjacent (111) planes were overlapping each other. The overlapping configuration provides a strong repulsive force, being a conceivable driving force to induce a chain reaction of atom displacements that collapses the SFT base portion. [Copyright &y& Elsevier]

The vacancy production behavior for a broad range of incident ion-target combinations were examined by using both the full cascade (F-C) and quick calculation of damage (Q-C) options of Stopping and Range of Ions in Matter (SRIM-2013) code. Vacancy production for each option was evaluated using both the convenient vacancy text file in SRIM, and by manual manipulation of the energy transfer, ionization, and phonon text file data to calculate the depth-dependent damage energy for the Norgett-Robinson-Torrens (NRT) defect production model. The F-C vacancy text file method was observed to overpredict vacancy production by a factor of ~1.1 to 4 times when compared to values calculated using the F-C damage energy method or either of the SRIM Q-C methods. Based on our extensive data set obtained for 142 unique bombarding ion-target combinations, we show that the SRIM F-C vacancy text file method should not be used for vacancy production calculations. This error appears to be due to mischaracterization within the SRIM F-C option of some near-threshold replacement events as vacancies instead of replacements. Use of the latest SRIM stopping powers in the SRIM F-C approach provides a better calculation of electronic and nuclear stopping compared to the Lindhard stopping power analytical approximation used to calculate the damage energy of recoil atoms in the SRIM Q-C option, and therefore SRIM F-C approach is deemed to provide the best accuracy for vacancy production (within the binary collision approximation) as long as the damage energy method is used. Alternately, the SRIM Q-C option using either the vacancy.txt or damage energy method provides relatively fast calculation speeds with moderate quantitative differences from the F-C damage energy results (~0 to ~ 30% for the investigated ion-target cases). [ABSTRACT FROM AUTHOR]

In the climb-enabled glide model of irradiation creep, the plastic deformation is defined by the elastic deflections of pinned dislocations, which is an inconsistency. We argue that this relation is incorrect; instead, as in other pinning-unpinning-type models, the dislocations move from one set of obstacles to another, so that the inter-obstacle spacing determines creep rate, whereas the dependence on the applied stress is only implicit in the unpinning time. [ABSTRACT FROM AUTHOR]

Resistance to swelling under irradiation and a low rate of corrosion in high temperature environments make Fe-Cr and Fe-Cr-Ni alloys promising structural materials for energy technologies. In this paper we report the results obtained using a combination of density functional theory (DFT) techniques: plane wave basis set solutions for pseudo-potentials and multiple scattering solutions for all electron potentials. We have found a very strong role of magnetism in the stability of screw dislocation cores in pure Fe and their interaction with Cr and Ni magnetic impurities. In particular, the screw dislocation quadrupole in Fe is stabilized only in the presence of ferromagnetism. In addition, Ni atoms, who's magnetic moment is oriented along the magnetization direction of the Fe matrix, prefer to occupy in core positions whereas Cr atoms, which couple anti-ferromagnetically with the Fe matrix, prefer out of the dislocation core positions. In effect, Ni impurities are attracted to, while Cr impurities are repelled by the dislocation core. Moreover, we demonstrate that this contrasting behavior can be explained only by the nature of magnetic coupling of the impurities to the Fe matrix. Specifically, Cr interaction with the dislocation core mirrors that of Ni if the Cr magnetic moment is constrained to be along the direction of Fe matrix magnetization. In addition, we have shown that the magnetic contribution can affect the impurity-impurity interaction at distances up to a few Burgers vectors. In particular, the distance between Cr atoms in Fe matrix should be at least 3–4 lattice parameters in order to eliminate finite size effects. [ABSTRACT FROM AUTHOR]

High entropy alloys (HEA) have unique properties including the potential to be radiation tolerant. These materials with extreme disorder could resist damage because disorder, stabilized by entropy, is the equilibrium thermodynamic state. Disorder also reduces electron and phonon conductivity keeping the damage energy longer at the deposition locations, eventually favoring defect recombination. In the short time-scales related to thermal spikes induced by collision cascades, phonons become the relevant energy carrier. In this work, we perform a systematic study of phonon thermal conductivity in multiple component solid solutions represented by Lennard-Jones (LJ) potentials. We explore the conditions that minimize phonon mean free path via extreme alloy complexity, by varying the composition and the elements (differing in mass, atomic radii, and cohesive energy). We show that alloy complexity can be tailored to modify the scattering mechanisms that control energy transport in the phonon subsystem. Our analysis provides a qualitative guidance for the selection criteria used in the design of HEA alloys with low phonon thermal conductivity. [ABSTRACT FROM AUTHOR]

The fragmentation of displacement cascades into subcascades in copper and iron has been investigated through the molecular dynamics technique. A two-point density correlation function has been used to analyze the cascades as a function of the primary knock-on (PKA) energy. This approach is used as a tool for detecting subcascade formation. The fragmentation can already be identified at the end of the ballistic phase. Its resulting evolution in the peak damage state discriminates between unconnected and connected subcascades. The damage zone at the end of the ballistic phase is the precursor of the extended regions that contain the surviving defects. A fractal analysis of the cascade exhibits a dependence on both the stage of the cascade development and the PKA energy. This type of analysis enables the minimum and maximum displacement spike energies together with the subcascade formation threshold energy to be determined. [ABSTRACT FROM AUTHOR]

The thermal conductivities of UO 2 single crystals containing nanoscale size pores and He gas bubbles are calculated using non-equilibrium molecular dynamics as a function of pore size and gas density in the bubble. As expected, the thermal conductivity decreases as pore size increases, while the decrease in thermal conductivity is determined to be more substantial than the predictions of traditional analytical models by Loeb and Maxwell-Eucken. However, the recent model of Alvarez, which is applicable when the phonon mean-free path is comparable to the pore size, is able to quantitatively reproduce the simulation results. The thermal conductivity of UO 2 of the small pores considered here is reduced further when the pore is filled with He gas. This surprising result is due to the penetration of the helium atoms into the lattice where they act as phonon scattering centers. [ABSTRACT FROM AUTHOR]

A scalar γ -parameter is proposed from the Nye tensor. Its maximum value occurs along a dislocation line, either straight or curved, when the coordinate system is purposely chosen. This parameter can be easily obtained from the Nye tensor calculated at each atom in atomistic modeling. Using the γ -parameter, a fully automated approach is developed to determine core atoms and the Burgers vectors of dislocations simultaneously. The approach is validated by revealing the smallest dislocation loop and by tracing the whole formation process of complicated dislocation networks on the fly. [ABSTRACT FROM AUTHOR]

A combination of density functional theory (DFT), kinetic Monte Carlo and mean-field rate theory is applied to analyze point defect migration and its effect on the observed growth of hexagonal close-packed (hcp) Zr under 1 MeV electron irradiation. DFT is used to study stability of various configurations of vacancies and self-interstitial atoms (SIAs) and migration barriers. The data are used in kinetic Monte Carlo modeling of defect diffusion at different temperatures. It is found that both defects exhibit anisotropic diffusion, predominantly parallel to the basal planes. The ratio of diffusion coefficients parallel and perpendicular to the basal planes is found to be higher for vacancies as compared to SIAs at temperatures below ∼600 K. This raises doubts that the observed radiation growth in Zr irradiated with 1 MeV electrons, namely positive strains in prismatic and negative strains in basal directions, and void alignment along basal planes, can be accounted for by the anisotropy of point defect diffusion, which predicts opposite strain signs. It is speculated that formation of small SIA clusters with higher diffusion anisotropy may be responsible for the experimental observations. [ABSTRACT FROM AUTHOR]

The alignment of vacancy loops and voids along basal planes observed in irradiated Zr and Zr alloys requires anisotropic point-defect transport with a dominant contribution along the basal plane. For neutron irradiation, this can be explained by one-dimensional mobility of self-interstitial atom (SIA) clusters, but experiments with electron irradiation indicate unambiguously that even single SIA should exhibit anisotropic diffusion. No experimental information is available on SIA properties in Zr and the previousab initiocalculations did not provide any evidence of anisotropic diffusion mechanisms. An extensive investigation of SIAs in Zr has been performed from first principles using two different codes. It was demonstrated that the simulation cell size, type of pseudopotential, exchange-correlation functional and thec/aratio are crucially important for determining the properties of interstitials in hcp Zr. The most stable SIA configurations lie in the basal plane, which should lead to SIA diffusion mainly along basal planes. [ABSTRACT FROM PUBLISHER]

Abstract: Atomic displacement cascades in solids are complex phenomena, the outcome of which can be statistically characterised by properties such as their spatial extent, morphology and the spatial correlation of defects. Some properties scale in a simple way with parameters such as the cascade energy, others have limited variability with energy, for example point defect cluster size distributions. Taking advantage of the latter invariance, we use object kinetic Monte Carlo simulations to demonstrate that most properties of displacement cascade play no significant role in the evolution of point defect cluster size distributions after long enough time. It is suggested that reliable long-term predictions are possible, when using only the self-interstitial and vacancy cluster size distributions from low energy displacement cascades as building blocks to represent the complete spectrum of cascade energies obtained under neutron irradiation conditions. This is shown on the basis of recursive properties of displacement cascades evidenced for the first time and taking only approximately into account the average volumes in which vacancies and self-interstitial atoms are confined. The model has been successfully used to simulate the evolution of point defect clusters in iron for displacement rates in the range of 10−6 dpa/s and doses of the order of 0.1dpa. The applicability beyond this range and to more complex materials is discussed. [Copyright &y& Elsevier]

Abstract: A molecular dynamics technique has been used to study the impact of single vacancies and small vacancy clusters/microvoids on thermal conductivity of Si and β-SiC. It is found that single vacancies reduce thermal conductivity more significantly than do microvoids with the same total number of vacancies in the crystal. The vacancy concentration dependence of the relative change of thermal resistivity of both Si and SiC changes from linear at low concentrations to square-root at higher values. In contrast, the dependence on the volume fraction of microvoids switches from square-root at small swelling values to nearly linear dependence at higher swelling. In the case of SiC the results obtained for vacancies and microvoids agree reasonably well with experimental values. The computational results are compared with the commonly used Debye–Callaway model. [Copyright &y& Elsevier]

Abstract: Post-irradiation tensile tests were conducted on alloy F82H and variants of this steels irradiated at 573K up to 19dpa in the High Flux Isotope Reactor (HFIR) in Oak Ridge National Laboratory. Post-irradiation tensile and hardness tests revealed that the strength of F82H steeply increased below 5dpa, and the total elongation decreased. The ductility of the variants, which showed more ductility in the unirradiated condition was the same as irradiated F82H, even though the magnitude of irradiation hardening is smaller than F82H. This suggests that the softened parts of the blanket, such as heat affected zones, could show more ductility loss at this temperature. The hardening behavior of F82H with 0.09% additional tantalum (mod3), which demonstrated microstructural stability under high temperature processing, was very similar to that of F82H. Therefore mod3 can be an attractive alternate structural material for a blanket when processed above 1373K. [Copyright &y& Elsevier]

Abstract: The status and key issues of reduced activation ferritic/martensitic (RAFM) steels R&D are reviewed as the primary candidate structural material for fusion energy demonstration reactor blankets. This includes manufacturing technology, the as-fabricated and irradiates material database and joining technologies. The review indicated that the manufacturing technology, joining technology and database accumulation including irradiation data are ready for initial design activity, and also identifies various issues that remain to be solved for engineering design activity and qualification of the material for international fusion material irradiation facility (IFMIF) irradiation experiments that will validate the data base. [Copyright &y& Elsevier]

Abstract: Knowledge of the long-term evolution of the microstructure after introduction of primary damage is an essential ingredient in understanding mechanical property changes that occur during irradiation. Within the European integrated project “PERFECT,” different techniques have been developed or improved to model microstructure evolution of Fe alloys under irradiation. This review paper aims to present the current state of the art of these techniques, as developed in the project, as well as the main results obtained. [ABSTRACT FROM AUTHOR]

Abstract: Irradiation hardening and embrittlement due to neutron irradiation around 573K are the important issues on RAF/M steels. It is expected that the improvement of irradiation hardening might be one of effective ways to control the mechanical properties of RAF/M after irradiation. In this study, the purposes are to investigate the effect of heat treatments on irradiation hardening of irradiated F82H variants and to compare the irradiation hardening based on ΔHardness with the irradiation hardening obtained by ΔYield Stress about F82H. Neutron irradiation was performed in HFIR at 573K. The ion-beam irradiation experiment at ∼573K was carried out at the TIARA facility of JAEA. For the results of tensile test and hardness test of F82H and F82H heat treatment variants neutron-irradiated at 573K, all specimens caused irradiation hardening. The irradiation hardening (ΔHardness) obtained by hardness test is almost same level for neutron- and ion-irradiated F82H specimens, however irradiation hardening (ΔYield Stress) of F82H Mod-1 A (two-steps heat treated F82H: high temperature tempering and then low temperature tempering) is smaller than that of F82H. [Copyright &y& Elsevier]

Abstract: Reduced activation ferritic/martensitic steels (RAFMs) are recognized as the primary candidate structural materials for fusion blanket systems. The RAFM F82H was developed in Japan with emphasis on high-temperature properties and weldability. Extensive irradiation studies have conducted on F82H, and it has the most extensive available database of irradiated and unirradiated properties of all RAFMs. The objective of this paper is to review the R&D status of F82H and to identify the key technical issues for the fabrication of an ITER test blanket module (TBM) suggested from the recent research achievements in Japan. This work clarified that the primary issues with F82H involve welding techniques and the mechanical properties of weld joints. This is the result of the distinctive nature of the joint caused by the phase transformation that occurs in the weld joint during cooling, and its impact on the design of a TBM will be discussed. [Copyright &y& Elsevier]

Abstract: The dependence of irradiation hardening of F82H steel on tempering conditions was examined by JMTR and HFIR irradiation experiments. The JMTR and HFIR irradiations were performed to 2dpa at 250°C and 5dpa at 300°C, respectively. Irradiation hardening measured at 25–400°C depended strongly on tempering conditions, and it basically increased with temperature and time of tempering. The increment of yield stress due to irradiation was negligible at 500°C in the specimens tempered at 750°C for 0.5–10h, but was measured to be 130–200MPa in the specimens tempered at 780 and 800°C for 0.5h. Irradiation hardening of F82H that was heat treated to produce a weld-like material was also examine. The weld-like material was annealed at 800, 860 and 920°C for 0.5h, and then continuously annealed at 700°C. The level of irradiation hardening depended on the heat treatment conditions. From these results, it is suggested that the control of heat treatments such as tempering is useful to resist irradiation hardening and embrittlement. [Copyright &y& Elsevier]

Abstract: Groups of displacement cascades calculated independently with different simulation models and computer codes are compared on a statistical basis. The parameters used for this comparison are the number of Frenkel pairs (FP) produced, the percentages of vacancies and self-interstitial atoms (SIAs) in clusters, the spatial extent and the aspect ratio of the vacancies and the SIAs formed in each cascade. One group of cascades was generated in the binary collision approximation (BCA) and all others by full molecular dynamics (MD). The MD results differ primarily due to the empirical interatomic potentials used and, to some extent, in code strategies. Cascades were generated in simulation boxes at different initial equilibrium temperatures. Only modest differences in the predicted numbers of FP are observed, but the other cascade parameters may differ by more than 100%. The consequences of these differences on long-term cluster growth in a radiation environment are examined by means of object kinetic Monte Carlo (OKMC) simulations. These were repeated with three different parameterizations of SIA and SIA cluster mobility. The differences encompassed low to high mobility, one- and three-dimensional migration of clusters, and complete immobility of large clusters. The OKMC evolution was followed until 0.1dpa was reached. With the range of OKMC parameters used, cluster populations after 0.1dpa differ by orders of magnitude. Using the groups of cascades from different sources induced no difference larger than a factor of 2 in the OKMC results. No correlation could be identified between the cascade parameters considered and the number densities of vacancies and SIAs predicted by OKMC to cluster in the long term. However, use of random point defect distributions instead of those obtained for displacement cascades as input for the OKMC modeling led to significantly different results. It is therefore suggested that although the displacement cascade characteristics considered do not correlate with cluster populations in the long term, other aspects of the internal structure of cascades do. [Copyright &y& Elsevier]

Abstract: The existence of an interplay between the structure of displacement cascades and point defect mobility that influences the long-term evolution of primary damage in α-Fe is revealed by applying an object kinetic Monte Carlo (OKMC) method. The investigation was carried out using different parameter sets, which primarily differ in the description of self-interstitial atom (SIA) cluster mobility. Two sets of molecular dynamics cascades (produced with the DYMOKA and the MOLDY codes, using different interatomic potentials) and one set of cascades produced in the binary collision approximation with the MARLOWE code, using a Ziegler–Biersack–Littmark (ZBL) potential were separately used as input for radiation damage simulation. The point defect cluster populations obtained after reaching 0.1dpa were analyzed in each case and compared. It turns out that the relative influence of using different input cascade datasets on the damage features that evolve depends on which OKMC parameter set is employed. [Copyright &y& Elsevier]

Fusion reactors, advanced fission reactors and high power accelerator spallation targets subject materials to damaging particle irradiation. Although these technologies derive their utility from different nuclear reactions and divergent applications, they experience many common features. Further, the physical mechanisms of radiation response are cross-cutting. For example, swelling, phase instability, hardening, flow localization, and embrittlement must be understood in order to estimate component lifetimes. Additional commonalities include reliance on the same classes of materials and sometimes on the identical alloy for critical components. In addition, databases supporting designs are mainly derived from the same relatively few irradiation facilities and from similar types of experiments. Opportunities are examined for coordinated efforts. Emphasis is placed on the development of fundamental knowledge to support alloy design strategies for resistance to irradiation and to form a scientific basis to develop better materials. [Copyright &y& Elsevier]

Abstract: Irradiation hardening and fracture toughness of reduced-activation ferritic/martensitic steel F82H after irradiation were investigated with a focus on changing the fracture toughness transition temperature as a result of several heat treatments. The specimens were standard F82H-IEA (IEA), F82H-IEA with several heat treatments (Mod1 series) and a heat of F82H (Mod3) containing 0.1% tantalum. The specimens were irradiated up to 20dpa at 300°C in the High Flux Isotope Reactor under a collaborative research program between JAEA/US-DOE. The results of hardness tests showed that irradiation hardening of IEA was comparable with that of Mod3. However, the fracture toughness-transition temperature of Mod3 was lower than that of IEA. The transition temperature of Mod1 was also lower than that of the IEA heat. These results suggest that optimization of specifications on the heat treatment condition and modification of the minor alloying elements seem to be effective to reduce the fracture toughness-transition temperature after irradiation. [Copyright &y& Elsevier]

Abstract: This work investigates irradiation response in the joints of F82H employed for a fusion breeding blanket. The joints, which were prepared using welding and diffusion welding, were irradiated up to 6dpa in the High Flux Isotope Reactor at the Oak Ridge National Laboratory. Post-irradiation tests revealed hardening in weldment (WM) and base metal (BM) greater than 300MPa. However, the heat affected zones (HAZ) exhibit about half that of WM and BM. Therefore, neutron irradiation decreased the strength of the HAZ, leaving it in danger of local deformation in this region. Further the hardening in WM made with an electron beam was larger than that in WM made with tungsten inert gas welding. However the mechanical properties of the diffusion-welded joint were very similar to those of BM even after the irradiation. [Copyright &y& Elsevier]

Abstract: Several types of reduced activation ferritic/martensitic (RAFM) steel have been developed over the past 30years in China, Europe, India, Japan, Russia and the USA for application in ITER test blanket modules (TBMs) and future fusion DEMO and power reactors. The progress has been particularly important during the past few years with evaluation of mechanical properties of these steels before and after irradiation and in contact with different cooling media. This paper presents recent RAFM steel results obtained in ITER partner countries in relation to different TBM and DEMO options. [Copyright &y& Elsevier]