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In this study, molecular dynamics (MD) simulations were used to systematically investigate the behaviour of hydrogen (H) and helium (He) around 1/2<111> screw dislocations (SDs) and 1/2<111>{110} edge dislocations (EDs) in tungsten (W). The binding energy distributions of H and He around these two dislocations were obtained by molecular statics methods. The results revealed that both types of dislocation cores strongly attracted H and He atoms, and capture strengths were stronger for EDs than SDs. The simulations predicted that two factors influenced the interaction: strain fields of dislocations (effects of dislocations on H/He atoms) and residual stresses of H/He atoms (influences of H/He atoms on dislocations), which were validated. In the selection of trapping sites, H was synergistically affected by hydrostatic compressive and shear stress fields, and He was influenced only by shear stress fields. Conversely, the insertion of H atoms reduced the local lattice distortion of the SDs and induced the EDs to glide. He atoms repelled neighbouring atoms to increase the free volume of the trapping site. The diffusion behaviour of H/He atoms around dislocation cores was studied through nudged elastic band (NEB) and MD methods. Both H and He atoms tended to diffuse along screw channels around SD lines and one-dimensional channels inclined to ED lines, preferentially migrating along the dislocation line. The results of this study provide new insight into the retention and diffusion of H and He atoms in W. However, we have not been able to determine the clear influence of dislocations on the retention amount and diffusion tendency of H or He atom. [ABSTRACT FROM AUTHOR]

First-principles theory is applied to identify the structure of γ-Al 2 O 3 first. Then, hydrogen dissolution and diffusion behavior was further studied to reveal the reason that permeation reduction factor (PRF) of γ-Al 2 O 3 is lower than that of α-Al 2 O 3. Our calculations show that the formation energy for Al vacancy at the octahedral interstitial site (V Al − OIS − 3) is lower than that at the tetrahedral interstitial site (V Al − TIS − 3), and are far lower than those of other defects in both defective spinel and non-spinel γ-Al 2 O 3. Thus, the stability of V Al − OIS − 3 is higher than that of V Al − TIS − 3 in both Al-rich and O-rich environments. However, when hydrogen atoms are introduced into γ-Al 2 O 3 , the stability of [V Al-TIS -H]−2 is higher than that of [V Al-OIS -H]−2 in the defective spinel γ-Al 2 O 3 structure, while the opposite is true for the non-spinel γ-Al 2 O 3 structure. The vibrational frequencies for OH− in [V Al-TIS -H]−2 and [V Al-OIS -H]−2 for defective γ-Al 2 O 3 are calculated to be 3608 cm−1 and 3374 cm−1, respectively, which is in excellent agreement with the infrared (IR) absorption peaks observed at ∼3500 and ∼3300 cm−1. The defective spinel model is more suitable for describing the γ-Al 2 O 3 structure. In defective spinel γ-Al 2 O 3 , there are many native Al vacancies, which are arranged in a straight line along the [ 2 1 ¯ 1 ¯ 0 ] direction. The migration barrier for H diffusion along these native Al vacancies is so low that these Al vacancies can provide a rapid diffusion channel for H. Thus, the permeability of H in γ-Al 2 O 3 is much higher than that in α-Al 2 O 3 leading to a lower permeation reduction factor (PRF). Our results can provide not only a sound theoretical explanation for the low PRF of γ-Al 2 O 3 but also a direction to improve the efficiency in preventing H permeation through FeAl/Al 2 O 3 tritium permeation barriers (TPBs). [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations have been performed to investigate the diffusion behavior of helium (He) clusters in tungsten (W), because their diffusion properties provide basic knowledge in understanding the He bubble formation. The binding energy between He and He cluster is shown to be positive, and thus, He is easy to form bubbles by self-trapping. The mean squared displacements (MSDs) were employed to determine the diffusivities of He clusters with different sizes at different temperatures. The He bubble formation at different temperatures with 1% He was also investigated. It is revealed that the formation of He bubbles is strongly associated with the temperature and the diffusivities of the He clusters in W. The results demonstrate the initial stage of the He bubble formation and growth in W. [ABSTRACT FROM AUTHOR]

Abstract: Molecular dynamics simulations have been performed to investigate the interactions between helium (He) and defects in tungsten (W). The binding energy between He and He cluster is shown to be positive, which increases with increasing He cluster size. Both the W self-interstitial atoms (SIAs) and the vacancy can promote the He cluster formation. The binding energies of a He, a vacancy and an SIA W to a He–vacancy cluster (He nVm ) in W are also investigated, which depend on the n/m ratio. According to these results, we propose the formation and growth mechanism of He bubbles, which involves the procedures of He–vacancy cluster formation, the capturing of vacancies, then He atoms, and vacancies again. The mechanism provides a good reference to understand the initial stage of the He bubble formation and growth in W. [Copyright &y& Elsevier]

Abstract: We have constructed W–He and H–He analytical bond-order potentials for a W–H–He system. In combination with the previously self-developed W–H potential [X.-C. Li, X. Shu, Y.-N. Liu, F. Gao, G.-H. Lu, J. Nucl. Mater. 408 (2011) 12] and the Hartree–Fock-dispersion pair potential (Aziz-potential) for He−He interactions, we demonstrate that such potentials behave well for reproducing various properties of the W–H–He system such as defect formation energies, structural properties, and diffusion barriers. Such potentials can be employed to model both the He behaviours and the H–He synergetic effects in the W–H–He system. [Copyright &y& Elsevier]

Abstract: We construct modified W–H and W–W analytical bond-order potentials for a W–H system. In combination with Brenner’s H–H potential, we demonstrate that such potentials can reproduce energetics and structural properties of W and W–H systems, including defect formation energies, surface energies and diffusion barriers as well as melting point determined from first-principles or experiments. The present potentials can be employed for modelling the behaviour of H in W containing defects such as vacancies and surfaces. [Copyright &y& Elsevier]

Experiments concerning deuterium gas-driven permeation through rolled tungsten foils in the temperature range of 850–950 K and subsequent deuterium retention have been performed. The steady state permeation flux of deuterium is proportional to the square root of the driving pressure. The permeability of deuterium is in an order of 10 −14 mol m −1 s −1 Pa −1/2 in this temperature range and the activation energy for permeation is 1.21 eV. Measurements of diffusivity are significantly affected by the driving pressure, which can be well explained by a saturable-trap model. Thermal desorption spectra of samples feature a single deuterium release peak at about 873 K. TMAP 4 modeling of this peak gives a detrapping energy of 1.70 eV, which fits the dissociation enthalpy of deuterium desorbing from the inner wall of vacancy clusters or pores in tungsten. [ABSTRACT FROM AUTHOR]

We systematically study the interaction behaviors between a point defect (PD) and two types of 1/2 〈111〉 {110} and 1/2 〈111〉 {112} edge dislocations using molecular dynamics (MD), elastic dipole tensor, and kinetic Monte Carlo (KMC) methods in tungsten (W). We first determine the capture region of a vacancy and four types of self-interstitial atoms (SIAs) by calculating their binding energies to both edge dislocations using MD. We then find anisotropic distributions of the diffusion energy barriers of PDs in dislocation systems employing the MD and elastic dipole tensor methods. Subsequently, we utilize the KMC method to obtain the lifetime of PDs and calculate the capture efficiencies of both edge dislocations to PDs at various temperatures and dislocation densities. Results show the capture efficiency of a vacancy decreases with increasing temperature, while it first increases and then decreases with increasing temperature for SIAs. As dislocation density increases, the capture efficiency of PDs increases. Ultimately, the swelling rate is estimated based on the capture efficiency and falls within the range of experimental data. The swelling rate increases first and then decreases with increasing temperature, and the maximum swelling rate occurs around 1200 K, while it increases with increasing the dislocation density. The present results are critical for understanding the interaction behaviors between PDs and 1/2 〈111〉 edge dislocations in W, which provide an essential database for large-scale simulation methods such as rate theory and phase-field simulations. [ABSTRACT FROM AUTHOR]

Dislocations are common line defects in the preparation and irradiation processes of tungsten (W), and they can serve as trapping sites for hydrogen (H) and helium (He) atoms. Existing studies have indicated that dislocations preferentially capture He atoms to form dislocation-He cluster structures. The focus of this study was on the effects of this new structure on the trapped and diffusion behavior of H atoms using molecular dynamics (MD) method. The study considered the number of He atoms from 1 to 150, and they were classified as small He cluster and large He cluster. By calculating the binding energy, the study revealed that the capture strength is positively correlated with the He clusters' size. However, it is negatively correlated with the distance from the dislocation core and the captured H atoms' number. This study output the stable configurations to analyze the growth patterns of He cluster in dislocations, and it was observed that the interstitial atoms caused by trap mutation preferentially aggregate into dislocation loops due to the dislocation core structure. Moreover, the study investigated the diffusion behavior of H atoms around the dislocation-He cluster structure at 600 K within 1 ns. The hydrogen atoms tend to migrate towards the He cluster and distribute around it uniformly. The distribution range narrows with the helium cluster's size, and the distribution pattern depends on the dislocations' type. This demonstrated the formation of dislocation-He cluster structure, promoting the increased H retention. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the trapping behaviors of hydrogen (H) in molybdenum (Mo) and tungsten (W) using a first-principles method with a zero point energy correction as well as the molecular dynamics (MD) method. The H trapping is found to generally satisfy the “optimal charge density” rule at the vacancy, and a monovacancy is shown to simultaneously trap 14 H with a H2 molecule formed at the vacancy center in both Mo and W. On the other hand, the MD simulation shows the temperature decreases the number of trapped H at the vacancy. We further propose a generic vacancy trapping mechanism for H bubble formation in metals. The H atoms will first saturate the internal surface of the vacancy (or other vacancy-type defects) to form a “screening layer”, which can screen the interaction between the further trapped H and the surrounding metal atoms. This leads to the formation of H2 molecule at the vacancy center, which can be considered as the preliminary stage of H bubble nucleation. [Copyright &y& Elsevier]

In this study, we investigated the diffusion and incidence of helium on tungsten surfaces using first principles and molecular dynamics methods. For three common tungsten surfaces (100, 111, 110), we found that the diffusion barrier of helium is positively/negatively correlated with the interatomic spacing/surface atomic density. The diffusion barriers of helium are relatively similar on the three surfaces, ranging from 5.82 to 6.50 eV. The diffusion barrier of helium on the tungsten surface represents the lower limit of the incidence barrier for helium implantation into tungsten. In our simulation, only helium with an incidence energy greater than 7 eV has the potential to be implanted into tungsten. Generally speaking, the implantation ratio of helium increases with increasing incidence energy and decreases with increasing substrate temperature (although the effect is minor). At a constant temperature, the implantation ratio of helium exhibits an exponential relationship with the incidence energy. The depth distribution of helium can be well described by Gamma distribution function, and the shape and scale parameters of the gamma distribution function can be determined based on the mean and variance of the helium depth distribution. The proposed fitting function allows for the quantitative description of the mean and variance of the helium depth distribution based on the incidence conditions (incident energy E and substrate temperature T), thereby determining the helium depth distribution. This has important implications for the in-depth study of helium-induced tungsten surface fuzz formation. [ABSTRACT FROM AUTHOR]

Owing to its excellent properties, high-entropy alloys (HEAs) have been regarded as a kind of promising candidate structural material for nuclear devices. For the safety concern, it is vital to figure out the compatibility of hydrogen isotopes with the HEAs. In this work, the hydrogen isotope permeation and retention behavior of a representative HEA, CoCrFeMnNi, has been mainly checked by a gas-driven permeation device and a thermal desorption spectra device. The hydrogen isotope permeation parameters have been obtained, which are quite similar to those of the 316L austenitic steels. The hydrogen isotope retention behavior has been compared with that for a reduced activation ferritic/martensitic steel, and the relatively high hydrogen isotope inventory in HEA has been explained by crystalline structure and high density of hydrogen trapping site. [ABSTRACT FROM AUTHOR]

Abstract Atomic-level studies have been performed to investigate the hydrogen behavior near a 1/2<111>{110} edge dislocation and a 1/2<111> screw dislocation in α iron. Molecular statics analysis has been carried out to determine the stress distributions and hydrogen binding energy around the dislocation cores. The results reveal that the hydrogen binding is more sensitive to the hydrostatics stress than the shear stress around the edge dislocation, while the shear stress plays a leading role on the hydrogen binding around the screw dislocation. In addition, nudged elastic band (NEB) calculations have been applied to explore different migration paths and the corresponding migration energy barriers of a single hydrogen atom at the dislocation cores. It is of interest to note that both edge and screw dislocations are not able to offer fast pipe diffusion of hydrogen atoms along the dislocation line, as normally considered. Instead, hydrogen atoms prefer to diffuse slowly along oblique paths crossing the edge dislocation line on the slip plane and spiral paths surrounding the screw dislocation line. Furthermore, molecular dynamics simulations have been performed to study the diffusion behavior of hydrogen at the dislocation cores, which verifies the results of NEB calculations. Highlights • The distribution of the stress and hydrogen binding energies around the dislocations together with their correlations. • Neither the edge nor screw dislocations can provide fast diffusion pipes for hydrogen in α iron. • The hydrogen diffusion mechanism near the dislocation cores at different temperatures. [ABSTRACT FROM AUTHOR]

Vanadium alloys are attractive candidate structural materials in future fusion reactors. To evaluate the effects of irradiation on the deuterium retention behavior in vanadium alloys, samples made of V-5Cr-5Ti were irradiated by 5.5 MeV carbon ions. Doppler broadening measurements of the positron annihilation radiation tests were carried out to investigate the defect properties in the irradiated samples. Then the irradiated and virgin samples were implanted with deuterium in an ECR (electron-cyclotron resonance) linear plasma device followed by thermal desorption spectroscopy experiments. It was found that the irradiation process introduced large density of vacancy-type defects and subsequently increased the deuterium retention in the V-5Cr-5Ti, which remains as a serious concern for the vanadium alloys in the application as the structural materials of the fusion blanket. [ABSTRACT FROM AUTHOR]

To evaluate the effects of re-deposited tungsten (W) on the surface modification and hydrogen isotope retention behavior of fusion structural materials, the plasma exposure behavior of re-deposited W samples prepared by magnetron sputtering on the F82H steel, the V-5Cr-5Ti alloy as well as bare substrate samples was investigated. All the samples were exposed to 367 shots of deuterium plasmas in the 2015 spring EAST campaign. After the plasma exposure, large area of W layer was exfoliated, while big blisters were found at the interface between the remaining W layer and the substrate materials. The deuterium retention behavior of the samples with re-deposited W layer was characterized by thermal desorption spectroscopy and compared with the bare substrate samples. [ABSTRACT FROM AUTHOR]

To evaluate the capability of Fe Cr Al ferritic steels as tritium permeation barrier in fusion systems, the oxidation behavior together with the permeation behavior of a Fe Cr Al steel was investigated. Gas driven permeation experiments were performed. The permeability of the oxidized Fe Cr Al steel was obtained and a reduced activation ferritic/martensitic steel CLF-1 was used as a comparison. In order to characterize the oxide layer, SEM, XPS, TEM, HRTEM were used. Al 2 O 3 was detected in the oxide film by XPS, and HRTEM showed that Al 2 O 3 in the α phase was found. The formation of α-Al 2 O 3 layer at a relatively low temperature may result from the formation of Cr 2 O 3 nuclei. [ABSTRACT FROM AUTHOR]

The influence of the transmutation element rhenium (Re) content with 3 and 25 wt.% on deuterium (D) permeation in tungsten (W) was investigated. It was found that the addition of Re increased D diffusion coefficient and decreased D permeability, and the D diffusion coefficient of W-3%Re was approximate to that of W-25%Re while the D permeability declined with the increase of Re content. The D retention behavior in the W-Re alloys have been studied after D plasmas exposure employing the Material and Plasma Evaluation System (MAPES) in Experimental Advanced Superconducting Tokamak (EAST). As for the higher D solution energy in W-Re alloys, the total D inventory in W was significantly decreased by Re addition. [ABSTRACT FROM AUTHOR]

The diffusion behavior of hydrogen (H) in tungsten (W) has been studied by molecular dynamics simulations. The diffusivities from 200 K to 3000 K are calculated and the diffusion equation is fitted to D = 5.13 × 10 −8 exp(−0.21 eV / kT) m 2 /s . The diffusion equations in different temperature ranges of 200–500 K, 500–2400 K and 2400–3000 K are also given separately and the results imply different H diffusion modes in different temperature ranges, which is proved by analyzing equilibrium H positions at low and high temperatures. The H atom jumps between different tetrahedral interstitial sites (TISs) at lower temperatures, but the transition to octahedral interstitial sites (OISs) is also observed at high temperatures. Moreover, with a vacancy present in the W system, vacancy trapping of H is observed, and it is shown that the vacancy trapping reduces with the increasing temperature. The H binding energy to monovacancy is obtained using three different methods including NEB and fitting both H diffusivity and time for H to detrap from the vacancy, which provides more information of the H behaviors with the existence of defects. [ABSTRACT FROM AUTHOR]

Pebbles made of lithium-containing ceramics with a few millimeters in diameter have been regarded as the main candidates for the tritium breeding materials. In this work, X-ray computed tomography (X-ray CT) technique has been employed to observe the microstructure of tritium breeding pebbles. The 3D pore network of the pebbles was presented and compared with normal 2D images. Based on the CT results, simulations of the heat transfer in one single pebble have been performed employing lattice Boltzmann method and showed that the gas atmospheres and microstructure have apparent influences on the thermal conductivities of pebbles. [ABSTRACT FROM AUTHOR]

• A possible mechanism for the formation of vacancy dislocation loops in α-Fe is proposed. • Vacancy-NH clusters prefer to shape into 2D clusters, especially on (211) planes. • The presence of H can greatly promote the growth of vacancy dislocation loops. Genetic algorithm was applied to search the energy minimization configuration of vacancy clusters in α-Fe. Molecular statics calculations and dynamics annealing relaxation were employed to calculate the formation and binding energies of vacancies and 3D vacancy or vacancy-hydrogen (H) clusters, as well as 2D vacancy or vacancy-H clusters on (111), (011) and (211) planes. Our calculations show that vacancies prefer to form 3D clusters and vacancy dislocation loops are difficult to form, while vacancy-H clusters prefer to shape into 2D clusters, especially on (211) planes. Since H prefers the directional bonding, a vacancy cluster with H atoms tend to form a vacancy dislocation loop with its slip direction along the 〈100〉 direction and on the (211) habit plane. Our results are consistent with the experimental observations, and provide a possible mechanism for the formation of vacancy dislocation loops in α-Fe. Furthermore, we have also explored how dislocation loops trap self-interstitial atoms, vacancies and H atoms. It is of interest to note that H atoms strongly bound to a 〈100〉 vacancy dislocation loop, and are able to enhance the ability for the dislocation loop to further trap vacancies and reduce its ability to absorb self-interstitial atoms, thus promoting the growth of the 〈100〉 vacancy dislocation loops. [ABSTRACT FROM AUTHOR]

Alumina is regarded as one of the most promising candidate tritium permeation barrier (TPB). Through thermal oxidization of Al-contained alloys Fe–Cr–Al, alumina layer with high tritium permeation reduction ability can be obtained. After bonding this kind of materials with structural materials such as reduced activation ferritic/martensitic (RAFM) steels, it can serve as TPB. In this work, efforts have been done to enhance the hydrogen isotope permeation reduction ability of the alumina layer on the Fe–Cr–Al ferritic steel by optimizing the oxidation process. The oxidation temperature of the Fe–Cr–Al ferritic steel is set to 973 K, which is lower than the final heat treatment temperature of the RAFM steel. Three different atmospheres have been employed for the oxidation process of the Fe–Cr–Al ferritic steel. Gas driven permeation (GDP) experiments have been performed to examine the deuterium permeability of the oxidized Fe–Cr–Al ferritic steel. The deuterium permeability of the Fe–Cr–Al ferritic steel oxidized in argon with 1700 ppm oxygen is 104 times lower than that of the RAFM steel at 823 K. The microstructure and chemical composition of the oxide layer of the Fe–Cr–Al ferritic steel oxidized in three different atmospheres has been clarified using Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) methods. It is suggested that oxidizing the Fe–Cr–Al ferritic steel at reduced oxygen atmosphere could inhibit the growth of the iron oxide and chromium oxide in the oxide layer, thereby leading to a dense and compact alumina layer that has excellent hydrogen permeation reduction performance. [ABSTRACT FROM AUTHOR]