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After metallographic etching, unetched white bright bands (WBB) are evident in the SA508-3/52(M) dissimilar metal weld joints. The WBBs in both joint without a buttering layer (JWOBL) and joint with a buttering layer (JWBL) are composed of austenite, with no presence of martensite. A pronounced elemental gradient is present within the WBB. The width of the WBB in JWOBL exceeds that in JWBL. Owing to differences in the welding thermal cycling process, precipitates are present in the WBB of JWOBL, whereas JWBL lacks such precipitates. These precipitates hinder dislocation movement and assist in the nucleation and growth of helium bubbles. Consequently, the nanoindentation hardness and helium bubble number density of the WBB in JWOBL are significantly higher than those in JWBL. Irradiation damage, including helium bubbles, induces irradiation hardening of the joints. At elevated irradiation temperatures, however, the extent of irradiation hardening diminishes.

Vanadium is a typical low-activation metal and has the advantages of lower neutron irradiation activation, better mechanical properties at high temperature, and higher compatibility with the liquid lithium blanket. However, the effect of helium on the formation of irradiation defects in vanadium has not been adequately explored at low temperatures (below 723 K). Helium ion irradiations of 18 keV up to 0.54 displacement per atom were employed to study the temperature-dependent behavior of irradiation defects in vanadium at 523, 623, and 723 K. Helium bubbles were observed in vanadium under irradiations at all temperatures, but no dislocation loops were observed. With the increase of irradiation temperature, the average size of helium bubbles and swelling increased, and the density of helium bubbles decreased. It is noteworthy that the average size of helium bubbles and swelling increased significantly when the irradiation temperature increased from 623 to 723 K. In addition, pentagonal helium bubbles, helium bubbles nucleated at the grain boundary, and combinations between helium bubbles were observed. [ABSTRACT FROM AUTHOR]

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A reduced cluster dynamics model is developed to investigate the microstructure evolution of irradiated single crystal tungsten. The model accounts for the generation and reaction of point defects, small-size defect clusters, helium clusters, as well as the nucleation and growth of large immobile defects, including the interstitial dislocation loops, voids and helium bubbles. Moreover, by incorporating an atomically informed loop punching mechanism for bubble growth, the model is able to accurately capture the evolution kinetics of radiation-induced defects with and without the helium implantation. The predicted densities and sizes of loops and voids/bubbles, the helium-to-vacancy ratio and the internal pressure of helium bubbles are all in good agreement with experimental data at different irradiation doses across a wide range of temperatures from 300 K up to 1200 K. This work aims to provide a robust tool for analyzing the microstructure of irradiated tungsten under both fission and fusion conditions.

Gaining insight into the underlying mechanisms driving nanoindentation-induced behavior in tungsten is essential for comprehending its mechanical properties. Although the plasticity of conventional pure tungsten under nanocontact conditions is well understood, these theoretical constructs may become inadequate when applied to irradiated tungsten due to the interplay between helium bubbles and dislocations. Here, we employed an integrated approach, combining crystal defect theories, molecular dynamics simulations, and machine learning, to explore the initiation and progression of dislocations in tungsten containing helium bubbles during nanoindentation, focusing on elucidating the impact of helium bubbles. In contrast to the typical dislocation nucleation in pure tungsten, the presence of helium bubbles mitigates local shear strain, thereby hindering dislocation nucleation and propagation, leading to a reduction of indentation force. Additionally, we conducted a comparative analysis between samples with and without helium bubbles, examining factors such as atomic displacement, strain localization parameters, dislocation line length, and stress component distribution. More importantly, a significant outcome of our study is the establishment of a relationship between the consistent alteration in the shape of helium bubbles and their size during micro-scale nanoindentation, achieved through machine learning techniques. Our findings reveal that the area occupied by helium bubbles post-nanoindentation is inversely correlated with indentation depth and directly linked to helium bubble size. These discoveries represent a notable advancement in understanding the effects of irradiation to a certain degree.

The study of metals and alloys containing helium has garnered significant attention within the nuclear energy community. However, there is limited research on the mechanical behavior of bulk alloys implanted with helium. This study investigates the mechanical properties of several Al-Boron alloys implanted with helium using controlled manipulation of helium doses via boron content under a consistent neutron dose. Results show that HemVn may contribute to strength by approximately 8.4–15 MPa and 16.8–23 MPa for helium doses 3.08 × 1019/cm3 and 6.84 × 1019/cm3, respectively, while lattice damages due to neutron-aluminum reaction contribute to strength by 24∼27 MPa. Subsequent annealing leads to the formation of helium bubbles, resulting in a slightly higher strengthening effect compared to HemVn. Additionally, the work hardening behavior of the alloys can be explained by the Voce model, drawing inspiration from the resemblance between helium bubbles and nanoprecipitates in 7xxx alloys. These findings provide insights to the nuclear energy community.

Tritium (T) is a costly radioactive element that, when retained in plasma-facing materials (PFMs), not only results in fuel loss but also raises issues of radioactive contamination. Hydrogen isotope exchange is a potential method for T removal in future fusion devices. However, in the nuclear fusion environment, PFMs will be subjected to low-energy and high-flux helium (He) plasma irradiation, forming a He bubble layer near the material surface. This greatly impacts the diffusion and retention behavior of hydrogen isotopes in PFMs. In this work, a multi-component hydrogen isotopes exchange model was developed considering the presence of He bubble layers. The influence of He bubbles location on hydrogen isotope retention and exchange behavior was investigated. It is found that when the hydrogen (H) particles implanted outside the He bubble layer, H entering the material would diffuse across the He bubble layer, resulting in a decrease of H amount entering the bulk. In other words, a thicker He bubble layer leads to lower H retention. As to isotope exchange, when there are deuterium (D) retained in the material, implanted H has a possibility to exchange with D, making D release from the materials. However, the D atoms exchanged out by H also need to diffuse across the He bubble layer, further reducing the D release rate. The results showed the barrier effect of the He bubble layer can have two distinct effects on hydrogen isotope exchange. One is that with enhancing the effect of the He bubble layer, the number of H entering the bulk will increase, resulting in an increase in the D removal. The other effect is that with strengthening the barrier effect of the He bubble layer, the D atoms exchanged out will be more inclined to diffuse into the bulk, leading to a decrease in the D removal. As a result, the D removal efficiency exhibits a peak considering the barrier effect of He bubble.

Both the nanoscale helium (He) bubble and grain boundaries (GBs) play important roles in the dynamic mechanical behavior of irradiated nanocrystalline materials. Using molecular dynamics simulations, we study the shock-induced deformation and spallation failure of bicrystal copper with a nanoscale He bubble. Two extreme loading directions (perpendicular or parallel to the GB plane) and various impact velocities (0.5–2.5 km/s) are considered. Our results reveal that the He bubble shows hindrance to the propagation of shock waves at lower impact velocities but will accelerate shock wave propagation at higher impact velocities due to the local compression wave generated by the collapse of the He bubble. The parallel loading direction is found to have a greater effect on He bubble deformation during shock compression. The He bubble will slightly reduce the spall strength of the material at lower impact velocities but has a limited effect on the spallation process, which is dominated by the evolution of the GB. At lower impact velocities, the mechanism of spall damage is dominated by the cleavage fracture along the GB plane for the perpendicular loading condition but dominated by the He bubble expansion and void growth for the parallel loading condition. At higher impact velocities, micro-spallation occurs for both loading conditions, and the effects of GBs and He bubbles can be ignored. [ABSTRACT FROM AUTHOR]

Future nuclear reactors and advanced power generators require materials with good stability and damage tolerance under harsh conditions, including high temperatures and high-dose radiation. Ti 3 SiC 2 MAX phase has good physical properties and mechanical strength. It can remain crystalline under serious microstructure damage due to the nanolaminate structure. In this study, the effects of helium in irradiated Ti 3 SiC 2 at up to 1100 °C were investigated by microstructural and chemical composition analysis. The concentrated helium can grow into large bubbles without significant confinement or capture by the nano-laminated layers. A new hexagonal to fcc phase transformation mechanism, driven mainly by the evolution of the helium bubbles accompanied by Si diffusion and depletion, is found and investigated. Si interstitials are forced to move out from the peak helium region by the helium evolution and segregate at the outermost surface, forming a thin Si-O layer, at 1100 °C. The formation of the fcc phase is the result of chemical compositional changes and local compressive stress contributed by He bubbles. [ABSTRACT FROM AUTHOR]

The AISI 316L austenitic stainless steel fabricated by selective laser melting (SLM) is considered to have great prospects for applications in nuclear systems. This study investigated the He-irradiation response of SLM 316L, and several possible reasons for the improved He-irradiation resistance of SLM 316L were systematically revealed and evaluated by using TEM and related techniques. The results show that the effects of unique sub-grain boundaries have primary contributions to the decreased bubble diameter in SLM 316L compared to that in the conventional 316L counterpart, while the effects of oxide particles on bubble growth are not the dominant factor in this study. Moreover, the He densities inside the bubbles were carefully measured using electron energy loss spectroscopy (EELS). The mechanism of stress-dominated He densities in bubbles was validated, and the corresponding reasons for the decrease in bubble diameter were freshly proposed in SLM 316L. These insights help to shed light on the evolution of He bubbles and contribute to the ongoing development of the steels fabricated by SLM for advanced nuclear applications. [ABSTRACT FROM AUTHOR]

• Evolutions of helium bubbles on different internal sinks and temperatures were studied in-situ. • The helium capture strength of precipitate is greater than that of grain boundary. • Helium injection causes larger bubbles at 673 K. • Swelling increases with irradiation dose and peaks at 673 K. Implanted helium resulting from fusion reactions can pose a significant threat to zirconium alloys used as fusion reactor materials. Intrinsic structures and temperatures in nuclear materials can significantly influence the behavior and distribution of these helium bubbles under irradiation. Therefore, the behavior of helium bubbles in zirconium alloys was investigated by in-situ He+ irradiation at 573 K, 623 K, 673 K, 723 K and 773 K. Results indicated that the smallest and densest bubbles are found at GBs, while the largest helium bubbles are observed near precipitates. The size of the helium bubbles first increased and then decreased with an increase in temperature, while the density of bubbles changed in the opposite trend. The swelling induced by irradiation was maximum at 673 K when the irradiation dose was constant, while at this temperature, the geometry of the helium bubble changed from spherical to polygonal with a larger size than those at other temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

A novel theoretical model based on modified diffusion rate equations is proposed to simulate the retention of hydrogen isotopes and the dynamics of bubble growth in tungsten (W) when exposed to simultaneous hydrogen (H) and helium (He) plasma irradiations. Simulation is conducted to assess the influence of temperature as well as simultaneous H and He irradiation at an increasing fluence. Not only to develop a holistic understanding but also to substantiate simulation findings about synergy between H and He plasma irradiation, a W sample is exposed sequentially to H and He plasma at 873 K using the large-power material irradiation experimental system. The topographical changes in the W sample are investigated using atomic force microscopy (AFM) after each plasma irradiation exposure sequence. Simulation results reveal that the ability of a bubble containing both H and He to trap adjacent H/He atoms is primarily governed by their individual partial pressure within the bubble. Furthermore, at elevated temperatures, the synergy between H and He significantly enhances the retention of H isotopes in W. AFM micrographs of the W sample exposed to both H and He plasma irradiation show a severely damaged and locally delaminated layer, absent in the sample exposed only to either H or He, conclusively establishing evidence of synergy between H and He irradiation effects. The average bubble radius computed using the model aligns excellently with experimentally determined values obtained through SEM/AFM analysis. The robustness of the proposed model is also assessed by comparing bubble radius and H isotopes retention at various temperatures with experimental data reported in the literature.

The helium effects in Chinese developed CN-G01 beryllium are important issues for its use in nuclear energy systems. In this work, the CN-G01 beryllium samples were irradiated with helium ions to fluences of 5.0 × 1016 ions/cm2 to 1.0 × 1018 ions/cm2 at room temperature and investigated by techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nano-indentation. It was found that the irradiation induced hardening of beryllium and the nano-hardness of the samples increased with increasing fluence of 5.0 × 1016 ions/cm2 to 1.0 × 1017 ions/cm2. When the fluence reached 5.0 × 1017 ions/cm2 and 1.0 × 1018 ions/cm2, helium irradiation induced serious surface blistering and its burst. TEM observation found that helium bubbles in the damage peak region became visible when the fluence reached 1.0 × 1017 ions/cm2. With increasing fluence, helium bubbles became larger and connected into large cracks. The underlying physical mechanisms are discussed based on the helium behavior at low temperatures and the contributions of helium induced defects. This work will provide some new understanding on the irradiation resistance of CN-G01 beryllium and the helium effects in beryllium at low temperatures. [ABSTRACT FROM AUTHOR]

In Tokamak fusion devices, tungsten divertor will be bombarded with low-energy high-flux helium ions, resulting in the formation of fuzzy nanostructures or pinhole-like surface damage on tungsten surface, which degrades the tungsten material properties and affects the steady-state operation of the plasma. It is now generally accepted that helium-induced surface damages are closely related to the formation and growth of helium bubbles under the tungsten surface. After helium irradiation, tungsten will form a high density of small helium bubbles near surface. The coalescence of helium bubbles is one of the major ways for the formation of large helium bubbles. In order to understand the effects of relative position, temperature, He/V and initial spacing of helium bubbles on helium bubble fusion, molecular dynamics method was used to simulate the fusion process of helium bubbles in tungsten. The results show that the coalescence of the helium bubbles is affected by the relative positions of the helium bubbles, the temperature, the helium-to-vacancy ratio (He/V), and the distance between helium bubbles, but the influence mechanism is not the same. Specifically, the relative position of the helium bubbles is the key factor affecting the coalescence of the helium bubbles. When the helium bubbles are arranged along the 〈100〉 direction, they tend to coalesce. In contrast, when they are arranged along the 〈111〉 direction, coalescence is not easy to happen. This is because there is an anisotropic stress field near the helium bubbles. The higher the temperature is, the faster and more sufficient relaxation of the helium bubbles will be obtained, resulting in promoted coalescence. Helium bubbles with higher He/V have higher pressure, so they are more likely to coalesce. When the temperature is 1 500 K, the maximum distance of coalescence is 0.96 nm for two helium bubbles with a radius of 1 nm and a He/V of 3, while the interaction distance between them can reach 1.28 nm or more. This study can promote the understanding of the coalescence mechanism of helium bubbles in tungsten and provide a possible explanation for the formation of large helium bubbles in tungsten. In addition, the results of this study can provide relevant input parameters for large-scale simulations (such as kinetic Monte Carlo, cluster dynamics) to study the long-time evolution of high-density helium bubbles.

Both the nanoscale helium (He) bubble and grain boundaries (GBs) play important roles in the dynamic mechanical behavior of irradiated nanocrystalline materials. Using molecular dynamics simulations, we study the shock-induced deformation and spallation failure of bicrystal copper with a nanoscale He bubble. Two extreme loading directions (perpendicular or parallel to the GB plane) and various impact velocities (0.5–2.5 km/s) are considered. Our results reveal that the He bubble shows hindrance to the propagation of shock waves at lower impact velocities but will accelerate shock wave propagation at higher impact velocities due to the local compression wave generated by the collapse of the He bubble. The parallel loading direction is found to have a greater effect on He bubble deformation during shock compression. The He bubble will slightly reduce the spall strength of the material at lower impact velocities but has a limited effect on the spallation process, which is dominated by the evolution of the GB. At lower impact velocities, the mechanism of spall damage is dominated by the cleavage fracture along the GB plane for the perpendicular loading condition but dominated by the He bubble expansion and void growth for the parallel loading condition. At higher impact velocities, micro-spallation occurs for both loading conditions, and the effects of GBs and He bubbles can be ignored.

The formation and migration of helium bubbles lead to performance degradation of structural materials in nuclear reactors. Intrinsic structures in nuclear materials can significantly influence the behavior and distribution of these helium bubbles under irradiation. Micro-nano composite 304L stainless steel (MN304-La) possesses three main kinds of intrinsic structures: grain boundaries (GBs), dislocations, and La-rich nano-precipitates (NPs). In this study, the helium bubbles on different intrinsic structures in MN304-La after He+ ions implantation were characterized and analyzed. We compared the capabilities of different intrinsic structures in capturing helium bubbles and confirmed that NPs exhibit the strongest strength in capturing helium bubbles among the three kinds of intrinsic structures. We found that the competition of intrinsic structures for capturing helium bubbles depends on both the capture strength of the intrinsic structures and the capture distance to the helium bubbles.

When tungsten (W) is used as a plasma-facing material in fusion reactors, it will suffer irradiation from the high-energy neutron and high-flux low-energy helium (He) plasma, affording voids and He bubbles with varying helium-to-vacancy ratios (He/V). With the accumulation of irradiation dose, an energetic neutron is very likely to trigger collision cascades close to the pre-existing voids/He bubbles. Therefore, in this study, we ran systematic molecular dynamics simulations of the cascades that overlapped with voids/He bubbles in W. We investigated the relationship between the evolution of the stress field and defect formation. Cascades overlapping with the bubbles with He/V values between 1 and 2 produced the least defects; alternatively, other bubbles could facilitate defect formation. Furthermore, two mechanisms for generating dislocation loops, the cascade-destruction and the over-pressured bubble-assist types, were identified and can be attributed to subcascade formation. When there are voids or under-pressured bubbles, the cascade-destruction mechanism can yield vacancy-type dislocation loops; however, the existence of He atoms can inhibit this behavior. In contrast, when the bubbles are over-pressured, the bubble-assist mechanism can contribute to the formation of self-interstitial atom-type dislocation loops. This study helps to improve our understanding of radiation damage in a complicated environment in experiments with multiple irradiations or fusion reactors.

In irradiated tungsten (W), the formation of numerous helium (He) bubbles has a significant impact on the diffusion behaviors of hydrogen (H) isotopes. To investigate the influence of the microstructure of He bubbles on the diffusion behaviors of deuterium (D), we utilize the reconstructed experimental data, the phase-field method, and the steady-state diffusion equation to calculate the effective diffusion coefficients of D based on four possible diffusion paths, i.e., D atoms diffuse in the bulk W, inside the He bubbles, and along the outer and inner surface of He bubbles. Simulation results based on the reconstructed depth-dependent distribution of He bubbles reveal that the diffusion paths remarkably affect the effective diffusion coefficient of D. By varying the radius and number density of He bubbles, the effective diffusion coefficient of D undergoes significant changes. Subsequently, we fit an empirical formula of the effective diffusion coefficient of D as a function of the radius and number density of He bubbles for different diffusion paths of D based on simulation data. Besides, the growth behaviors of He bubbles are simulated by the phase-field model, and the effective diffusion coefficient of D as a function of the evolutionary time for different diffusion paths is also discussed. At last, we investigate the influence of the D concentration and temperature on the effective diffusion coefficient of D along different diffusion paths. These results indicate that the effective diffusion coefficient of D increases with decreasing the D concentration and increasing the temperature for four diffusion paths. The current study provides a reference for investigating the diffusion behaviors of D in W in the presence of He bubbles, and may benefit the efforts of developing low D retention W-based materials. [ABSTRACT FROM AUTHOR]

Plutonium and its derivatives have been demonstrated with a wide range of research and applications in nuclear energy, nuclear devices, radioactive waste storage, basic science, etc. As a radioactive element, plutonium inexorably undergoes alpha decay, which makes the properties of Pu-based materials (e.g. , structure, composition, and density) change over time, namely, aging. Those properties can also be influenced by external chemical corrosion and internal phase stability of the materials, which are aging processes, either. Therefore, the application performances of Pu-related materials are dramatically affected by the aging of plutonium. This review covers the important progress of Pu aging and comprehensively introduces the fundamental mechanisms and the affected material properties. Future perspectives are discussed on the opportunities and challenges in deeply investigating Pu aging. [ABSTRACT FROM AUTHOR]

• The gradient convergence method and a novel generative model were proposed to train a highly accurate model with limited and unlabeled data. • Proposed a novel fine-tuning strategy, achieving highly stable precision and recall performance on helium bubble images of additive manufacturing and traditionally cast 304 L stainless steel under different conditions. • For evaluating material irradiation performance, our method greatly enhanced the image analysis process, achieving a several hundred-fold speedups and significantly improving accuracy. • Integrated artificial intelligence into the workflow of manufacturing research, greatly improved efficiency and can lead to the discovery of new results. Additive manufacturing technology has received significant attention in the field of nuclear materials due to its potential to improve the radiation resistance of materials and components. In our research, we propose to use 304 L stainless steel fabricated by Selective Laser Melting (SLM) as a replacement for traditional casting due to its superior performance and optimized manufacturing technique. During testing of its radiation resistance, we found that analyzing helium bubbles captured by transmission electron microscopy (TEM) is crucial for understanding and predicting material behavior under irradiation. However, this task typically involves manual counting, which is both time-consuming and prone to errors due to the fatigue of human annotators. To address this challenge, we present a novel machine learning model for automatic helium bubbles detection and counting through images. Our model leverages a fusion of traditional computer vision techniques with cutting-edge machine learning methods to achieve superior detection of helium bubbles in nuclear materials. This approach is based on two core designs: a novel generative model to generate training data and a gradient convergence layer to limit the range of parameters. Experiments show that our model outperforms previous state-of-the-art unsupervised detectors, and achieves highly accurate helium bubble segmentation in a few-shot scenario, with an F1 score typically exceeding 90 % and an average size error less than 0.1 nm. Our model achieves over 100 times more efficiency than manual counting, which takes approximately 20 s per image. Our work demonstrates a successful collaboration between the fields of material characterization and artificial intelligence (AI). By leveraging the respective strengths of material science and machine learning, we have achieved surprising results that could have a significant impact on the development of new materials and components for nuclear applications. [ABSTRACT FROM AUTHOR]

Helium bubbles are known to form in nuclear reactor structural components when displacement damage occurs in conjunction with helium exposure and/or transmutation. If left unchecked, bubble production can cause swelling, blistering, and embrittlement, all of which substantially degrade materials and—moreover—diminish mechanical properties. On the mission to produce more robust materials, nanocrystalline (NC) metals show great potential and are postulated to exhibit superior radiation resistance due to their high defect and particle sink densities; however, much is still unknown about the mechanisms of defect evolution in these systems under extreme conditions. Here, the performances of NC nickel (Ni) and iron (Fe) are investigated under helium bombardment via transmission electron microscopy (TEM). Bubble density statistics are measured as a function of grain size in specimens implanted under similar conditions. While the overall trends revealed an increase in bubble density up to saturation in both samples, bubble density in Fe was over 300% greater than in Ni. To interrogate the kinetics of helium diffusion and trapping, a rate theory model is developed that substantiates that helium is more readily captured within grains in helium-vacancy complexes in NC Fe, whereas helium is more prone to traversing the grain matrices and migrating to GBs in NC Ni. Our results suggest that (1) grain boundaries can affect bubble swelling in grain matrices significantly and can have a dominant effect over crystal structure, and (2) an NC-Ni-based material can yield superior resistance to irradiation-induced bubble growth compared to an NC-Fe-based material and exhibits high potential for use in extreme environments where swelling due to He bubble formation is of significant concern. [ABSTRACT FROM AUTHOR]

Two different of Sm-loading fluorapatite (Ca10−2xNaxSmx(PO4)6F2, x = 1 and 2) glass-ceramics were synthesized by a two-step melt sintering method. The samples were irradiated with 50 keV He+ ions with a fluence of 2.6 × 1016 ions/cm2 at 593 K. The irradiation induced microstructural evolution were characterized by grazing incidence X-ray diffraction and cross-sectional transmission electron microscopy. For the smaller Sm-doping samples, no phase transformation is observed. Meanwhile, in the lager Sm-doping samples, the irradiation induced the crystals into smaller nanocrystals. The mechanism of the transformation of the crystalline phase was also analyzed and discussed. [ABSTRACT FROM AUTHOR]

Investigation of the influence of helium bubbles on the dynamic strength has drawn continuous attention. In this article, the phase field method (PFM) is applied to investigate the evolution of helium bubbles at an early stage in shock-loaded aluminum based on its advantage to describe the interface evolution. PFM is coupled with the crystal plasticity finite element method (CPFEM), which makes it possible to investigate the interaction between the helium bubbles and the collective behaviors of dislocation assembles. It is found that the heterogeneity of helium bubbles induces a local concentration of plastic deformation, which leads to a rarefaction wave along the propagation direction of the shock wave. From an energy perspective, it is inferred that both the growth of helium bubbles and the plastic deformation are driven by the strain energy, which indicates that these two processes may compete with each other. [ABSTRACT FROM AUTHOR]

In present study, Li 4 SiO 4 ceramics were sequentially irradiated at room temperature using 540 keV He+ and 250 keV H+ with ion fluences of 1 × 1016 ions/cm2 and 1 × 1017 ions/cm2, corresponding to 2.14 displacements per atom (dpa) and 21.4 dpa, respectively. The change of featured structures and the long-term evolution of helium bubbles in irradiated Li 4 SiO 4 were investigated by grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and transmission electron microscope (TEM). For the as-irradiated Li 4 SiO 4 , the results revealed that the lattice parameter and the amorphous fraction increased and the crystallinity decreased with the increasing irradiation dose, and the anionic disordering was the mainly induced damage. When the irradiation dose reached to 21.4 dpa, high-density bubbles with a mean diameter of 1.6 nm were observed in Li 4 SiO 4. The following 480-day aging at room temperature rendered bubble growth due to surface diffusion, which was demonstrated by the fact of an increase in bubble size (mean diameter of 18.4 nm) and a decrease in bubble density. The formation and growth of bubbles could benefit from the presence of amorphous structures and hydrogen atoms. Furthermore, the dynamic evolution of bubbles during annealing at 723 K was monitored by in-situ TEM, and the results indicated that the migration and coalescence mechanism dominated the growth of bubbles in Li 4 SiO 4 under relatively low temperatures. [ABSTRACT FROM AUTHOR]

The helium effects in Chinese developed CN-G01 beryllium are important issues for its use in nuclear energy systems. In this work, the CN-G01 beryllium samples were irradiated with helium ions to fluences of 5.0 × 1016 ions/cm2 to 1.0 × 1018 ions/cm2 at room temperature and investigated by techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nano-indentation. It was found that the irradiation induced hardening of beryllium and the nano-hardness of the samples increased with increasing fluence of 5.0 × 1016 ions/cm2 to 1.0 × 1017 ions/cm2. When the fluence reached 5.0 × 1017 ions/cm2 and 1.0 × 1018 ions/cm2, helium irradiation induced serious surface blistering and its burst. TEM observation found that helium bubbles in the damage peak region became visible when the fluence reached 1.0 × 1017 ions/cm2. With increasing fluence, helium bubbles became larger and connected into large cracks. The underlying physical mechanisms are discussed based on the helium behavior at low temperatures and the contributions of helium induced defects. This work will provide some new understanding on the irradiation resistance of CN-G01 beryllium and the helium effects in beryllium at low temperatures.

This study aims to reduce the size and density of He bubbles and dislocation loops during the irradiation process by constructing nanostructure engineering through precipitation. The Ni 35 (CoFe) 55 V 5 Nb 5 high-entropy alloy (HEA) with solid solution and aging states underwent He2+ ions irradiation (500 keV at a fluence of 1017 ions∙cm−2) at 500 °C. The results indicate that the preexisting nanoprecipitates increased the surface area for He atoms adsorption, and improved the ability of the alloy to suppress the coarsening of He bubbles. In the aged HEA, He bubble size decreased (∼3.3 ± 0.4 nm), and density significantly reduced (0.58 ± 0.14 × 1023 m−3) compared to the solid solution HEA (∼6.1 ± 0.3 nm, 2.42 ± 0.15 × 1023 m−3). Moreover, the preexisting nanoprecipitates coordinate the movement and recombination of irradiation defects, which overwhelms the energy driving force that improves the stability and growth of dislocation loops due to the reduction of stacking fault energy, thereby suppressing the formation and growth of dislocation loops. The aged HEA exhibited reduced size and density of dislocation loops (∼6.3 ± 0.5 nm and 0.78 ± 0.15 × 1020 m−3) compared to the solid solution HEA (∼8.5 ± 0.6 nm, 3.83 ± 0.13 × 1020 m−3). • The effect of He2+ ion irradiation at 500 °C on the microstructure evolution of a high entropy alloy was investigated. • The high-density nanoprecipitates can increase the surface area of particles capable of adsorbing helium atoms. • The nanoprecipitates acting as sink points for radiation defects suppress the accumulation of radiation defects. [ABSTRACT FROM AUTHOR]

Increasing the interface density in nuclear-material systems has been proposed to be a promising strategy for annihilating defects introduced by ion irradiation. The role of interface characteristics between the WTaTiVCr refractory high entropy alloy (RHEA) and metal tungsten (W) on the irradiation resistance is rather unclear because of the distinct defect energy that exists in these two kinds of materials. Here, the nano-crystalline and amorphous mixed structures were obtained in WTaTiVCr/W multilayers to investigate the irradiation tolerance and mechanical properties. After being exposed to a 60 keV helium (He) ion irradiation platform at a fluence of 1 × 1017 cm−2, the grain sizes in irradiated multilayers were slightly decreased from 12.4 nm to 7.2 nm with increasing individual thickness, which was explained from the perspective of He+ energy deposition. The dependence of bubble growth in multilayers on the individual thickness was disclosed based on the analysis of bubble size and its pressure in the RHEA and W layers. The phase structures and element distributions in WTaTiVCr/W multilayers were also revealed by the defect formation and crystallization. Also, a maximum irradiation hardening of 1.91 GPa in the RHEA(10)/W(7) multilayer was disclosed, and the irradiation hardening in the multilayers exhibited a decreasing trend with increasing monolayer thickness, and the corresponding mechanisms about the hardening behaviors in these multilayers were also discussed based on the interface density and irradiation-induced defects. • Irradiation induced grain growth in the multilayers depended on the individual thickness. • Bubble sizes and pressures in the multilayers were analyzed. • The hardening in the irradiated multilayers exhibit a decreasing trend with increasing individual thickness. [ABSTRACT FROM AUTHOR]

FeCoCrNiAlx-based high entropy alloys (HEAs) exhibit excellent high-temperature strength, good radiation tolerance and corrosion resistance. However, studies on light-ion irradiation damage are relatively lacking. The influence of hydrogen and helium effect on HEAs deserves further study, given their complex interaction during different sequence irradiation at different irradiation conditions. In the present study, FeCoCrNiAl 0.3 HEA was irradiated under two different modes at 723 K, one is single He2+ and the other is pre-iradiation of H+ followed by He2+. Microstructure evolution and irradiation hardening were studied by transmission electron microscopy and nanoindentation technique. There is no distinct difference of the irradiation hardening rate between sequential H+-He2+ and single He2+ irradiated samples. A large number of irradiation-induced nano-defects were observed after pre-irradiation of H+, which act as sinks to trap the subsequent helium atoms in the H+-He2+ irradiated sample. Fine helium bubbles and dislocation loops with high density dispersed in both irradiated samples. Compared with single He2+ irradiation, pre-irradiation of H+ resulted in a decrease in bubble density and an increase in loop density in H+-He2+ irradiated sample, but had little effect on their size. It is speculated that the critical concentration of bubble formation originating from H-He-V clusters is supposed to be higher than that of He V clusters at elevated temperatures in FeCoCrNiAl 0.3 alloy, leading to the aggregation and redistribution of He atoms at the H-He-V clusters rather than the formation of stable bubble nucleus after succeeding He2+ irradiation. The influence of pre-irradiation of H+ on FeCoCrNiAl 0.3 alloy on the formation of helium bubbles and the relationship between bubble, loop and irradiation hardening are discussed. • The pre-irradiation of H formed interstitial-type dislocation loops and much SFTs were observed after H-He irradiation. • Fine helium bubbles and dislocation loops with high density dispersed in both irradiated samples. • The average size of loops is similar while the density in H-He irradiated sample is about twice that of He irradiated one. • Little difference in average size of bubbles while density of H-He irradiated sample is much lower than that of He irradiation. [ABSTRACT FROM AUTHOR]

We use molecular dynamics to assess the effect of grain boundaries and rigid intergranular inclusions on plastic deformation in nickel (Ni) containing helium (He) bubbles and self-interstitial atom clusters. Our simulations show that plasticity in Ni bicrystals is relatively uniform, with no localized slip bands or nano-twins. We attribute this behavior to grain boundaries, which block dislocations and favor activation of new sources over persistent slip along a single plane. While there is no initiation of intergranular cracks, He bubbles at matrix/inclusion interfaces elongate along the tensile axis and migrate towards regions of high tension, potentially setting the conditions for formation of crack-like flaws via bubble coalescence. We discuss the implications of our work for understanding degradation of mechanical properties in Ni-base alloys in nuclear reactors. [ABSTRACT FROM AUTHOR]

• Helium bubble behavior under dry storage environment was studied in Al-B 4 C absorber. • Helium bubble formation was confirmed at the lowest dose (0.01 dpa) from 150 °C. • Abrupt helium bubble growth was observed above 270 °C. • Severe helium bubble agglomeration was observed at the interfacial boundaries. • Gap found in bubble density between high T irradiation and post-annealing case. Al-B 4 C neutron absorber in spent fuel dry storage could suffer significant radiation damage at elevated temperature (∼150–300 °C) mainly via 10B(n, α)7Li reactions. Several recent studies reported significant bubble formation in the absorber, however only from wet storage environment at near ambient temperature (< ∼90 °C). In this study, we irradiated a commercially widely used Al-B 4 C absorber of interest utilizing 120-keV helium ion-beam accelerator with twelve different conditions; three different doses (0.01, 0.1, and 1 dpa) combined with four different temperatures (room temperature, 150, 270, and 400 °C). This was to investigate the irradiation-induced microstructural evolution of the absorber in dry storage environment in expedited manner instead of time-consuming neutron irradiation test. The effects of irradiation dose and temperature on helium bubble nucleation and growth were investigated by image analysis on BF-TEM images, which showed a typical tendency of increasing bubble size with increasing temperature at the expense of decreasing bubble number density. Helium bubble morphology at grain boundaries was quite similar with the ones reported from the previous studies on the surveillance coupons used in the wet environment, elliptic helium bubbles around B 4 C particles. Notable difference was the prominent formation of numerous helium bubbles even from the lowest dose (0.01 dpa) at slightly elevated temperature (150 °C), perhaps due to higher irradiation temperature than that of spent fuel pool. This study may experimentally confirm that significant bubble formation and growth in the absorber could be the case from the early stage of the dry storage, even without the participation of hydrogen produced from the absorber corrosion. Particularly in the case of non-clad Al-B 4 C absorber, possible B 4 C particle detachment may need to be concerned since the bubble coalescence was concentrated at the interface between Al alloy matrix and B 4 C particle which would be microcracked under neutron irradiation. [Display omitted] [ABSTRACT FROM AUTHOR]

The unusual mechanical and radiation properties found in concentrated solid–solution alloys (CSAs) make them promising candidate materials in nuclear applications. In this work, He behavior in a typical CSA, NiCFeCr, is assessed by first-principles calculations. Our results suggest that the energy difference of He in substitutional and interstitial sites is greatly reduced. The energy barriers of He through both interstitial and substitutional diffusion are much higher than those in pure Ni, indicating slower He dynamics in CSAs. The calculated activation energies show that the dissociation mechanism is more favorable in NiCoFeCr, which helps to suppress He growth as observed experimentally.

Helium bubbles produced by neutron irradiation can induce property deterioration in ferritic/martensitic (F/M) steels serving in nuclear reactor. To investigate helium-to-vacancy (He/V) ratio of helium bubbles during nucleation and growth in α-Fe, molecular dynamics (MD) method was used to study the dynamic evolution of small helium bubbles under different concentrations of Frenkel pairs (0 ~ 6000 appm) and fixed helium atoms concentration of 2000 appm. Molecular statics (MS) method and Metropolis Monte Carlo (MMC) algorithm were combined to investigate the energetics of HenVm clusters with different He/V ratios and sizes to analyze underlying evolutionary mechanisms. The MD simulation results show that the He/V ratios increase firstly to about 1.4 and then fluctuate within an interval of about ± 0.15 with the increasing of bubble size, corresponding to the main peak of He/V ratio distribution of helium bubbles. And the dynamic evolution results suggest that the equilibrium He/V ratios decrease as Frenkel pairs concentrations increase. The energetics mechanism analysis also shows that the He/V ratio of the most stable helium bubbles is around 1.5 ± 0.1, the crossover of binding energy curve of self-interstitial atoms (SIA) and vacancy when the SIA and vacancy have identical ability to bind with the bubbles under neutron irradiation. Moreover, the simulation results of the He/V ratio of small helium bubbles are consistent with a previous experiment result on F/M steels irradiated in the spallation neutron source.

Two different of Sm-loading fluorapatite (Ca10−2xNaxSmx(PO4)6F2, x = 1 and 2) glass-ceramics were synthesized by a two-step melt sintering method. The samples were irradiated with 50 keV He+ ions with a fluence of 2.6 × 1016 ions/cm2 at 593 K. The irradiation induced microstructural evolution were characterized by grazing incidence X-ray diffraction and cross-sectional transmission electron microscopy. For the smaller Sm-doping samples, no phase transformation is observed. Meanwhile, in the lager Sm-doping samples, the irradiation induced the crystals into smaller nanocrystals. The mechanism of the transformation of the crystalline phase was also analyzed and discussed.

This work, with an emphasis on helium irradiation rates appropriate for fusion-plasma conditions, advances the understanding of helium evolution at grain boundaries in W, an important consideration in the understanding of W as a plasma-facing component. Using accelerated molecular dynamics, helium bubble nucleation and growth at a symmetric Σ5[100](310) tilt grain-boundary in W is studied. The simulations reveal that the growth mode associated with bubble growth at the grain-boundary leads to a suppression of the helium supply to the bubble and hence to arrested growth. Such an unconventional bubble growth mode may dominate in materials with a high density of sinks.

Subject of Research.The paper presents the study of the scheme with customizable dissipative properties for compressible multicomponent flows in case of an interaction between a shockwave and a helium bubble. Method. We chose a two-step TVD Runge-Kutta time-marching scheme. The spatial difference operator is splitting by the physical processes at each time step using an adaptive artificial viscosity of the Christensen type and TVD-reconstruction of flows by a weighted linear combination of upwind and central approximations of convective terms with flux limiter. To suppress the oscillations at the gases interface we used an Abgrall nonconservative advection equation. Main Results. Numerical convergence in the norm L1 is shown on the example of the one-dimensional Karni and Quirk test problem. We have performed a comparison of the proposed scheme and the finite-volume WENO type method of Coralic and Colonius on the same resolution grids and for the same Courant number. The presented scheme requires significantly lower computational costs for the resolution of the shock-wave pattern and vortex formation details. Practical Relevance. The scheme with customizable dissipative properties can be recommended for practical calculations of the interaction between shockwaves and gas interfaces of different physical properties, wave interference and vortex formation.

Nanocrystalline reduced activation ferritic/martensitic (RAFM) steel samples were prepared using surface mechanical attrition treatment (SMAT). Un-SMATed and SMATed reduced activation ferritic/martensitic samples were irradiated by helium ions at 200°C and 350°C with 2 dpa and 8 dpa, respectively, to investigate the effects of grain boundaries (GBs) and temperature on the formation of He bubbles during irradiation. Experimental results show that He bubbles are preferentially trapped at GBs in all the irradiated samples. Bubble denuded zones are clearly observed near the GBs at 350°C, whereas the bubble denuded zones are not obvious in the samples irradiated at 200°C. The average bubble size increases and the bubble density decreases with an increasing irradiation temperature from 200°C to 350°C. Both the average size and density of the bubbles increase with an increasing irradiation dose from 2 dpa to 8 dpa. Bubbles with smaller size and lower density were observed in the SMATed samples but not in the un-SMATed samples irradiated in the same conditions, which indicate that GBs play an important role during irradiation, and sink strength increases as grain size decreases.

Oxide dispersion strengthened (ODS) FeCrAl alloys were successfully fabricated by various mechanical alloying time (10h and 40h), combined with spark plasma sintering (SPS) technique. To understand their helium irradiation resistance, two kinds of ODS FeCrAl alloys were implanted with helium ions at 400 keV in the fluence of 1 × 1021/m2 at 350 °C, 400 °C, and 450 °C to investigate the effect of high concentration helium on the materials in advanced nuclear reactors. The distributions of helium bubbles associated with the microstructural features were characterized by transmission electron microscope (TEM). The experimental results showed that both the helium bubble sizes and volume fractions increased with increasing the irradiation temperatures. The high sink strength of nano-oxides and small inter-oxides distance contributed to the good suppression of helium bubbles growth. In particular, we identified that the high-density nano-oxides with the small sizes (<10 nm) could sequester the helium into several (1–3) tiny bubbles to inhibit bubbles accumulating and coarsening both in elevated temperatures and high helium concentration. Additionally, grain boundaries demonstrated the weaker ability than nano-oxides to disperse helium bubbles, when compared helium irradiation resistance in ODS FeCrAl alloys and FeCrAl alloys. Eventually, ODS FeCrAl alloys in this study revealed the superior helium irradiation resistance than other studied alloys, making it promising to be one of the potential structural materials applicated in advanced nuclear reactors. • The size and volume fraction of helium bubbles increased with increasing the irradiation temperature (350 °C, 400 °C, and 450 °C), when helium ions were implanted into ODS FeCrAl alloys to a fluence of 1 × 1021/m2. • High sink strength of nano-oxides and small inter-oxides distance contributed to the good suppression of helium bubbles growth. • High-density nano-oxides with the small sizes (<10 nm) could sequester the helium into several (1–3) tiny bubbles to inhibit bubbles accumulating and coarsening both in elevated temperatures and high helium concentration. [ABSTRACT FROM AUTHOR]

Powder metallurgy-hot isostatic pressing (PM-HIPing) is a promising technology for the manufacturing of components for nuclear reactors. In this study, Ni-based alloy GH3535 for molten salt reactor applications, was processed via PM-HIPing, and irradiation damage behaviors of the PM-HIPed GH3535 along with its forged counterparts were studied with He+ ions irradiation and microstructural characterization. The irradiation-induced defects in the samples, such as helium bubbles and dislocation loops, were characterized using transmission electron microscope (TEM). The mechanical responses of irradiated specimens were studied by nano-indentation. The results show that the PM-HIPed GH3535 exhibits a comparable irradiation resistance to its forged counterparts. Size of the helium bubbles and dislocation loops are larger for samples irradiated at high temperature than those irradiated at room temperature, irrespective of processing methods. However, number density of these defects is smaller for the high temperature irradiated samples than those irradiated at room temperature, which is particularly evident for the forged sample. The samples show significant hardening after irradiation, evidenced by the nano-indentation hardness test results. The experimental results and the calculations based on dispersed barrier hardening (DBH) model are in general agreement. This study will provide important data to support the application of the PM-HIPed materials or components in the nuclear reactors. [ABSTRACT FROM AUTHOR]

As important irradiation defects, helium bubbles will threaten the stability and reliability of reactor structural materials. Extensive research has been conducted on the dynamic response of helium bubbles under thermal annealing, irradiation, or tension, but the behavior and mechanisms of helium bubble evolution under compression remain to be understood. Here, by adopting in-situ cyclic nanocompression testing on Fe9Cr1.5W0.4Si F/M steel containing helium bubbles at 673 K, we present direct experimental evidence for the shrinkage and coalescence behavior of helium bubbles under the coupling effect of compression stress and temperature for the first time. Unlike the stress-free condition where the bubble size shows an increasing trend, compressive stress causes the bubble to shrink. Moreover, the large-sized bubbles show a greater change in size than small-sized bubbles. At 673 K, there is no thermal motion of the bubbles in F/M steel, and it is the deformation and mutual squeezing of bubbles under compression that causes the coalescence of tightly arranged bubbles. Furthermore, helium bubbles can act as dislocation sources to emit dislocations under compression, and the emitted dislocations can in turn envelope around the helium bubbles, thus hindering bubble coalescence and leading to the formation of deformed bubbles. [ABSTRACT FROM AUTHOR]